Question

1. A freight car carrying sand moves along a frictionless level railroad track at constant speed. An additional amount of sand is thrown straight down onto the car as it passes by a storage building. What happens to the velocity of the car?

A) It decreases.

B) It remains the same.

C) It increases.

D) cannot be determined from the information given

2. A 27g tennis ball heads straight toward a racket at 21 m/s. A player swings the racket, hitting the tennis ball and sending it back in the other direction at 25 m/s. If the racket is i contact with the ball for 0.06 second, what is the average force exerted by the racket on the ball?

3.8 N       B) 0.16 N              C) 43.7 N              D) none of the above

3. An object is lowered into a hole deep below the surface of the Earth. What can be said about the object's gravitational potential energy?

A) it is definitely positive B) it is definitely negative C) it is zero D) none of these are necessarily true

4. Which of the following statements is always true?

A) If a system of two particles has total momentum of zero, it also has total kinetic energy of zero.

B) If a system of two particles has total kinetic energy of zero, it also has total momentum of zero.

C) both of these statements are always true

D) neither of these statements is always true

4. A forklift moves a 100 N box at constant speed from one shelf 1 meter high to another shelf 10 meters away at the same height. How much work did the forklift do on the box?

A) less than 100J, but more than 0 J.

B) 100 J.

C) more than 100 J.                                                                          

D) zero.

5. Two equal mass balls (one red and the other blue) are dropped from the same height. The red ball collides elastically with the floor. The blue ball collides completely inelastically with the floor. Which ball experiences the greatest momentum change during its collision with the floor?

A) It's impossible to tell since the time intervals and forces are unknown.

B) Both balls were subjected to the same magnitude momentum change

C) the blue ball

D) the red ball

6. A sled starting from rest slides down a frictionless hill that has an incline of 30 degrees and reaches a speed of 56 m/s. What distance along the slope did the sled travel?

A) 110 m      B) 160 m               C) 320 m               D) 640 m

7. A 1000 kg cannon rests on a horizontal frictionless surface. It fires a 40 kg cannon ball horizontally. Which experiences the greater magnitude momentum change?

A) Neither; both experienced the same magnitude momentum change

B) the cannon ball

C) the cannon

D) It's impossible to tell since the velocities after the ball is fired are unknown

Answer 1

(1) Correct option is (A) Velocity of the car decreases.

Explanation -

The car is moving with constant speed means, the car engine is applying just enough force to overcome the hindering forces. Now, mass of the car increases means hindering force increases, so it slows down.

(2) Option (D) is the correct answer.

Explanation:

Change in momentum, P = (27x10^-3)(21+25) = 1.242 kg.m/s

So, the average force, F = P/t = 1.242 / 0.06 = 20.7 N

(3) Option (A) is the correct answer.

Explanation:

Down below the surface of earth means value of gravitational acceleration shall be low but it is definitely positive. So, potential energy shall be less.

(4) (A) False.

Zero momentum does not means ZERO kinetic energy.

(4) (B) (D)

(4) (C)

Work done, W = F*d = 100*10 = 1000 J

(5) (B)

Momentum is always conserved whether it is elastic or inelastic collision.

(6) Option (C) is the correct answer.

Apply conservation of energy –

mgh = (1/2)mv^2

=> h = v^2 / (2g) = 56^2 / (2*9.81) = 159.84 m.

So, length of the slope, L = h / sin30 = 320 m

(7) (D)

The momentum of the system means cannon and cannon ball is conserved.

So, about individual momentum cannot be determined because enough data is not given.