# Proof for projectile motion

Tags: kinematics, projectile motion
 P: 158 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.
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P: 41,465
 Quote by Jadaav When the body falls, gravity acts upon it. So why don't we consider gravity ?
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.
 P: 158 Then range = horizontal speed * time is false ?
Emeritus
PF Gold
P: 29,238
Proof for projectile motion

 Quote by Jadaav Then range = horizontal speed * time is false ?
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.
 P: 158 Ok thanks for clearing my doubt on the subject. I didn't give much thought on the time which is already affected by gravity.

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