1. A lump of clay is thrown at a wall. It hits and sticks. The mass of the clay
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Question
1. A lump of clay is thrown at a wall. It hits and sticks. The mass of the clay is 5kg, and the velocity before impact is 12 m/s, After the impact, the momentum of the clay plus wall is:
a. 12x5 kgm/s
b. 12/5
c.5/12
d. can't be determined.
2. A mass of 20 kg with velocity 40 m/s, hits a mass of 40 kg, with velocity -10m/s. The masses fuse. What is its velocity after the collision?
3. The Earth exerts a force of 600 N on a 60 kg object. The Earth weighs 10exp25 kg. What is the acceleration of the Earth as result?
a. same as that of object.
b. 60 X 10 exp 25 m/ss
c. 60/10exp25
d. 600/10exp25
4. The moon is 240,000 miles from the Earth It experiences a gravitatioanl acceleration due to Earth's field. Far form the moon, suppose a pebble is 240,000 miles form the Earth. Its acceleration is?
a. In the direct ratio of its mass than that of the moon.
b.Same as that of the moon.
c. in inverse ratio of its mass to that of the moon.
d. so small as to be negligible.
5. A bullet is fired horizontally. velocity after leaving the rifle is 1000m/s, and mass is 0.01kg. The height of the rifle is 5m above the surface. At the same time, a 500 kg mass is dropped from a height of 5m. The 5000 kg mass hits the gorund
a. first
b. last
c. at the same time as the bullet.
6. You are in an elevator. The cable breaks. What acceleration are you under? If you are standing on the scale, what does the scale read?
7. A bat hits a baseball. The ball is moving at -30m/s, and has a mass of 0.5 kg. After the collision the ball moves in the opposite direction at 40 m/s. What can you say about he approximate speed of the bat, if its mass is 3kg and its average velocity before impact is 5 m/s?
8. A truck with a weight equal to itself. It accelerates at 5m/ss. Suppose the load abruptly falls off. What is the acceleration of the truck then?
a. 10 m/ss
b. 2.5 m/ss
c. 5m/ss
Please show work and explain.
Explanation / Answer
1. A lump of clay is thrown at a wall. It hits and sticks. The mass of the clay is 5kg, and the velocity before impact is 12 m/s, After the impact, the momentum of the clay plus wall is:
d. can't be determined.
Reason: The wall acts as a body of infinite mass. Thus in this inelastic collision the momentum can''t be determined since it is not possible to determine the initial momentum of the wall since the mass is unknown / indeterminable.
2. A mass of 20 kg with velocity 40 m/s, hits a mass of 40 kg, with velocity -10m/s. The masses fuse. What is its velocity after the collision?
Ans: Momentum is conserved. Thus,
20(40) + 40 (-10) = 800 - 400 = 400 kg m/s = initial momentum
final momntum = initial momentum = 400
velocity = 400 / (20 + 40) = 400 / 60 = 20/3 or 6.67 m/s
3. The Earth exerts a force of 600 N on a 60 kg object. The Earth weighs 10exp25 kg. What is the acceleration of the Earth as result?
d. 600/10exp25
reason : Acceleration = Force / Mass
4. The moon is 240,000 miles from the Earth It experiences a gravitatioanl acceleration due to Earth's field. Far form the moon, suppose a pebble is 240,000 miles form the Earth. Its acceleration is?
b.Same as that of the moon.
Reason : Acceleration due to gravity on any object due to earth = GM / r2 which is independent of mass of the object.
.
5. A bullet is fired horizontally. velocity after leaving the rifle is 1000m/s, and mass is 0.01kg. The height of the rifle is 5m above the surface. At the same time, a 500 kg mass is dropped from a height of 5m. The 5000 kg mass hits the gorund
a. first
b. last
c. at the same time as the bullet.
Reason:
Time to fall vertically downwards is dependent on initial height and not the mass since the only force acting is gravity.
6. You are in an elevator. The cable breaks. What acceleration are you under? If you are standing on the scale, what does the scale read?
Ans: We are in a free fall or a = g.
The scale would read our weight as 0 N as it measures the normal reaction. However when a = g, N = 0.
.
7. A bat hits a baseball. The ball is moving at -30m/s, and has a mass of 0.5 kg. After the collision the ball moves in the opposite direction at 40 m/s. What can you say about he approximate speed of the bat, if its mass is 3kg and its average velocity before impact is 5 m/s?
Initial momentum of system = Bat + Ball = 3 * (+5) + 0.5 ( -30) = 0
Momentum should be conserved,
So, final momentum = 0 = (3) (Vbat) + 40 ( 0.5) = 0
Vbat = -20 / 3 = -6.67 m/s
8. A truck with a weight equal to itself. It accelerates at 5m/ss. Suppose the load abruptly falls off. What is the acceleration of the truck then?
a. 10 m/ss
b. 2.5 m/ss
c. 5m/ss
Reason : The force acting on truck would remain constant
Thus, since Force = mass * acceleration
As mass gets halved, acceleration gets doubled.
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