# Homework Help: Special Relativity, Mechanics

1. Sep 22, 2011

### wuffle

1. The problem statement, all variables and given/known data

Hey guys! I'm at loss what to do for these 2 problems, have no idea how to start and what equations to use.
First
Person standing on a bridge overlooking a highway drops an apple over the railing just as the front end of a truck passes directly below the railing. If the vehicle is moving at 55 km/h and is 12 m long, how far above the truck must the railing be if the apple just misses hitting the rear end of the truck?

Second.
There is a boy and a girl, both have mirrors and a skateboard, the boy does not move while the girl moves with the use of skateboard. Boy observes the girl's time to stop in the limit in which she moves at the speed of light and she also comes to the same conclusion about the boy

How does the boy(and the girl) come to that conclusion?

3. The attempt at a solution
Obviously we should use the equations for position velocity and acceleration, problem is, how do I apply those?

Last edited: Sep 23, 2011
2. Sep 22, 2011

### Ignea_unda

For the first problem, think of how the apple will move. More specifically think of the position, velocity, and acceleration when it rolls of the railing. Once you think of this, see if you can solve it. If not, perhaps we can get you another hint.

This doesn't have a question to it. I don't know how to help you without a question.

3. Sep 22, 2011

### wuffle

Fixed OP with a question, sorry my bad!

4. Sep 22, 2011

### wuffle

what I have most troubles with is picturing the problem, what i mean by that is i have no idea how to imagine this problem and how to apply physics to it.

if an apple is hit from the bridge, the position is
x=v0t-((gt^2):2), no?
Isnt an apple a freely falling body?in that case a=g so my formula should be correct.
but what then? where do i apply velocity of the truck and its height?

5. Sep 22, 2011

### wuffle

http://www.mediafire.com/imageview.php?quickkey=naki9m78phfyuzp
basically what my friend drew to illustrate the problem
so as i understood correctly, a guy on the bridge throws an apple just as front of the car reaches the point where the apple is being shot at, and at the end, apple barely touches the end of the truck, and we have to measure distance between truck and railing
do i understand this correctly?

6. Sep 22, 2011

### Ignea_unda

Yes, you do.

7. Sep 22, 2011

### wuffle

pretty much my thinking(or absence of it)
so while ball was thrown down, the car has moved 12 meters, if we find the interval during which the car drove those 12 meters, we can apply this time to time which the ball has been in air, and by knowing time, we can find V0(innitial velocity) , after that we can find
distance by S=v0t-0.5gt^2
i know im wrong(cuz my calculations are stupid), but please point out where i am wrong, also how do we find time by which car has moved 12 meters since we only know his velocity and not acceleration?

edit:
well i found the S to be around 3.08 meters
t=12 m /15.27 m/s
g=v0/t
v0=7.70 m/s
s=v0t-0.5gt^2=3.08 m

Last edited: Sep 23, 2011