A cricket ball has mass 0.155 kg.

Question

1. If the velocity of a bowled ball has a magnitude of 55.0 m/s and the batted ball's velocity is 60.0 m/s in the opposite direction, find the magnitude of the change in momentum of the ball.
2. Find the magnitude of the impulse applied to it by the bat.
3. If the ball remains in contact with the bat for 2.50 ms , find the magnitude of the average force applied by the bat.

 

I did my calculations for change in momentum 

=o.155*55.0-(-60.0)

=68.525 kg m/s

 

im not sure if it is the rigth process to get what i want. 

Answer 1

Hi Valentina!

 

I think your general process is correct, though there is one (relatively minor) correction.

 

Δp = p_f - p_i = mv_f - mv_i = m(v_f - v_i)

 

There is an understandable tendency to want to put the starting quantity first, but in this case, you need to put the final velocity first.  If we call the final direction positive (and thus the initial direction negative), this still leads to:

 

Δp = 0.155*(60-(-55))

 

which will lead to the same result as your setup, IF the algebra is done in the correct order.  Note the parentheses.  You have to compute the change in velocity first, THEN multiply by the mass.  I think you multiplied the mass into your first term and then added the second, which is the wrong order (by PEMDAS).  I think if you correct that, and put the final and initial velocities where they belong, you will be right on.

 

I hope this helps!  Let me know if you have any other questions.