Homework Help: Impulse on ball involving angles

1. Dec 2, 2008

1. A ball of mass 0.40kg is moving to the left at a velocity v0=-20 m/s. It is kicked and given a velocity, at 45 degrees upwards to the right, of vf30[/sub] m/s.

1) Find the impulse of the force

2) Find the average force

The collision time is 0.010s

2. I = (delta) P

I = F (delta)t

3.

1) before p = 0.4x-20= -8
after 30x0.4=12

Change in p = 20Ns Where do the angles some in?

2) F=(delta)P/t=2000

Which is obviously wrong. They didn't include the angles for fun, so there something wrong in my working. Do I have to choose a line to work out the momentum across? ie, linear momentum. A bit confused. Any help appreciated.

Last edited: Dec 2, 2008
2. Dec 2, 2008

Staff: Mentor

Momentum and velocity are vectors and must be treated as such.

$$\Delta \vec{v} = \vec{v}_f - \vec{v}_i$$

Hint: Break the vectors into components, then subtract them. That's where the angles come in.

3. Dec 2, 2008

OK, so, 30cos45=21.2

21.2-(-20)=41.2

correct?

4. Dec 2, 2008

Staff: Mentor

So far, so good. You've correctly found the change in the horizontal component of the velocity. What about the vertical component? (Then find the magnitude of the total change in velocity.)

5. Dec 2, 2008

well, sin 45 and cos45 are the same, so the vertical change is just sin45=21.2

So the total change in velocity is the total of the two, 21.2+41.2=62.4

So no to find the change in momentum I times 62.4 by the mass to get 24.96 ? which is the impulse?

6. Dec 2, 2008

Staff: Mentor

OK, the change in the vertical component of velocity is 21.2 - 0 = 21.2.
No, you can't just add horizontal and vertical components of a vector. How do you find the magnitude of a vector from its components?

7. Dec 2, 2008

Pythagorus. So $$\sqrt{41.2^2+21.2^2}=46.3$$

so I = 46.3x0.4 = 18.5

8. Dec 2, 2008

Staff: Mentor

Good. (Impulse will have the same units as momentum, thus kg-m/s.)

9. Dec 2, 2008

And my working out tghe average force will be correct with this new value? ie,

F=18.5/0.01=1850N.

Thanks for you help btw. This has got to be handed in in 20 minutes! Worth 25 marks too. Silly really for how easy it is, once you've got started, i'm just too hungover to think properly and needed a nudge to get going. Not even sure there are 25 stages in the question to mark.

Last edited: Dec 2, 2008
10. Dec 2, 2008

Looks good.