# Homework Help: Understanding Momentum and Impulse

1. Feb 24, 2015

### Sonny18n

1. The problem statement, all variables and given/known data
1) A hockey player makes a slap shot, exerting a force of 30.0N on the hockey pock for 0.16 seconds. What impulse is given to the puck?

2) The hockey puck shot in exercise 1 has a mass of 0.115 kh and was at rest before the shot. With what speed does it head toward the goal.

2. Relevant equations
p=mv

(F)triangle(t)= Triangle(p)

3. The attempt at a solution
1. If I'm using impulse formula correctly than multiplying 30N and 0.16 sec is 4.8

2) I have a harder time understanding this, I'm not even sure if it's asking me to use the formula for momentum

2. Feb 24, 2015

### Staff: Mentor

Units?

What is the relation between momentum and speed?

3. Feb 24, 2015

### dean barry

The solution can be found starting with with newtons classic:
f = m * a
Think about the definition for acceleration (a).

4. Feb 24, 2015

### haruspex

Yes, but given the first part of the question I would think the intent is to use momentum.

5. Feb 25, 2015

### Sonny18n

Ok so how do I find the acceleration

6. Feb 25, 2015

### haruspex

You don't need to. See DrClaude's post.

7. Feb 25, 2015

### Sonny18n

That's all the question says, the "at rest" part is kind of throwing me off

8. Feb 25, 2015

### haruspex

Why? Suppose it moves off at speed v. How much is the increase in speed? What is the increase in momentum?

9. Feb 25, 2015

### dean barry

Force * time gives the value for impulse.
The result equals the change in momentum, during the impulse.
The original momentum is 0

10. Feb 25, 2015

### haruspex

Sonny18n already did that bit in part 1. DrClaude and I are trying to get Sonny18n to use that result together with the puck mass to solve part 2 directly rather than going back to the force given. I (we?) feel this is the intent of the question, and clearly something Sonny18n needs to learn how to do. The force * time approach is not always available, because it only works when the force is constant. Yes, it's constant here, but in other momentum questions it won't be.

11. Feb 27, 2015

### Sonny18n

At rest means 0m/s right? So it won't affect the equation in a major way. But it's asking me to relate the answer from question 1 but it's not clear to me how impulse I can find the speed it takes.

12. Feb 27, 2015

### haruspex

Impulse is momentum. How do you normally assess the momentum of an object?

13. Feb 27, 2015

### Sonny18n

Mass times velocity?

14. Feb 27, 2015

### haruspex

Yes.

15. Feb 27, 2015

### Sonny18n

So the velocity is 0m/s then or..?

16. Feb 27, 2015

### haruspex

If the momentum is mass times velocity, what do you think a change in momentum for an object equals?

17. Feb 27, 2015

### Sonny18n

But I don't have momentum. Only mass and no velocity

18. Feb 27, 2015

### haruspex

You have change in momentum and mass, and you want change in velocity.

19. Feb 27, 2015

### yuganes warman

the first question is asking about impusle which the formula is ft=mv-mu. So in order to get the impulse just multiply the force given and the time. You may ask where does ft=mv-mu came from, it is derived from the F=ma. Acceleration equals to v-u/t. So when yiu substitute v-u/t into the acceleration , you will get F=mv-mu/t. The t is taken to the left side of the equation and it becomes Ft=mv-mu. mv-mu is the impulse and so ft is also impulse

20. Feb 27, 2015

Keep in mind that momentum is conserved, meaning that it can not change unless there is an outside influence acting on it- an impulse.

21. Feb 27, 2015

### Staff: Mentor

Your two equations tell the whole story. If you combine them, you get:
$$FΔt=Δ(mv)$$
What is F and Δt in your problem?
What is mv to begin with?
You are trying to find the final value of v from this equation. You do know how to express Δ(mv) algebraically in terms of the initial and final velocities of the puck, correct?

Chet

22. Feb 28, 2015

### dean barry

You could go by another route, use newtons classic relationship: f = m * a
( transpose for acceleration a )
Then find the velocity after time t
Calculate the momentum.
( m * v )