Deck 8: Systems of Particles and Conservation of Linear Momentum

Question 1

A boy and girl on ice skates face each other. The girl has a mass of 20 kg and the boy has a mass of 30 kg. The boy pushes the girl backward at a speed of 3.0 m/s. As a result of the push, the speed of the boy is

A) zero
B) 2.0 m/s
C) 3.0 m/s
D) 4.5 m/s
E) 9.0 m/s

Answer 1

2.0 m/s

Question 2

Calculate the impulse by the force, as shown in the figure below.

A) 1.0 mN.s
B) 2.0 mN.s
C) 4.0 mN.s
D) 10.0 mN.s
E) 40.0 mN.s

Answer 2

4.0 mN.s

Question 3

If the momentum of a mass M is doubled, its kinetic energy will be multiplied by a factor of

A)
B) 2
C)
D) 4
E)

Answer 3

4

Question 4

For this question, assume that all velocities are horizontal and that there is no friction. Two skaters A and B are on an ice surface. A and B have the same mass M = 90.5 kg. A throws a ball with mass m = 200 g toward B with a speed v = 21.5 m/s relative to the ice. B catches the ball and throws it back to A with the same speed. After A catches the ball, his speed with respect to the ice is

A) 4.3 $\times$ 10³ m/s
B) 4.3 m/s
C) 4.8 $\times$ 10^-2 m/s
D) 9.5 $\times$ 10^-2 m/s
E) 0.34 m/s

Question 5

The SI units of momentum are

A) kg · m · s
B) m²/s
C) kg · s/m
D) kg/J
E) kg · m/s

Question 6

Two particles, each of mass m, are moving with velocity and The kinetic energy at the center-of-mass is

A) mv²
B) mv²
C) mv²
D) 2mv²
E) 4mv²

Question 7

In any and all collisions of short duration and for which it is true that no external forces act on the collision participants,

A) kinetic energy is conserved.
B) both momentum and kinetic energy are conserved.
C) neither momentum nor kinetic energy is conserved.
D) the relative velocities before and after impact are equal and oppositely directed.
E) momentum is conserved.

Question 8

An automobile of mass 1300 kg has an initial velocity of 7.20 m/s toward the north and a final velocity of 6.50 m/s toward the west. The magnitude and direction of the change in momentum of the car are

A) 1.26 $\times$ 10⁴ kg · m/s at 48º S of E
B) 1.26 $\times$ 10⁴ kg · m/s at 48º S of W
C) 1.26 $\times$ 10⁴ kg · m/s at 48º N of W
D) 1.78 $\times$ 10⁴ kg · m/s at 48º N of W
E) 910 kg · m/s at 48º S of E

Question 9

If you take the derivative of the kinetic energy of a particle with respect to its velocity, you get

A) force.
B) momentum.
C) acceleration.
D) mass.
E) potential energy.

Question 10

Momentum is conserved in which of the following?

A) elastic collisions
B) inelastic collisions
C) explosions
D) collisions between automobiles
E) All of these are correct.

Question 11

For a system consisting of two particles that undergo an elastic collision,

A) momentum is conserved but the total energy is not conserved.
B) neither the kinetic energy nor the momentum is conserved.
C) neither the total energy nor the momentum is necessarily conserved.
D) the mechanical energy is conserved but momentum is not conserved.
E) both kinetic energy and momentum are conserved.

Question 12

A golf ball and a Ping-Pong ball are dropped in a vacuum chamber. When they have fallen halfway to the floor, they have the same

A) speed.
B) potential energy.
C) kinetic energy.
D) momentum.
E) speed, potential energy, kinetic energy, and momentum.

Question 13

Two identical bodies of mass M move with equal speeds v. The direction of their velocities is illustrated above. The magnitude of the linear momentum of the system is

A) 2Mv
B) Mv
C) 4Mv
D) Mv
E) Mv

Question 14

A superball of mass m is dropped vertically from a height, h. If the impact time with the floor is $\Delta$
t, what is the average force on the ball? (Assume that the superball bounces back to the same height.)

A)
B)
C)
D)
E) none of the above

Question 15

Two students, sitting on frictionless carts, push against each other. Both are initially at rest and the mass of student 1 and the cart is M, and that of student 2 and the cart is 1.5M. If student 1 pushes student 2 so that she recoils with velocity , what is the velocity of student 2.

A)
B)
C)
D)
E)

Question 16

A car of mass 1850 kg is traveling at 22.5 m/s in a straight line. A truck of mass 3170 kg has the same momentum as the car. The speed of the truck is

A) 38 m/s
B) 13 m/s
C) 10 m/s
D) 40 m/s
E) 27 m/s

Question 17

If a body moves in such a way that its linear momentum is constant, then

A) its kinetic energy is zero.
B) the sum of all the forces acting on it must be zero.
C) its acceleration is greater than zero and is constant.
D) its center of mass remains at rest.
E) the sum of all the forces acting on the body is constant and nonzero.

Question 18

Two masses M and 5M rest on a horizontal frictionless table with a compressed spring of negligible mass between them. When the spring is released, the energy of the spring is shared between the two masses in such a way that

A) M gets 3/5 of the energy.
B) M gets 1/6 of the energy.
C) M gets 1/5 of the energy.
D) M gets 4/5 of the energy.
E) None of these will occur.

Question 19

The condition necessary for the Conservation of Linear Momentum in a given system is that

A) energy is conserved.
B) one body is at rest.
C) the net external force is zero.
D) internal forces equal external forces.
E) None of these is correct.

Question 20

Two students, sitting on frictionless carts, push against each other. Both are initially at rest and the mass of student 1 and the cart is M, and that of student 2 and the cart is 1.5M. If student 1 pushes student 2 so that she recoils with velocity , what is the change in momentum of the two students?

A)
B)
C) 0
D)
E)

Question 21

Two equal masses travel in opposite directions with equal speed. If they collide in a perfectly elastic collision, then, just after the collision, their velocities will be

A) zero.
B) equal to their original velocities.
C) equal in magnitude but opposite in direction to their original velocities.
D) less in magnitude and in the same direction as their original velocities.
E) less in magnitude and opposite in direction to their original velocities.

Question 22

A toy car of mass 2.0 kg moving to the right with a speed of 8.0 m/s collides perfectly inelastically with another toy car of mass 3.0 kg that is moving to the left with a speed of 2.0 m/s. Immediately after the collision the velocity of the system is

A) 4.4 m/s to the right.
B) 2.0 m/s to the right.
C) 0 m/s
D) -2.0 m/s to the right.
E) 10 m/s to the right.

Question 23

A 40-kg girl, standing at rest on the ice, gives a 60-kg boy, who is also standing at rest on the ice, a shove. After the shove, the boy is moving backward at 2.0 m/s. Ignore friction. The girl's speed is

A) zero
B) 1.3 m/s
C) 2.0 m/s
D) 3.0 m/s
E) 6.0 m/s

Question 24

The balls shown in the figure are strung on a taut wire and slide without friction. If the balls are of equal mass, the diagram that best represents an elastic collision is

A) 1
B) 2
C) 3
D) 4
E) 5

Question 25

A bullet of mass m and velocity strikes and becomes imbedded in a wooden block of mass M, which is initially at rest on a frictionless surface. The ratio of the velocity of the system after collision to the initial velocity of the bullet is

A) (M + m)/m
B) (M + m)/M
C) M/(m + M)
D) m/(m + M)
E) M/(m - M)

Question 26

Two bodies A and B move toward each other with speeds of 80 cm/s and 20 cm/s, respectively. The mass of A is 140 g and that of B is 60 g. After a head-on, perfectly elastic collision, the speed of B is

A) 8.0 cm/s
B) 20 cm/s
C) 92 cm/s
D) 1.2 m/s
E) 1.3 m/s

Question 27

Two equal masses travel in opposite directions with equal speeds. They collide in a collision that is between elastic and inelastic. Just after the collision, their velocities are

A) zero.
B) equal to their original velocities.
C) equal in magnitude but opposite in direction to their original velocities.
D) less in magnitude and in the same direction as their original velocities.
E) less in magnitude and opposite in direction to their original velocities.

Question 28

A mass m₁ = 2.5 kg is connected to another mass m₂ = 4.0 kg by a compressed spring. Both masses are at rest on a frictionless surface. When the spring is released, the masses are pushed apart and a total energy of 16.8 J is given to the two masses. The speed of mass m₁ is

A) 3.2 m/s
B) 2.9 m/s
C) 1.8 m/s
D) 8.3 m/s
E) 5.4 m/s

Question 29

A block of wood with a mass M = 4.65 kg is resting on a horizontal surface when a bullet with a mass m = 18 g and moving with a speed v = 725 m/s strikes it. The coefficient of friction between the block and the surface is µ = 0.35. The distance the block moves across the surface is

A) 1.1 m
B) 3.3 m
C) 0.41 m
D) 11 m
E) None of these is correct.

Question 30

A force acting on an object is approximated as F = 4 sin (100 $\pi$ t) N from t = 0 to t = 10 ms. Calculate the impulse on the object.

A) 1/(25 $\pi$ ) N.s
B) 2/(25 $\pi$ ) N.s
C) 1/(200 $\pi$ ) N.s
D) 1/(400 $\pi$ ) N.s
E) 1/(800 $\pi$ ) N.s

Question 31

An object of mass M₁ is moving with a speed v on a straight, level, frictionless track when it collides with another mass M₂ that is at rest on the track. After the collision, M₁ and M₂ stick together and move with a speed of

A) v
B) M₁v
C) (M₁ + M₂)v/M₁
D) M₁v/(M₁ + M₂)
E) M₁v/M₂

Question 32

Two cars of equal mass travel in opposite directions at equal speeds. They collide in a perfectly inelastic collision. Just after the collision, their velocities are

A) zero.
B) equal to their original velocities.
C) equal in magnitude but opposite in direction to their original velocities.
D) less in magnitude and in the same direction as their original velocities.
E) less in magnitude and opposite in direction to their original velocities.

Question 33

Five billiard balls are in contact and at rest on a wire that passes through their centers. Two billiard balls are slammed into one end of the row of five at a velocity . If the balls are free to slide but not roll and if the collision is elastic, which of the following is most likely to take place?

A) One ball at each end goes off with a speed v.
B) One ball on the side opposite the striking balls goes off with a speed of 2v.
C) The five balls move off together with a speed of 2v/5.
D) Two balls on the side opposite the striking balls go off with a speed of v.
E) None of these will occur.

Question 34

A railway car having a total mass of 5.8 $\times$ 10⁵ kg, moving with a speed of 9.1 km/h, strikes another car that has a mass of 8.7 $\times$ 10⁵ kg and is initially at rest. The speed of the coupled cars after the collision is

A) 9.1 km/h
B) 7.2 km/h
C) 3.6 km/h
D) 1.8 km/h
E) 4.2 km/h

Question 35

Glider A, traveling at 10 m/s on an air track, collides elastically with glider B traveling at 8.0 m/s in the same direction. The gliders are of equal mass. The final speed of glider B is

A) 8.4 m/s
B) 10 m/s
C) 8.0 m/s
D) 4.0 m/s
E) 12 m/s

Question 36

A 5.0-kg ball and a 10.0-kg ball approach each other with equal speeds of 20 m/s. If they collide inelastically, the speed of the balls just after the collision is approximately

A) 1.0 m/s
B) 20 m/s
C) 6.7 m/s
D) 1.5 m/s
E) zero

Question 37

In riot control, the riot squad uses a water hose that shoots water at a rate of 5 m/s and volume of 40 L/s. What is the average force exerted on a person assuming that the water splashes sideways in all directions? The density of water is 1000 kg/m³.

A) 5 N
B) 40 N
C) 80 N
D) 100 N
E) 200 N

Question 38

Consider an inelastic collision between balls of mass m₁ and m₂. The velocities before the collision are ₁ and ₂, respectively, and the velocities after the collision are ₁ and ₂, respectively. Which of the following statements concerning this collision is true?

A)
B) ₂ - ₁ > ₁ - ₂
C) ₂ - ₁ = ₁ - ₂
D) ₂ - ₁ < ₁ - ₂
E)

Question 39

A bullet with a mass of 20 g and a speed of 960 m/s strikes a block of wood of mass 4.5 kg resting on a horizontal surface. The bullet gets embedded in the block. The speed of the block immediately after the collision

A) cannot be found because we don't know whether the surface is frictionless.
B) is 0.21 km/s.
C) is 65 m/s.
D) is 9.3 m/s.
E) None of these is correct.

Question 40

A moving particle is stopped by a single head-on collision with a second, stationary particle if the moving particle undergoes

A) an elastic collision with a second particle of much smaller mass.
B) an elastic collision with a second particle of much greater mass.
C) an elastic collision with a second particle of equal mass.
D) an inelastic collision with a second particle of any mass.
E) any type of collision in which the coefficient of restitution is zero.

Question 41

An 1810-kg truck traveling eastward at 64.4 km/h collides at an intersection with a 905-kg automobile traveling northward at 96.5 km/h. The vehicles lock together and immediately after the collision are headed in which direction?

A) 30º N of E
B) 37º N of E
C) 45º N of E
D) 53º N of E
E) 67º N of E

Question 42

A bullet, m = 0.500 kg, traveling with a velocity of 100 m/s strikes and embeds itself in the bob of a ballistic pendulum, M = 9.50 kg. The combined masses rise to a height h = 1.28 m. The speed V_f of the combined masses immediately following impact is

A) 5.00 m/s
B) 5.26 m/s
C) 9.10 m/s
D) 10.0 m/s
E) 22.3 m/s

Question 43

A car having a total mass of 2250 kg and traveling at 72.0 km/h smashes into a tree. The car is stopped in 0.250 s. The average force acting on the car during the collision is

A) 1.80 $\times$ 10⁵ N
B) 80.0 N
C) 1.80 $\times$ 10² N
D) zero
E) 2.20 $\times$ 10⁴ N

Question 44

The momentum vectors ₁ and ₁ of two bodies are shown before collision. After the collision, body A is at rest. The vector that correctly represents the momentum of body B after the collision is

A)
B)
C)
D)
E)

Question 45

You shoot an arrow with a mass of 0.54 kg from a bow. The bow exerts a force of 125 N for 0.65 s. The speed of the arrow as it leaves the bow is

A) 0.23 km/s
B) 0.10 km/s
C) 0.15 km/s
D) 0.30 km/s
E) 0.27 km/s

Question 46

Vectors and are the momentum vectors of two equal masses, m₁ and m₂. A third equal mass, m₃, has a momentum such that when the three masses collide and stick together the combined mass has zero velocity. The vector that best represents the momentum of mass m₃ is

A)
B)
C)
D)
E)

Question 47

Two identical masses are hung on strings of the same length. One mass is released from a height h above its free-hanging position and strikes the second mass; the two stick together and move off. They rise to a height H given by

A) 3h/4
B) h/4
C) h/2
D) h
E) None of these is correct.

Question 48

A block that has a mass M = 4.25 kg is hanging at rest on a light string. A projectile with a mass m = 250 g moving horizontally strikes M and sticks to it. The block and its projectile swing up and the center of mass rises a distance h = 12.0 cm. The speed of the projectile is approximately

A) 28 m/s
B) 42 m/s
C) 0.28 km/s
D) 26 m/s
E) 14 m/s

Question 49

You shoot an arrow with a mass of 0.54 kg at 45º above the horizontal. The bow exerts a force of 125 N for 0.65 s. With no air resistance, the maximum height the arrow reaches is

A) 1.2 km
B) 5.4 m
C) 0.57 km
D) 0.29 km
E) 0.61 km

Question 50

A particle of mass 2m is moving to the right in projectile motion. At the top of its trajectory, an explosion breaks the particle into two equal parts. After the explosion, one part falls straight down with no horizontal motion. What is the direction of the motion of the other part just after the explosion?

A) up and to the left
B) stops moving
C) up and to the right
D) straight up
E) down and to the right

Question 51

A 40 kg boy, on a stunt, jumps from one ice sled to another sled placed next to the first one, and then quickly jumps back to the first one. The horizontal speed relative to the ice for each jump is 2 m/s and the mass of each sled is 20 kg. What is the speed of the second sled after the second jump? Assume that there is no friction between the sled and the ice.

A) 1 m/s
B) 2 m/s
C) 3 m/s
D) 4 m/s
E) 8 m/s

Question 52

A projectile with a mass 6M is fired at a speed of 400 m/s at an angle of 60º above the horizontal. At the highest point of its trajectory, the projectile is broken into two equal pieces by an internal explosion. Just after the explosion, one of the two pieces is known to be traveling vertically downward at a speed of 300 m/s. The magnitude of the velocity of the other half of the projectile is

A) 500 m/s
B) 1.50 km/s
C) 400 m/s
D) 710 m/s
E) 123 m/s

Question 53

Two objects, one of mass m₁ = 2 kg and the second of unknown mass, are connected by a compressed spring with negligible mass. The system is at rest on a frictionless table. Both objects are released simultaneously. m₁ is observed to recoil with velocity v₁ = 2 m/s , and m₂ shots forward with velocity v₂ = 4 m/s . What is the mass of m₂?

A) 0.5 kg
B) 1 kg
C) 2 kg
D) 4 kg
E) cannot be determined

Question 54

The figure shows a ballistic pendulum in three states. The system (considered to be the ball and the pendulum) will move in such a way that

A) the kinetic energy is conserved during the collision.
B) the linear momentum is conserved after the collision.
C) the linear momentum is not conserved during the collision.
D) the total mechanical energy is conserved during the collision.
E) the total mechanical energy is conserved after the collision.

Question 55

Two identical cars approach an intersection. One is traveling east at 18 m/s. The second is traveling north at 24 m/s. They collide violently, sticking together. Immediately after the crash they are moving

A) 30 m/s, 37º N of E
B) 30 m/s, 37º E of N
C) 15 m/s, 37º N of E
D) 15 m/s, 37º E of N
E) 42 m/s, 37º N of E

Question 56

A particle with speed v₁ = 2.64 $\times$ 10⁶ m/s makes a glancing elastic collision with another particle that is at rest. Both particles have the same mass. After the collision, the struck particle moves off at 45º to v₁. The speed of the struck particle after the collision is approximately

A) 3.4 $\times$ 10⁶ m/s
B) 1.3 $\times$ 10⁶ m/s
C) 0.53 $\times$ 10⁶ m/s
D) 1.9 $\times$ 10⁶ m/s
E) 6.4 $\times$ 10⁶ m/s

Question 57

In a real collision,

A) kinetic energy is conserved.
B) linear momentum is conserved in the absence of external forces.
C) both momentum and kinetic energy are conserved.
D) neither momentum nor kinetic energy is conserved.
E) the extent to which momentum and kinetic energy are conserved depends on the coefficient of restitution.

Question 58

A particle of mass m moving at 5.0 m/s in the positive x direction makes a glancing elastic collision with a particle of mass 2m that is at rest before the collision. After the collision, m moves off at an angle of 45º to the x axis and 2m moves off at 60º to the x axis. The speed of m after the collision is

A) 4.5 m/s
B) 2.5 m/s
C) 3.3 m/s
D) 1.8 m/s
E) 1.1 m/s

Question 59

In an elastic collision of two objects,

A) momentum is not conserved.
B) momentum is conserved, and the kinetic energy after the collision is less than its value before the collision.
C) momentum is conserved, and the kinetic energy after the collision is the same as the kinetic energy before the collision.
D) momentum is not conserved, and the kinetic energy of the system after the collision differs from the kinetic energy of the system before the collision.
E) the kinetic energy of the system after the collision depends on the masses of the objects.

Question 60

A 7000-kg coal car of a train coasts at 7.0 m/s on a frictionless track when a 3000-kg load of coal is dropped vertically onto the car. The coal car's speed after the coal is added is

A) 2.1 m/s
B) 3.0 m/s
C) 4.9 m/s
D) 7.0 m/s
E) 16 m/s

Question 61

What is the physical significance of the area under the curve shown in the figure?

A) work
B) impulse
C) velocity
D) acceleration
E) mass

Question 62

A 4.0-kg block, initially at rest, experiences a force that varies with time as shown in the figure. When t = 6.0 ms, the speed of the block is

A) 3.0 m/s
B) 5.0 m/s
C) 6.0 m/s
D) 12 m/s
E) 6.0 km/s

Question 63

While in horizontal flight at a speed of 20 m/s, a baseball of mass 0.11 kg is struck by a bat. After leaving the bat, the baseball has a speed of 29 m/s in a direction opposite to its original direction. The magnitude of the impulse given the ball is

A) 0.99 kg · m/s
B) 5.4 kg · m/s
C) 2.2 kg · m/s
D) 3.2 kg · m/s
E) 0.55 kg · m/s

Question 64

Two balls of equal mass are thrown against a massive wall with equal velocities. The first rebounds with a speed equal to its striking speed, and the second sticks to the wall. The impulse that the first ball transmits to the wall, relative to the second, is

A) twice as great.
B) half as great.
C) the same.
D) four times as great.
E) one-fourth as great.

Question 65

A ball with a mass of 50 g is dropped from a point 5.41 m above a sidewalk. The ball is in contact with the sidewalk for 8.1 $\times$ 10^-3 s. What is the magnitude of the average force exerted on the ball?

A) 0.12 kN
B) 89 N
C) 9.2 N
D) 0.49 N
E) It cannot be determined without knowing how high the ball bounces.

Question 66

A helium atom (mass = 4m) moving with speed V collides elastically with a tritium (hydrogen 3 ) atom (mass = 3m) at rest. Calculate the speed of the tritium atom after the collision.

A) 0.86 V
B) 1.33 V
C) 1.14 V
D) 1.25 V
E) 1.00 V

Question 67

A pitcher throws a baseball with a velocity of 27 m/s. After being struck by a bat the ball travels in the opposite direction with a velocity of 40 m/s. If the ball has a mass of 0.11 kg and is in contact with the bat for 3.0 ms, the average force exerted by the bat on the ball is

A) 0.99 kN
B) 4.8 kN
C) 1.5 kN
D) 7.4 kN
E) 2.5 kN

Question 68

The force exerted on a 10-kg mass is given by
F = 10 + 2t
Where the units are SI. If the mass starts from rest, its velocity after 2 s is

A) 14 m/s
B) 2.0 m/s
C) 2.4 m/s
D) 0.20 m/s
E) 0.24 km/s

Question 69

The force exerted on a body of mass 10 kg varies with time according to
F = 20t + 10
Where the units are SI. If the velocity of the body was zero at t = 0, its velocity at t = 5 s is

A) 11 m/s
B) 16 m/s
C) 23 m/s
D) 30 m/s
E) 0.11 km/s

Question 70

A ball of mass m strikes a wall that is perpendicular to its path at speed +v and rebounds in the opposite direction with a speed -v. The impulse imparted to the ball by the wall is

A) 2mv
B) mv
C) zero
D) -mv
E) -2mv

Question 71

A ball of mass m and velocity ₁ collides with and sticks to a ball of mass M and velocity ₂. If the pair moves on with velocity , the impulse given the ball of mass m during the collision must have been

A) m( - ₁)
B) m( + ₁)
C) (m + M)
D) (m + M)( - ₂)
E) zero

Question 72

A body is acted on by an impulsive force from time t = 0 to time t = 10 ms. During this time, the force decreases uniformly from 10³ N to zero as shown in the graph. The change in momentum of the body during this interval is

A) 10 kg · m/s
B) 5.0 kg · m/s
C) 0.16 kg · m/s
D) 10⁵ kg · m/s
E) a value that cannot be determined from this graph.

Question 73

The graph shows the momentum of a body as a function of time. The time at which the force acting on the body is greatest is

A) 0.5 s
B) 2.5 s
C) 4.0 s
D) 1.5 s
E) 5.0 s

Question 74

Two balls are dropped from a height of 6 m. Ball A bounces up to a height of 4 m whereas ball B bounces up to 2 m. Which ball experiences the larger impulse during its collision with the floor?

A) ball A
B) ball B
C) They both experience the same impulse.
D) It is impossible to tell without knowing the masses of the balls.
E) It is impossible to tell without knowing the durations of the collisions.

Question 75

A 20-g bullet with an initial velocity of 3.0 $\times$ 10⁴ cm/s penetrates 6.0 cm into a stationary wall. The average force exerted on the bullet by the wall in bringing it to rest is approximately

A) 1.5 $\times$ 10⁴ N
B) 2.3 $\times$ 10³ N
C) 9.8 $\times$ 10^-3 N
D) 2.0 $\times$ 10² N
E) 4.7 $\times$ 10^-2 N

Question 76

A helium atom (mass = 4m) moving with speed collides elastically with a deuterium (hydrogen 2 ) atom (mass = 2m) at rest. Calculate the percentage change in the kinetic energy of the helium atom after the collision.

A) 11%
B) 36%
C) 50%
D) 64%
E) 89%

Question 77

An automatic rifle fires 0.040-kg projectiles at a speed of 800 m/s. If the gunner holding the rifle in her hands can exert an average force of 160 N against the gun, the maximum number of projectiles she can fire in one minute is

A) 15
B) 300
C) 800
D) 4000
E) 48,000

Question 78

A particle of unknown mass has a momentum of 73 kg · m/s. At a time 7.3 s later, the momentum of the particle is 38 kg · m/s. What is the magnitude of the force acting on the particle during the interval, assuming the motion is in a straight line?

A) It cannot be determined because the mass of the particle is not given.
B) 4.8 N
C) 10 N
D) 5.3 N
E) 9.4 N

Question 79

Using a mallet, you strike a ball of mass 0.50 kg that is initially at rest. The force F on the ball as a function of time is plotted in the figure. At t = 2.0 ms, the speed of the ball is

A) 10 m/s
B) 8.0 m/s
C) 6.0 m/s
D) 4.0 m/s
E) 2.0 m/s

Question 80

A car having a total mass of 2250 kg and traveling at 72 km/h smashes into a tree. The car is stopped in 0.25 s. The driver of the car is not held in place by a seat belt or any other restraining device. Just after the impact but before the driver hits any part of the car, the acceleration of the driver is

A) 80 m/s² toward the tree.
B) zero with respect to the tree.
C) 80 m/s² away from the tree.
D) 1.8 $\times$ 10²m/s² with respect to the car.
E) 37 m/s² away from the tree.