# Impulse Momentum for Linear

1. Aug 8, 2012

### freshbox

1. The problem statement, all variables and given/known data
Hi guys, just want to check is there any problem with the answer for part c and d of the question? I would really appreciate if you can help me double check, thank you.

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2. Aug 8, 2012

### Staff: Mentor

Do you get their answers for (a) and (b)?

3. Aug 8, 2012

### freshbox

yes but not for c and d

4. Aug 9, 2012

### Staff: Mentor

I don't get anywhere near their answers for (a) or (b).

I get their answers only if I change the force to 120 MN.

5. Aug 9, 2012

### ehild

For c) : Count gravity to the force that acts on the hammer+pile.

ehild

6. Aug 9, 2012

### ehild

Have you used 8000 kg as mass of the hammer? (Mg = 103 kg)

ehild

7. Aug 9, 2012

### Staff: Mentor

Ah, with that correction all of my answers now are the same as those given alongside the question.

8. Aug 9, 2012

### freshbox

Part C

M1=mv
=(9000)(8.82)
=79380

M2=mv
=0

I1-2=Force x Time
=(F)(0.1)
=0.1F

M1+I1-2=M2

79380-0.1F=0
79380=0.1F
F=793800 Ans Wrong

Counting gravity force into working

I1-2= Force x Time
=(9.81F)(0.1)
=0.981F

79380=0.981F
F=80917.43 Ans Wrong

Last edited: Aug 9, 2012
9. Aug 9, 2012

### Staff: Mentor

That's the net force acting. Use that to find the resistive force.

10. Aug 9, 2012

### freshbox

Is I1-2= Force x Time this force always the net force?

11. Aug 9, 2012

### Staff: Mentor

I'd say so.

12. Aug 9, 2012

### freshbox

Can you guide me on how to use the net force to find the resistive force? Is there a formula to it?

13. Aug 9, 2012

### Staff: Mentor

Draw a free body diagram and analyze the forces acting on the hammer+pile system.

14. Aug 9, 2012

### freshbox

There is an action and reaction force. Action :+793800 Reaction-793800

15. Aug 9, 2012

### Staff: Mentor

For one thing, action and reaction forces act on different bodies. What you want are the forces acting on the hammer+pile.

Second, that 793800 N force is a net force, not an individual force. So even talking about it as an "action" is meaningless.

What forces act on the hammer+pile? Hint: Only two forces act.

16. Aug 9, 2012

### Staff: Mentor

That's the opposing force to bring about the deceleration. In addition, you also have to oppose their weights, as well.

17. Aug 9, 2012

### freshbox

Gravity and the 120kN external force?

18. Aug 9, 2012

### Staff: Mentor

The specs indicate that the 120kN force is turned off by the time the soil opposes the moving pile.

19. Aug 9, 2012

### freshbox

I just can't think of any other forces besides gravity.. and the FBD i drew looks abit weird to me too haha..

20. Aug 9, 2012

### Staff: Mentor

How many newtons force UP must be exerted to balance their weights acting DOWNWARD?