Deck 8: Force

Question 1

A box slides down a frictionless plane inclined at an angle θ above the horizontal.The gravitational force on the box is directed

A) parallel to the plane in the same direction as the movement of the box.
B) parallel to the plane in the opposite direction as the movement of the box.
C) perpendicular to the plane.
D) vertically.
E) at an angle θ below the inclined plane.

Answer 1

vertically.

Question 2

A 615 N student standing on a scale in an elevator notices that the scale reads 645 N.From this information,the student knows that the elevator must be moving

A) downward.
B) upward.
C) You cannot tell if it is moving upward or downward.

Answer 2

You cannot tell if it is moving upward or downward.

Question 3

In order to lift a bucket of concrete,you must pull up harder on the bucket than it pulls down on you.

Answer 3

False

Question 4

Two bodies P and Q on a smooth horizontal surface are connected by a light cord.The mass of P is greater than that of Q.A horizontal force (of magnitude F) is applied to Q as shown in the figure, accelerating the bodies to the right. The magnitude of the force exerted by the connecting cord on body P will be

A) zero.
B) less than F but not zero.
C) equal to F.
D) greater than F.

Question 5

Consider what happens when you jump up in the air.Which of the following is the most accurate statement?

A) It is the upward force exerted by the ground that pushes you up, but this force cannot exceed your weight.
B) You are able to spring up because the earth exerts a force upward on you that is greater than the downward force you exert on the earth.
C) Since the ground is stationary, it cannot exert the upward force necessary to propel you into the air. Instead, it is the internal forces of your muscles acting on your body itself that propels your body into the air.
D) When you push down on the earth with a force greater than your weight, the earth will push back with the same magnitude force and thus propel you into the air.
E) When you jump up the earth exerts a force F₁ on you and you exert a force F₂ on the earth. You go up because F₁ > F₂.

Question 6

A stalled car is being pushed up a hill at constant velocity by three people.The net force on the car is

A) zero.
B) up the hill and equal to the weight of the car.
C) down the hill and equal to the weight of the car.
D) up the hill and greater than the weight of the car.
E) down the hill and greater than the weight of the car.

Question 7

You are standing in a moving bus,facing forward,and you suddenly fall forward as the bus comes to an immediate stop.The force acting on you that causes you to fall forward is

A) the force of gravity.
B) the normal force due to your contact with the floor of the bus.
C) the force due to static friction between you and the floor of the bus.
D) the force due to kinetic friction between you and the floor of the bus.
E) No forces were acting on you to cause you to fall.

Question 8

An object is moving to the right,and experiencing a net force that is directed to the right.The magnitude of the force is decreasing with time.The speed of the object is

A) increasing.
B) decreasing.
C) constant in time.

Question 9

Which one of the following free-body diagrams best represents the free-body diagram,with correct relative force magnitudes,of a person in an elevator that is traveling upward but is gradually slowing down at a rate of 9 m/s²? _f is the force of the floor on the person and
_g is the force of gravity on the person.

A)

B)

C)

D)

E)

Question 10

Two objects having masses m₁ and m₂ are connected to each other as shown in the figure and are released from rest.There is no friction on the table surface or in the pulley.The masses of the pulley and the string connecting the objects are completely negligible.What must be true about the tension T in the string just after the objects are released?

A) T = m₂g
B) T > m₂g
C) T < m₂g
D) T = m₁g
E) T > m₁g

Question 11

Two objects,each of weight W,hang vertically by spring scales as shown in the figure.The pulleys and the strings attached to the objects have negligible weight,and there is no appreciable friction in the pulleys.The reading in each scale is

A) W.
B) more than W, but not quite twice as much.
C) less than W.
D) 2W.
E) more than 2W.

Question 12

Two weights are connected by a massless wire and pulled upward with a constant speed of 1.50 m/s by a vertical pull P.The tension in the wire is T (see figure).Which one of the following relationships between T and P must be true?

A) T > P
B) T = P
C) P + T = 125 N
D) P = T + 25 N
E) P = T + 100 N

Question 13

Which one of the following free-body diagrams best represents the free-body diagram,with correct relative force magnitudes,of a person in an elevator that is traveling upward with an unchanging velocity? _f is the force of the floor on the person and
_g is the force of gravity on the person.

A)

B)

C)

D)

E)

Question 14

Two unequal masses M and m (M > m)are connected by a light cord passing over a pulley of negligible mass,as shown in the figure.When released,the system accelerates.Friction is negligible.Which figure below gives the correct free-body force diagrams for the two masses in the moving system?

A)

B)

C)

D)

Question 15

You swing a bat and hit a heavy box with a force of 1500 N.The force the box exerts on the bat is

A) exactly 1500 N only if the box does not move.
B) exactly 1500 N whether or not the box moves.
C) greater than 1500 N if the box moves.
D) less than 1500 N if the box moves.
E) greater than 1500 N if the bat bounces back.

Question 16

A 20-ton truck collides with a 1500-lb car and causes a lot of damage to the car.During the collision

A) the force on the truck due to the collision is slightly greater than the force on the car.
B) the force of on the truck due to the collision is exactly equal to the force on the car.
C) the force on the car due to the collision is much greater than the force on the truck.
D) the car and the truck have the same magnitude acceleration.

Question 17

A fish weighing 16 N is weighed using two spring scales,each of negligible weight,as shown in the figure.What will be the readings of the scales?

A) The bottom scale will read 16 N, and the top scale will read zero.
B) Each scale will read 16 N.
C) The top scale will read 16 N, and the bottom scale will read zero.
D) The scales will have different readings, but the sum of the two readings will be 16 N.
E) Each scale will read 8 N.

Question 18

Point P in the figure indicates the position of an object traveling at constant speed clockwise around the circle.Which arrow best represent the direction the object would travel if the net external force on it were suddenly reduced to zero?

A)

B)

C)
D)

E)

Question 19

In order to get an object moving,you must push harder on it than it pushes back on you.

Question 20

A car is being towed at constant velocity on a horizontal road using a horizontal chain.The tension in the chain must be equal to the weight of the car in order to maintain constant velocity.

Question 21

A ball is tossed vertically upward.When it reaches its highest point (before falling back downward)

A) the velocity is zero, the acceleration is directed downward, and the force of gravity acting on the ball is directed downward.
B) the velocity is zero, the acceleration is zero, and the force of gravity acting on the ball is zero.
C) the velocity is zero, the acceleration is zero, and the force of gravity acting on the ball is directed downward.
D) the velocity and acceleration reverse direction, but the force of gravity on the ball remains downward.
E) the velocity, acceleration, and the force of gravity on the ball all reverse direction.

Question 22

A block lies on a horizontal frictionless surface. A horizontal force of 100 N is applied to the block giving rise to an acceleration of 3.0 m/s².
(a) Determine the mass of the block.
(b) Calculate the distance the block will travel if the force is applied for 10 s.
(c) Calculate the speed of the block after the force has been applied for 10 s.

Question 23

A woman is straining to lift a large crate,without success because it is too heavy.We denote the forces on the crate as follows: P is the upward force the woman exerts on the crate,C is the vertical contact force exerted on the crate by the floor,and W is the weight of the crate.How are the magnitudes of these forces related while the woman is trying unsuccessfully to lift the crate?

A) P + C = W
B) P + C < W
C) P + C > W
D) P = C

Question 24

Suppose you are playing hockey on a new-age ice surface for which there is no friction between the ice and the hockey puck.You wind up and hit the puck as hard as you can.After the puck loses contact with your stick,the puck will

A) start to slow down.
B) not slow down or speed up.
C) speed up a little, and then slow down.
D) speed up a little, and then move at a constant speed.

Question 25

The figure shows an acceleration-versus-force graph for three objects pulled by rubber bands.The mass of object 2 is 36 kg.What are the masses of objects 1 and 3?

A) 14 kg and 90 kg
B) 72 kg and 18 kg
C) 90 kg and 18 kg
D) 14 kg and 72 kg

Question 26

If a 5.0 kg box is pulled simultaneously by a 10.0 N force and a 5.0 N force,then its acceleration must be

A) 3.0 m/s².
B) 2.2 m/s².
C) 1.0 m/s².
D) We cannot tell from the information given.

Question 27

A crate is sliding down an inclined ramp at a constant speed of 0.55 m/s.The vector sum of all the forces acting on this crate must point down the ramp.

Question 28

Which of the graphs in the figure illustrates Hooke's Law?

A) Graph a
B) Graph b
C) Graph c
D) Graph d

Question 29

A 50.0-N box is sliding on a rough horizontal floor,and the only horizontal force acting on it is friction.You observe that at one instant the box is sliding to the right at 1.75 m/s and that it stops in 2.25 s with uniform acceleration.What magnitude force does friction exert on this box?

A) 3.97 N
B) 8.93 N
C) 38.9 N
D) 50.0 N
E) 490 N

Question 30

You are seated in a bus and notice that a hand strap that is hanging from the ceiling hangs away from the vertical in the backward direction.From this observation,you can conclude that

A) the velocity of the bus is forward.
B) the velocity of the bus is backward.
C) You cannot conclude anything about the direction of the velocity of the bus.

Question 31

Consider a plot of the displacement (x)as a function of the applied force (F)for an ideal elastic spring.The slope of the curve would be

A) the spring constant.
B) the reciprocal of the spring constant.
C) the acceleration due to gravity.
D) the reciprocal of the acceleration of gravity.
E) the mass of the object attached to the spring.

Question 32

Which of the graphs in the figure represents a spring that gets less stiff the more it is stretched?

A) Graph a
B) Graph b
C) Graph c
D) Graph d

Question 33

A small car has a head-on collision with a large truck.Which of the following statements concerning the magnitude of the average force due to the collision is correct?

A) The truck experiences the greater average force.
B) The small car experiences the greater average force.
C) The small car and the truck experience the same average force.
D) It is impossible to tell since the masses are not given.
E) It is impossible to tell since the velocities are not given.

Question 34

A 7.0-kg object is acted on by two forces. One of the forces is 10.0 N acting toward the east. Which of the following forces is the other force if the acceleration of the object is 1.0 m/s² toward the east?

A) 6.0 N east
B) 3.0 N west
C) 12 N east
D) 9.0 N west
E) 7.0 N west

Question 35

An object is moving forward with a constant velocity.Which statement about this object MUST be true?

A) The net force on the object is zero.
B) The net force on the object is in the forward direction.
C) No forces are acting on the object.
D) The acceleration of the object is in the forward direction.

Question 36

Consider two less-than-desirable options.In the first you are driving 30 mph and crash head-on into an identical car also going 30 mph.In the second option you are driving 30 mph and crash head-on into a stationary brick wall.In neither case does your car bounce off the thing it hits,and the collision time is the same in both cases.Which of these two situations would result in the greatest impact force?

A) hitting the other car
B) hitting the brick wall
C) The force would be the same in both cases.
D) We cannot answer this question without more information.
E) None of these is true.

Question 37

On a horizontal frictionless floor,a worker of weight 0.900 kN pushes horizontally with a force of 0.200 kN on a box weighing 1.80 kN.As a result of this push,which statement could be true?

A) The box will not move because the push is less than its weight.
B) The worker and box will both have an acceleration of 1.08 m/s², but in opposite directions.
C) The worker and box will both have an acceleration of 2.17 m/s², but in opposite directions.
D) The worker will accelerate at 1.08 m/s² and the box will accelerate at 2.17 m/s², but in opposite directions.
E) The worker will accelerate at 2.17 m/s² and the box will accelerate at 1.08 m/s², but in opposite directions.

Question 38

A dog is standing in the bed of a pickup truck.The bed is coated with ice,causing the force of friction between the dog and the truck to be zero.The truck is initially at rest,and then accelerates to the right,moving along a flat road.As seen from a stationary observer (watching the truck move to the right),the dog

A) does not move left or right, but the back of the truck moves towards the dog.
B) moves to the right, but not as quickly as the truck is moving to the right, causing it to slide towards the back of the truck.
C) moves to the right at the same rate as the truck, so it doesn't slide.
D) moves to the left, as the truck moves to the right, causing the dog to slide towards the back of the truck.

Question 39

A block is on a frictionless horizontal table,on earth.This block accelerates at 1.9 m/s² when a 90 N horizontal force is applied to it.The block and table are then set up on the moon where the acceleration due to gravity is 1.62 m/s².What is the weight of the block on the moon?

A) 77 N
B) 67 N
C) 58 N
D) 48 N
E) 39 N

Question 40

The International Space Station has a mass of 1.8 × 10⁵ kg. A 70.0-kg astronaut inside the station pushes off one wall of the station so she accelerates at 1.50 m/s². What is the magnitude of the acceleration of the space station as the astronaut is pushing off the wall? Give your answer relative to an observer who is space walking and therefore does not accelerate with the space station due to the push.

A) 5.8 × 10^-4 m/s²
B) 1.50 m/s²
C) 4.7 × 10^-4 m/s²
D) zero
E) 3.9 × 10^-3 m/s²

Question 41

An 1100-kg car traveling at 27.0 m/s starts to slow down and comes to a complete stop in 578 m.What is the magnitude of the average braking force acting on the car?

A) 690 N
B) 550 N
C) 410 N
D) 340 N

Question 42

A 10,000-kg rocket blasts off from earth with a uniform upward acceleration of 2.00 m/s² and feels no air resistance. The upward thrust force its engines must provide during this acceleration is closest to

A) 20,000 N.
B) 980,000 N.
C) 118,000 N.
D) 78,000 N.

Question 43

On its own,a certain tow-truck has a maximum acceleration of 3.0 m/s². What would be the maximum acceleration when this truck was towing a bus of twice its own mass?

A) 2.5 m/s²
B) 2.0 m/s²
C) 1.5 m/s²
D) 1.0 m/s²

Question 44

The figure shows a 100-kg block being released from rest from a height of 1.0 m.It then takes it 0.90 s to reach the floor.What is the mass m of the other block? The pulley has no appreciable mass or friction.

A) 60 kg
B) 54 kg
C) 48 kg
D) 42 kg

Question 45

A 60.0-kg person rides in an elevator while standing on a scale. The scale reads 400 N. The acceleration of the elevator is closest to

A) 3.13 m/s² downward.
B) 6.67 m/s² downward.
C) zero.
D) 9.80 m/s² downward.
E) 6.67 m/s² upward.

Question 46

The figure shows two 1.0 kg-blocks connected by a rope.A second rope hangs beneath the lower block.Both ropes have a mass of 250 g.The entire assembly is accelerated upward at 2.3 m/s² by force .What is the tension at the top end of rope 1?

A) 18 N
B) 15 N
C) 2.9 N
D) 3.5 N

Question 47

A box of mass 50 kg is at rest on a horizontal frictionless surface.A constant horizontal force F then acts on the box and accelerates it to the right.It is observed that it takes the box 6.9 seconds to travel 28 meters.What is the magnitude of the force?

Question 48

Two objects are connected by a very light flexible string as shown in the figure,where M = 0.60 kg and m = 0.40 kg. You can ignore friction and the mass of the pulley.
(a) Draw free-body diagrams for each object.
(b) Calculate the magnitude of the acceleration of each object.
(c) Calculate the tension in the string.

Question 49

A series of weights connected by very light cords are given an upward acceleration of 4.00 m/s² by a pull P,as shown in the figure.A,B,and C are the tensions in the connecting cords.The SMALLEST of the three tensions,A,B,and C,is closest to

A) 80.0 N.
B) 196 N.
C) 276 N.
D) 483 N.
E) 621 N.

Question 50

A 6.00-kg block is in contact with a 4.00-kg block on a horizontal frictionless surface as shown in the figure.The 6.00-kg block is being pushed by a horizontal 20.0-N force as shown. What is the magnitude of the force that the 6.00-kg block exerts on the 4.00-kg block?

A) 6.00 N
B) 20.0 N
C) 8.00 N
D) 4.00 N
E) 10.0 N

Question 51

15)Bumpers on cars are not of much use in a collision.To see why,calculate the average force a bumper would have to exert if it brought a 1200-kg car (a so-called compact model)to a rest in 15 cm when the car had an initial speed of 2.0 m/s (about 4.5 mph).(Bumpers are built with springs that compress to provide a stopping force without,hopefully,denting the metal.)

A) 1.8 × 10⁴ N
B) 1.6 × 10⁴ N
C) 5.4 × 10⁴ N
D) 6.5 × 10⁵ N
E) 3.2 × 10⁴ N

Question 52

A series of weights connected by very light cords are given an upward acceleration of 4.00 m/s² by a pull P,as shown in the figure.A,B,and C are the tensions in the connecting cords.The pull P is closest to

A) 690 N.
B) 490 N.
C) 290 N.
D) 200 N.
E) 50 N.

Question 53

Three boxes in contact rest side-by-side on a smooth,horizontal floor.Their masses are 5.0-kg,3.0-kg,and 2.0-kg,with the 3.0-kg box in the center.A force of 50 N pushes on the 5.0-kg box,which pushes against the other two boxes.
(a) Draw the free-body diagrams for each of the boxes.
(b) What magnitude force does the 3.0-kg box exert on the 5.0-kg box?
(c) What magnitude force does the 3.0-kg box exert on the 2.0-kg box?

Question 54

A 1000-kg car is driving toward the north along a straight horizontal road at a speed of 20.0 m/s. The driver applies the brakes and the car comes to a rest uniformly in a distance of 200 m.What are the magnitude and direction of the net force applied to the car to bring it to rest?

A) 1.00 N north
B) 10.0 × 10³ N south
C) 1.00 × 10³ N south
D) 1.00 N south
E) 100 N south

Question 55

A construction worker pulls a box of tools on a smooth horizontal floor with a force of 100 N in a direction of 37.0° above the horizontal. The mass of the box and the tools is 40.0 kg.
(a) Draw a free-body diagram for the box.
(b) Calculate the acceleration of the box.
(c) How hard does the floor push up on the box?

Question 56

Three objects are connected by massless wires over a massless frictionless pulley as shown in the figure.The tension in the wire connecting the 10.0-kg and 15.0-kg objects is measured to be 133 N.What is the mass M?

A) 8.33 kg
B) 33.9 kg
C) 35.0 kg
D) 52.8 kg
E) 95.0 kg

Question 57

A locomotive is pulling 19 freight cars,each of which is loaded with the same amount of weight.The mass of each freight car (with its load)is 37,000 kg.If the train is accelerating at 0.22 m/s² on a level track,what is the tension in the coupling between the second and third cars? (The car nearest the locomotive is counted as the first car,and friction is negligible.)

Question 58

A block is on a frictionless horizontal table,on earth.This block accelerates at 3.6 m/s² when a 90 N horizontal force is applied to it. The block and table are then set up on the moon where the acceleration due to gravity is 1.62 m/s². A horizontal force of 45 N is applied to the block when it is on the moon.What acceleration does this force impart to the block?

A) 1.8 m/s²
B) 1.6 m/s²
C) 2.0 m/s²
D) 2.2 m/s²
E) 2.3 m/s²

Question 59

In a ballistics test,a 1.50-g bullet is fired through a 28.0-kg block traveling horizontally toward the bullet.In this test,the bullet takes 11.4 ms to pass through the block as it reverses the block's velocity from 1.75 m/s to the right to 1.20 m/s to the left with constant acceleration. Find the magnitude of the force that the bullet exerts on the block during this ballistics test.

Question 60

A 60.0-kg person rides in elevator while standing on a scale. The elevator is traveling downward but slowing down at a rate of 2.00 m/s². The reading on the scale is closest to

A) 589 N.
B) 708 N.
C) 469 N.
D) 120 N.
E) 349 N.

Question 61

A 20.0-N box rests on a 50.0-N box on a perfectly smooth horizontal floor.When a horizontal 15.0-N pull to the right is exerted on the lower box (see figure),both boxes move together.Find the magnitude and direction of the net external force on the upper box.

Question 62

An object attached to an ideal massless spring is pulled across a frictionless surface.If the spring constant is 45 N/m and the spring is stretched by 0.88 m when the object is accelerating at 2.0 m/s²,what is the mass of the object?

A) 20 kg
B) 17 kg
C) 22 kg
D) 26 kg

Question 63

The graph in the figure shows the x component of the acceleration of a 2.4-kg object as a function of time (in ms).
(a) At what time(s)does the x component of the net force on the object reach its maximum magnitude,and what is that maximum magnitude?
(b) What is the x component of the net force on the object at time t = 0.0 ms and at t = 4.0 ms?

Question 64

A spring stretches by 21.0 cm when a 135 N object is attached.What is the weight of a fish that would stretch the spring by 31.0 cm?

A) 199 N
B) 91.0 N
C) 145 N
D) 279 N

Question 65

A block of mass m = 8.40 kg,moving on a horizontal frictionless surface with a speed 4.20 m/s,makes a perfectly elastic collision with a block of mass M at rest.After the collision,the 8.40 block recoils with a speed of 0.400 m/s. In the figure,the blocks are in contact for 0.200 s.The magnitude of the average force on the 8.40-kg block,while the two blocks are in contact,is closest to

A) 193 N
B) 185 N
C) 176 N
D) 168 N
E) 160 N

Question 66

Two weights are connected by a massless wire and pulled upward with a constant speed of 1.50 m/s by a vertical pull P.The tension in the wire is T (see figure).P is closest to

A) 25 N.
B) 125 N.
C) 187.5 N.
D) 245 N.
E) 1225 N.

Question 67

The figure shows a graph of the acceleration of an object as a function of the net force acting on it. The mass of this object,in grams,is closest to

A) 130.
B) 11.
C) 89.
D) 8000.

Question 68

The figure shows two forces acting on an object,with magnitudes F₁ = 78 N and F₂ = 26 N.What third force will cause the object to be in equilibrium (acceleration equals zero)?

A) 52 N pointing down
B) 52 N pointing up
C) 82 N pointing down
D) 82 N pointing up

Question 69

The graph in the figure shows the net force acting on a 3.0-kg object as a function of time.
(a) What is the acceleration of this object at time t = 2.0 s?
(b) Draw,to scale,a graph of the acceleration of this object as a function of time over the range t = 0.00 s to t = 7.0 s.

Question 70

In the figure,two identical ideal massless springs have unstretched lengths of 0.25 m and spring constants of 700 N/m.The springs are attached to a small cube and stretched to a length L of 0.30 m as in Figure A.An external force P pulls the cube a distance D = 0.020 m to the right and holds it there.(See Figure B.)The external force P,that holds the cube in place in Figure B,is closest to

A) 28 N.
B) 25 N.
C) 21 N.
D) 18 N.
E) 14 N.

Question 71

The figure shows a graph of the acceleration of a 125-g object as a function of the net force acting on it.What is the acceleration at points A and B?

Question 72

A stationary 1.67-kg object is struck by a stick.The object experiences a horizontal force given by
F = at - bt²,where t is the time in milliseconds from the instant the stick first contacts the object.
If a = 1500 N/(ms)and b = 20 N/(ms)²,what is the speed of the object just after it comes away from the stick at t = 2.74 ms?

A) 3.3 m/s
B) 22 m/s
C) 3.7 m/s
D) 25 m/s

Question 73

A 0.500-kg ball traveling horizontally on a frictionless surface approaches a very massive stone at 20.0 m/s perpendicular to wall and rebounds with 70.0% of its initial kinetic energy. What is the magnitude of the change in momentum of the stone?

A) 18.4 kg•m/s
B) 14.0 kg•m/s
C) 3.00 kg•m/s
D) 1.63 kg•m/s
E) 0.000 kg•m/s

Question 74

A time-varying horizontal force F(t)= At⁴ + Bt² acts for 0.500 s on a 12.25-kg object,starting at time
t = 1.00 s.In the SI system,A has the numerical value 4.50 and B has the numerical value 8.75.
(a)What are the SI units of A and B?
(b)What impulse does this force impart to the object?

Question 75

During a collision with a wall,the velocity of a 0.200-kg ball changes from 20.0 m/s toward the wall to 12.0 m/s away from the wall. If the time the ball was in contact with the wall was 60.0 ms,what was the magnitude of the average force applied to the ball?

A) 40.0 N
B) 107 N
C) 16.7 N
D) 26.7 N
E) 13.3 N

Question 76

Three objects are connected by massless wires over a massless frictionless pulley as shown in the figure.The tension in the wire connecting the 10.0-kg and 15.0-kg objects is measured to be 133 N.What is the tension in wire A?

A) 87.5 N
B) 245 N
C) 280 N
D) 333 N
E) 517 N