# Projectile and relative motion help

Torpedo007

## Homework Statement

A boy is standing on a long railroad car throws a ball straight upwards.The car is moving on a horizontal road with an accelaration of 1m/s^2. and the projection velocity is 9.8m/s.How far behind the boy will the ball fall on the car.

## Homework Equations

Not entirely sure what to use but i tried using the basic equations of motions and relative velocity but without an actual velocity for the rail car i found it really hard

## The Attempt at a Solution

I fist used the basic V=U+at and found the time taken to go up.

Hence time taken to come down will be 2t.In this same the car should have travelled a certain distance.Now this is where im stuck since i only have the acceleration not the speed of the car D:

I juggled around but couldnt get the proper answer which was 2 meters

Any help would be much appreciated!

Last edited:

Homework Helper
Welcome to PF!

Hi Torpedo007! Welcome to PF! (try using the X2 icon just above the Reply box )

Show us what you've tried, and where you're stuck, and then we'll know how to help! Torpedo007

Hi Torpedo007! Welcome to PF! (try using the X2 icon just above the Reply box )

Show us what you've tried, and where you're stuck, and then we'll know how to help! Hey thanks for the reply ;D

I fist used the basic V=U+at and found the time taken to go up.

Hence time taken to come down will be 2t.In this same the car should have travelled a certain distance.Now this is where im stuck since i only have the acceleration not the speed of the car D:

Homework Helper
Hey Torpedo007! I fist used the basic V=U+at and found the time taken to go up.

Hence time taken to come down will be 2t.

(btw, you could have used V = -U and found the total time, or 0 = Ut + at2/2 )
In this same the car should have travelled a certain distance.Now this is where im stuck since i only have the acceleration not the speed of the car D:

Call the speed of the car c, put it in the x equation, and you should find it cancels at the end. Torpedo007
Call the speed of the car c, put it in the x equation, and you should find it cancels at the end. Sorry to be a bother but can you just elaborate a bit more please , i dont understand D:

Homework Helper
The boy has initial speed c0, and acceleration 1.

The ball has initial distance x = 0 and initial horizontal speed vx,0 = c0.

So when the ball lands, where is the ball, and where is the boy?

Torpedo007
The boy has initial speed c0, and acceleration 1.

The ball has initial distance x = 0 and initial horizontal speed vx,0 = c0.

So when the ball lands, where is the ball, and where is the boy?

Ah i think i finally i got it ;D thanks a load