# How to calc the force needed to change direction of a moving object?

Dastan

## Homework Statement

A ball [Weight 5 kg & velocity 20 m/s] coming straight at me. How much force I would need to change its direction at 0 degree on left side & sent 20 meters?

I simply don't know what formula I should use.Let alone how to calc it.

Mentor
How much force I would need to change its direction at 0 degree on left side & sent 20 meters?
How do you mean that? (Can you draw a sketch?)

The answer will certainly use F=ma, but to determine the necessary acceleration you have to determine which path the ball should take.

Dastan
Suppose the ball [Red Dot] is coming from above at said speed at 90 degree.

http://img27.imageshack.us/img27/8700/3e0a.png [Broken]

I have to use force to send it to green dot [after it reaches me] while it was still moving.

Shouldn't I have to overpower the ball's KE?:hmm

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Homework Helper
The "zero degrees" before was misleading because you had not specified a coordinate system. Apparently you are assuming a coordinate system in which 0 degrees is to your left and 90 degrees directly ahead. In other words you want to change its direction by 90 degrees.

But then you say "and sent 20 meters" That makes no sense because you can only change it velocity. Unless you are assuming specific air resistance, which you do not give, there is no reason for it to stop at any distance.

Mentor
In addition, if you deflect the ball like this, the force will depend significantly on the exact path the ball takes during that deflection.

Enigman
Shouldn't I have to overpower the ball's KE?:hmm
You can't 'overpower' Kinetic Energy; whatever you may mean by it...
You can absorb K.E. You can give it more K.E. Overpower, mmm..nope.
The change in direction of velocity relates to change in momentum.
Force is rate of change of momentum wrt time.
With given information you can at best calculate change in momentum.
Also as others have mentioned the ball won't stop after 20m.
Have you been given air resistance, or the height at which ball is at?
And it may that by 20m you really mean 20m/s?