9.2 Impact forces

Question 1

If a ball is initially stationary and the impact causes it to accelerate to speed v in time t, the gain of momentum of the ball due to the impact = mv

THEREFORE:

Answer 1

The force of the impact F = change of momentum/contact time

ie. F=mv/t

Question 2

If a ball is moving with an initial velocity u, and the impact changes its velocity to v in time t, the change of momentum of the ball = mv-mu

THEREFORE:

Answer 2

The force of impact F - change of momentum/contact time

ie. F=mv-mu / t

Question 3

How can the variation of an impact force with time on a ball be recorded

Answer 3

By using a force sensor connected using long wires or a radio link to a computer
Force sensor is attached to the object (eg. a bat) that causes the impact
As the force on the bat is equal and opposite to the force on the ball during impact, the force on the ball due to the bat varies in the same way as the force on the bat due to the ball. Variation showed on computer

Question 4

What is the rebound like if a ball hits a wall normally

Answer 4

It rebounds normally so the direction of its momentum is reversed. Therefore, opposite sign Eg. + for towards wall and - for away from wall

Question 5

Things to note:

Answer 5

If there is no loss of speed on impact, v=u so F=-2mu/t

If the impact is oblique (not perpendicular) the normal components of velocity must be used.For an impact for which the initial and final direction of ball are at same angle to normal and no loss of speed, the change of momentum is -2mucosθ