# Homework Help: Continuing My Quest - Physics Plebe

1. Oct 15, 2004

### PhysicsNovice

Hello Everyone. This is my fourth posting. I am basically a non-science person learning some basics physics with a on-line program. Please review my homework questions and my answers. I know that I am confused on some concepts. Any help will be appreciated greatly.

Physics Questions – Round 4

1. A sports car with a mass of 1000 kg travels down the road with a speed of 2 m/s. Which is larger, its momentum or its kinetic energy?
a. momentum
b. kinetic energy*
c. they are equal
d. the two cannot be compared

2. Which of the following objects has the largest kinetic energy? A mass of _____with a speed of _________.
a. 6 kg .....2 m/s
b. 4 kg .....3 m/s
c. 2 kg .....5 m/s
d. 6 kg .....3 m/s*

3. Under what conditions is the kinetic energy conserved in a collision?
a. It is always conserved*
b. When the collision is totally elastic
c. When there is no outside force
d. When there is no friction

4. A 3-kg toy car with a speed of 6 m/s collides head-on with a 2 -kg car traveling in the opposite direction with a speed of 4 m/s. If the cars are locked together after the collision with a speed of 2 m/s, how much kinetic energy is lost?
a. 10 J
b. 28 J*
c. 60 J
d. 70 J

5. A tennis ball on the end of a string travels in a horizontal circle at a constant speed. The circle has a radius of 2 m, the ball has speed of 3 m/s, and the centripetal force in 1.5 N. How much work is done on the ball each time it goes around?
a. zero
b. 6 J
c. 10 J*
d. 12 J

6. Which of the following properties of a ball is conserved as it falls freely in a vacuum?
a. kinetic energy*
b. gravitational potential energy
c. momentum
d. mechanical energy

7. If a 0.5-kg ball is dropped from a height of 6 m, what is its kinetic energy when it hits the ground?
a. 3 J*
b. 9 J
c. 30 J
d. There is not enough information to say

8. An elephant, an ant, and a professor jump from a lecture table. Assuming no frictional losses, which of the following could be said just before they hit the floor?
a. They all have the same kinetic energy
b. They all started with the same potential energy
c. They will all experience the same force on stopping
d. They all have the same speed*

9. A 1-kg ball falling freely through a distance of one meter loses 10 J of gravitational energy. How much does the kinetic energy of the ball change if this occurs in vacuum?
a. gain of 1 J
b. gain of 10 J*
c. loss of 1 J
d. loss of 10 J

10. A block of wood loses 160 J of gravitational potential energy as it slides down a ramp. If it has 90 J of kinetic energy at the bottom of the ramp, we can conclude that:
a. mechanical energy is conserved
b. momentum is conserved
c. 250 J of energy was lost
d. 70 J of energy was lost*

Thanks for taking the time to help a true novice.

2. Oct 15, 2004

### arildno

1. It is meaningless to compare the "largeness" of quantities which have different dimensions!!
3. Why do you think that?
4.Why do you think that?

3. Oct 18, 2004

### PhysicsNovice

Hello arildno. Thanks for the input. My revised answer to #1 is (d.) the two can not be compared. To calculate momentum and kinetic energy in this problem uses different formalas that require different values that are not related. Question #3 - Maybe I am confusing momentum and kinetic energy. I thought that the collison of the two objects would result in an equivalent value based upon the new speed and mass (if the objects stayed together) of the resulting collison. Question #4 - I am not sure what I am thinking. I am confused and will review the tutorial of kinetic energy and momentum. Thanks for starting me off on this assignment.

4. Oct 18, 2004

### arildno

You ARE confusing momentum and kinetic energy in 3)
Just a question; Do you know what "totally elastic" means?
At 4)
Compute the total kinetic energy prior to the collision
Compute the total kinetic energy after to the collision
You're welcome to post your reasoning, and how you go about to do the computations, and whatever other questions you might have