TBR momentum problem

[greenseeking]

---

Members don't see this ad.

Please see attached for problem and passage.

Correct answer is C.

I chose D because impulse is J= p(final) - p (initial)
In experiment I, it's an elastic collision so m(bob) * V(bob)=m(block) * V (block)

In Experiement II, it's an inelastic collision so m(bob) * V(bob) = [m(Block) + m (Bob)] V (both objects)

So, in experiment I, the J would be 0 because p initial and p final would be equal to each other.
In experiment I, the J would be greater than 0 because p initial and p final would be different.

Can anyone please clarify/give any insights?

 

[monkeyvokes]

Please see attached for problem and passage.

Correct answer is D.

I chose C because impulse is J= p(final) - p (initial)
In experiment I, it's an elastic collision so m(bob) * V(bob)=m(block) * V (block)

In Experiement II, it's an inelastic collision so m(bob) * V(bob) = [m(Block) + m (Bob)] V (both objects)

So, in experiment I, the J would be 0 because p initial and p final would be equal to each other.
In experiment I, the J would be greater than 0 because p initial and p final would be different.

Can anyone please clarify/give any insights?

Yeah, after looking at your image I was going to say exactly what you did. You seem to understand what is happening, what do you feel needs clarification?

 

[greenseeking]

Well the answer is D not C. I chose C because I thought that since in Exp 1 (elastic collision) m1v1=m2v2; that J=pf-pi= m1v1-m2v2=0. Thus, the magnitude of impulse would be 0 and less than the magnitude of impulse in Exp 2. This is wrong according to the answer key.

 

[phys2med]

Well the answer is D not C. I chose C because I thought that since in Exp 1 (elastic collision) m1v1=m2v2; that J=pf-pi= m1v1-m2v2=0. Thus, the magnitude of impulse would be 0 and less than the magnitude of impulse in Exp 2. This is wrong according to the answer key.

I must be missing something. So J for Exp 1 is 0 which means it is less than J for Exp 2. Answer D says just that. "we find that the magnitude of the impulse is greater in Exp D). 2" Isn't that all?

 

[greenseeking]

Omg I'm so sorry. I chose D but the correct answer is C.

 

[phys2med]

Omg I'm so sorry. I chose D but the correct answer is C.

No problem I'm just trying to make sure I understand what you're saying.

So, conceptually, elastic collisions have a greater impulse than inelastic. The initial object is moving in one direction and then the collision causes it to move in the opposite direction (or in this case, to stay put which still requires a force in the opposite direction). The second object then gains momentum also. So we have one object that changes direction and one that starts moving. For inelastic, the initial object is still moving in the same initial direction with the second object so there isn't as big a change as with the elastic.

 

[greenseeking]

OK. That explanation makes a lot more sense than what I tried to do. It makes more sense to think about it conceptually. The first object's momentum would change by mv in exp1 (since it goes from mv to 0) vs. in the second exp, the first object's momentum will change by less than mv (since it goes form mv to mv' where v' is less than v) Thanks alot!