# Help with simple HW problems please

1. Feb 2, 2010

### SoulInNeed

Help with simple HW problems please!!

1. Two cars drive off a cliff. The road was completely horizontal until it ended (launching the cars off the cliff with an initail velocity in the horizontal direction). Car A was driving at 80 m/s, and Car B was driving at 20 m/s when they left the cliff. Assume you can ignore wind resistance and friction in this problem.

a) Which car (if any) hits the water at the bottom of the cliff first? Why?

b) Which car (if any) travels further in the air before it hits the water? Why?

2. If the cliff is 17 m high, calculate how long each car is in the air.

3. A bullet is shot out of a gun at a 45 degree angle above horizontal. We will call the height of the gun zero height. When the bullet reaches its highest point in its trajectory, which of the following must be true?

The acceleration of the bullet is zero.
The velocity of the bullet is zero.
The height of the bullet is zero
All of the above
None of the above

Relevant Equations x=x(0) + v(0x)t
y=y(0) + v(0y)t - (.5)g(t^2)

The attempt at a solution

1. a) They will both hit the water at the same time, because they both fall at the same rate of acceleration (ay=-9.8 m/s^2).

b) Car A will have a higher range of travel, because it starts with a higher horizontal velocity.

2. 0=17-(.50)(9.80)t^2
-17=(-4.9)t^2
3.5=t^2
t=1.9 seconds

This applies to both cars, because both leave the cliff with the same rate of acceleration in the y axis (ay=-9.80), and the same starting velocity in the y axis (ay=0).

3. None of the Above

4. The acceleration of the bullet is 0 in the x-axis, but it remains -9.80 in the y axis.

The velocity of the bullet is zero in the y-axis when it is at its highest point, but its velocity in the x-axis is constant.

The height of the bullet is not zero if the height of the gun is considered zero height.

2. Feb 2, 2010

### mgb_phys

Re: Help with simple HW problems please!!

Correct

3. Feb 2, 2010

### Delphi51

Re: Help with simple HW problems please!!

All answers look good to me.

4. Feb 2, 2010

### SoulInNeed

Re: Help with simple HW problems please!!

Thanks!