- #1
TomK
- 69
- 14
- Homework Statement
- ENGAA 2017 - Question 52
- Relevant Equations
- Force = rate of change of momentum
Please scroll-sown to Question 52: https://www.undergraduate.study.cam.ac.uk/files/publications/engineering_s1_qp_2017.pdf
The correct answer is 'B'. This is the working I did:
F = (change in momentum) / (change in time)
change in momentum = mv - mu, where v = final velocity and u = initial velocity
initial velocity = +u, final velocity = -v (as velocity reverses direction after hitting racket)
change in time = t2 - t1m (-v - (+u)) = F (t2 - t1)
-mv - mu = Ft2 - Ft1
mv = Ft1 - Ft2 - mu
mv = F (t1 - t2) - mu
As you can see, my answer is wrong. I don't understand why. What's strange is that if you assume final velocity = +v and initial velocity = -u, you'll get the change in momentum to be m (v - (-u)) = mv + mu. Then, you'll get the right answer after rearranging.
The correct answer is 'B'. This is the working I did:
F = (change in momentum) / (change in time)
change in momentum = mv - mu, where v = final velocity and u = initial velocity
initial velocity = +u, final velocity = -v (as velocity reverses direction after hitting racket)
change in time = t2 - t1m (-v - (+u)) = F (t2 - t1)
-mv - mu = Ft2 - Ft1
mv = Ft1 - Ft2 - mu
mv = F (t1 - t2) - mu
As you can see, my answer is wrong. I don't understand why. What's strange is that if you assume final velocity = +v and initial velocity = -u, you'll get the change in momentum to be m (v - (-u)) = mv + mu. Then, you'll get the right answer after rearranging.