Deck 7: Newtons Third Law

Question 1

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.

Answer 1

exactly 1500 N whether or not the box moves.

Question 2

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.

Answer 2

the force of on the truck due to the collision is exactly equal to the force on the car.

Question 3

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.

Answer 3

7.25 × 10³ N

Question 4

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 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 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 7

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 8

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 9

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 10

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 11

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 12

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

Question 13

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 14

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 15

A wooden block A of mass 4.0 kg slides on a frictionless table when pulled using a massless string and pulley array by a hanging box B of mass 5.0 kg, as shown in the figure. What is the acceleration of block A as it slides on the frictionless table? Hint: Think carefully about the acceleration constraint.

A) 4.1 m/s²
B) 3.5 m/s²
C) 3.1 m/s²
D) 2.7 m/s²

Question 16

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 17

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 18

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 19

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

Question 20

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 21

Two blocks are connected by a string that goes over an ideal pulley as shown in the figure. Block A has a mass of 3.00 kg and can slide over a rough plane inclined 30.0° to the horizontal. The coefficient of kinetic friction between block A and the plane is 0.400. Block B has a mass of 2.77 kg. What is the acceleration of the blocks?

A) 0.392 m/s²
B) 1.96 m/s²
C) 3.12 m/s²
D) 5.35 m/s²
E) 0.00 m/s²

Question 22

In the figure, two wooden blocks each of 0.30 kg mass are connected by a string that passes over a very light frictionless pulley. One block slides on a horizontal table, while the other hangs suspended by the string, as shown in the figure. At time t = 0, the suspended block is 0.80 m over the floor, and the blocks are released from rest. After 2.5 s, the suspended block reaches the floor. What is the coefficient of kinetic friction between the table and the sliding block?

A) 0.35
B) 0.52
C) 0.84
D) 0.65
E) 0.95

Question 23

A 150-N box is being pulled horizontally in a wagon accelerating uniformly at 3.00 m/s². The box does not move relative to the wagon, the coefficient of static friction between the box and the wagon's surface is 0.600, and the coefficient of kinetic friction is 0.400. The friction force on this box is closest to

A) 450 N.
B) 90.0 N.
C) 60.0 N.
D) 45.9 N.

Question 24

A 4.00-kg box sits atop a 10.0-kg box on a horizontal table. The coefficient of kinetic friction between the two boxes and between the lower box and the table is 0.600, while the coefficient of static friction between these same surfaces is 0.800. A horizontal pull to the right is exerted on the lower box, as shown in the figure, and the boxes move together. What is the friction force on the UPPER box?

A) 19.3 N to the right
B) 19.3 N to the left
C) 23.5 N to the right
D) 31.4 N to the right
E) 31.4 N to the left

Question 25

The figure shows a 2000 kg cable car descending a high hill. A counterweight of mass 1800 kg on the other side of the hill aids the brakes in controlling the cable car's speed. The rolling friction of both the cable car and the counterweight are negligible. How much braking force does the cable car need to descend at constant speed?

A) 3800 N
B) 2900 N
C) 2000 N
D) 980 N

Question 26

Two boxes are connected by a weightless cord running over a very light frictionless pulley as shown in the figure. Box A, of mass 8.0 kg, is initially at rest on the top of the table. The coefficient of kinetic friction between box A and the table is 0.10. Box B has a mass of 15.0 kg, and the system begins to move just after it is released.
(a) Draw the free-body diagrams for each of the boxes, identifying all of the forces acting on each one.
(b) Calculate the acceleration of each box.
(c) What is the tension in the cord?

Question 27

Two blocks are connected by a string that goes over an ideal pulley as shown in the figure and pulls on block A parallel to the surface of the plane. Block A has a mass of 3.00 kg and can slide along a rough plane inclined 30.0° to the horizontal. The coefficient of static friction between block A and the plane is 0.400. What mass should block B have in order to start block A sliding up the plane?

Question 28

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 29

A pickup truck is moving at 25 m/s with a toolbox of mass m resting on the bed of the truck 2.5 m behind the cab. Suddenly the brakes are applied, causing the toolbox to slide, and the truck comes to a stop in 4.7 s. The coefficient of kinetic friction between the toolbox and the bed of the truck is 0.28. After the brakes are applied, how much time elapses before the toolbox strikes the cab?

Question 30

Three objects are connected as shown in the figure. The strings and frictionless pulleys have negligible masses, and the coefficient of kinetic friction between the 2.0-kg block and the table is 0.25. What is the acceleration of the 2.0-kg block?

A) 2.5 m/s²
B) 1.7 m/s²
C) 3.2 m/s²
D) 4.0 m/s²

Question 31

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 32

A 4.00-kg block rests between the floor and a 3.00-kg block as shown in the figure. The 3.00-kg block is tied to a wall by a horizontal rope. If the coefficient of static friction is 0.800 between each pair of surfaces in contact, what horizontal force F must be applied to the 4.00-kg block to make it move?

A) 16.2 N
B) 54.9 N
C) 21.1 N
D) 23.5 N
E) 78.4 N

Question 33

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 34

A system comprised blocks, a light frictionless pulley, and connecting ropes is shown in the figure. The 9.0-kg block is on a perfectly smooth horizontal table. The surfaces of the 12-kg block are rough, with µ_k = 0.30 between the block and the table. If the 5.0-kg block accelerates downward when it is released, find its acceleration.

A) 1.0 m/s²
B) 1.2 m/s²
C) 1.4 m/s²
D) 1.6 m/s²
E) 1.8 m/s²

Question 35

A system comprising blocks, a light frictionless pulley, a frictionless incline, and connecting ropes is shown in the figure. The 9.0-kg block accelerates downward when the system is released from rest. The tension in the rope connecting the 6.0-kg block and the 4.0-kg block is closest to

A) 30 N.
B) 33 N.
C) 36 N.
D) 39 N.
E) 42 N.

Question 36

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 37

Block A of mass 5.0 kg and block X are attached to a rope which passes over a pulley, as shown in the figure. An 80-N force P is applied horizontally to block A, keeping it in contact with a rough vertical face. The coefficients of static and kinetic friction between the wall and block A are μ_s = 0.40 and μ_k = 0.30. The pulley is light and frictionless. The mass of block X is adjusted until block A moves upward with an acceleration of 1.6 m/s². What is the mass of block X?

A) 9.9 kg
B) 9.3 kg
C) 8.7 kg
D) 8.1 kg
E) 7.5 kg

Question 38

The figure shows two packages that start sliding down a 20° ramp from rest a distance d = 6.6 m along the ramp from the bottom. Package A has a mass of 5.0 kg and a coefficient of kinetic friction 0.20 between it and the ramp. Package B has a mass of 10 kg and a coefficient of kinetic friction 0.15 between it and the ramp. How long does it take package A to reach the bottom?

A) 2.7 s
B) 3.0 s
C) 3.2 s
D) 3.5 s

Question 39

A rope pulls on the lower block in the figure with a tension force of 20 N. The coefficient of kinetic friction between the lower block and the surface is 0.16. The coefficient of kinetic friction between the lower block and the upper block is also 0.16. The pulley has no appreciable mass or friction. What is the acceleration of the 2.0 kg block?

A) 4.1 m/s²
B) 5.1 m/s²
C) 8.4 m/s²
D) 9.2 m/s²

Question 40

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

Question 41

Block A of mass 8.0 kg and block X are attached to a rope that passes over a pulley. A 50-N force P is applied horizontally to block A, keeping it in contact with a rough vertical face. The coefficients of static and kinetic friction between the wall and block A are μ_s = 0.40 and μ_k = 0.30. The pulley is light and frictionless. In the figure, the mass of block X is adjusted until block A descends at constant velocity of 4.75 cm/s when it is set into motion. What is the mass of block X?

A) 6.5 kg
B) 7.2 kg
C) 8.0 kg
D) 8.8 kg
E) 9.5 kg

Question 42

A system of blocks and a frictionless pulley is shown in the figure. Block A has a mass of 2.0 kg and is on a rough horizontal surface for which μ_s = 0.40 between the surface and block A. The rope pulls horizontally on block A. Block C has a mass of 1.0 kg. An external force P = 23.0 N, applied vertically to block A, maintains the system at rest as shown in the figure. What is the friction force on block A?

A) 6.3 N
B) 6.9 N
C) 7.5 N
D) 5.7 N
E) 5.1 N

Question 43

A 1520-N crate is to be held in place on a ramp that rises at 30.0° above the horizontal (see figure). The massless rope attached to the crate makes a 22.0° angle above the surface of the ramp. The coefficients of friction between the crate and the surface of the ramp are µ_k = 0.450 and µ_s = 0.650. The pulley has no appreciable mass or friction. What is the MAXIMUM weight w that can be used to hold this crate stationary on the ramp?