# Proof for projectile motion

1. Feb 24, 2013

A body is thrown horizontally from a cliff with an initial horizontal speed. It hits the ground at a distance X from its initial position. It is said that the displacement = horizontal speed * time.

When the body falls, gravity acts upon it. So why don't we consider gravity ?

Any proof that the displacement would be the same as above if the same body would have moved with the same initial speed on a flat horizontal path for the same time period ?

Of course in the 2nd situation, it would continue to move infinitely.

2. Feb 24, 2013

### Staff: Mentor

Of course you must consider gravity. The vertical motion under gravity determines how quickly the body reaches the ground and thus how far it's able to get horizontally before it lands.

3. Feb 24, 2013

Then range = horizontal speed * time is false ?

4. Feb 24, 2013

### ZapperZ

Staff Emeritus
What determines the "time"? It is the fall of the object due to gravity!

While gravity doesn't affect the motion in the horizontal direction, it affects the time taken for the object to move! That is how gravity comes in!

You should go back and resolve this problem, and pay attention to what factors determine the time of motion that you used for the horizontal component.

Zz.

5. Feb 26, 2013