How Is Force Calculated When Momentum Change Appears Zero?

[roam]

Homework Statement

A 100 g snooker ball with speed 50 cm/s strikes a cushion at an angle of 45° and bounces back off the same speed and angle. If the ball is in contact with the cushion for 0.05 s, what is the average force exerted by the cushion on the ball?

The Attempt at a Solution

I get final momentum=initial momentum because the angle, mass and the speed are the same before and after.

$$p_i=(0.1)(0.5 cos 45) = p_f$$

The change in momentum is 0.

So, the impulse is equal to $$I=\Delta \vec{p}$$. To find the force I think we can use the formula

$$\vec{I}=\sum \vec{F} \Delta t$$

But how can I find force when the change in momentum is zero? Is there something wrong with my strategy or did I make some mistakes?

 

[pat666]

You need to approach these questions systematically
ie.
Vi=50cm/s* cos45i+50cm/s* sin45j
vf=-50cm/s*cos45+50cm/s* sin45j
j=pf-pi
=0.5cos45i*2*100i+0.5cos45*2*100j as cos 45=sin 45
=0.0707i+0.0707j
J=0.1=Ft
F=0.05/0.1
F=0.5N

Did you understand the other momentum problem from before??

 

[Jokerhelper]

You need to approach these questions systematically
ie.
Vi=50cm/s* cos45i+50cm/s* sin45j
vf=-50cm/s*cos45+50cm/s* sin45j
j=pf-pi
=0.5cos45i*2*100i+0.5cos45*2*100j as cos 45=sin 45
=0.0707i+0.0707j
J=0.1=Ft
F=0.05/0.1
F=0.5N




Did you understand the other momentum problem from before??

I think you made a mistake when subtracting the initial and final momenta

 

[pat666]

whys that what answer did you get? the plus is there as its a double negative

 

[Jokerhelper]

whys that what answer did you get? the plus is there as its a double negative

you made a mistake when subtracting the j-components for the impulse. If you notice, it's equal for both the final and initial momenta so the change in the j-direction is zero.

 

[pat666]

yep your right,,,, . that's my bad sorry but the basic principals are still right. agree??

 

[Jokerhelper]

yep your right,,,, . that's my bad sorry but the basic principals are still right. agree??

Yes, that's right.