[Physics] Force to deflect ball colliding with wall

collisionforcesmomentumnewtonian-mechanics

Suppose I have a ball with a certain coefficient of restitution. The wall can be considered indeformable and with infinite mass. Everything's in 3 dimensions, and the ball can hit the wall at any angle.

The ball has a certain velocity at the time of contact, and we need to calculate the new velocity after the impact.

So far, I use simple linear algebra to get a reflected vector using the normal with the wall – scaling it by the restitution coefficient. This gives me a velocity "deflected" away from the wall with a smaller magnitude than before the impact.

However I can't figure out how to calculate a Force vector instead of a new velocity.

How do you get the force impressed by the wall upon the collision? Most formulas need a Dt parameter, the time the impact lasted: I don't know that, it should be obtainable with the ball characteristics.

Best Answer

Given the mass m of the ball, the incident normal speed v, and the coefficient of restitution $\rho$, Then the integral of F over the duration of the collision $\Delta t$ is $$\int_0^{\Delta t} F dt = \frac{m(1 + \rho)v }{\Delta t}$$ assuning no rotational effects are incurred.

This follows from the fact that at any instant the acceleration of the ball away from the wall is F/m, so the integrated acceleration over the duration of the collision is the total change in normal velocity of the ball. Since the post-bounce velocity is simply the approach velocity times the coefficient of restitution, the total velocity change is as indicated.

If you want to simplify the force profile to assume a constant force FB during the collision (which is clearly not accurate) then $$\frac{(FB)(\Delta t)}{m} = (1 + \rho)v $$

The collision clearly does not produce a uniform force level over the duration of the collision, because the elastic forces on the ball will vary with the amount of deformation of the ball.