# Find the force to stop a ball at a given speed

## Homework Statement

Find the force to stop a 5kg ball at 20m/s instantly. Dont take in account resistance.

## Homework Equations

kinetic energy = 0.5*mass*velocity^2
Energy = force * distance
force = energy / distance

## The Attempt at a Solution

0.5 * 5kg * 20m/s^2 = 1000J
1000J / 0 = ??????

As seen it does not work if you want to find the force to counter it exactly...
What am I doing wrong?

Thanks

nrqed
Homework Helper
Gold Member

## Homework Statement

Find the force to stop a 5kg ball at 20m/s instantly. Dont take in account resistance.

## Homework Equations

kinetic energy = 0.5*mass*velocity^2
Energy = force * distance
force = energy / distance

## The Attempt at a Solution

0.5 * 5kg * 20m/s^2 = 1000J
1000J / 0 = ??????

As seen it does not work if you want to find the force to counter it exactly...
What am I doing wrong?

Thanks
You are not doing it wrong. It is unphysical to ask to stop an object instantly. In the limit as the time goes to zero, the force goes to infinity (using non relativistic formula or even relativistic ones) so it is simply ill-defined.