BR Physics Chapter IV/Passage I --Momentum Help

[Biebs]

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Hello Everyone,

I was wondering if you could help me with some of the momentum problems in this passage. For some reason, I can't really wrap my head around momentum. I understand that momentum is always conserved, and KE is ONLY conserved in cases of elastic collisions (When objects do not stick together). I am particularly having a hard time with the first passage in section IV in the BR review book.

The passage states: "A young girl decides to play on her sled, which is resting on a frozen and frictionless pond. The 15kg girl runs and then slides across the ice at a speed of 2 m/s until she collides with the 20-kg sled. Now stuck to the sled, she continues to slide until she and the sled hit a 50-kg log that is embedded in the ice." --There is a picture of the girl running _______ sled ______ log.

Question 1 asks: What is the approximate speed of the center of mass of the child/sled system, after the collision.

I set this up to be an inelastic collision, because I assume that the upon impact, the girl will just stop.

so: m1v1 i + m2v2 i = (m1 + m2) v
solve for v.

So I set it up like this:

v= m1v1 i / (m1+m2)
v= (35)(2) / (15+20)
v= 2

However the answer is 1 m/s. I see that they did not factor in the sleds weight when finding the initial momentum. Why is this? Aren't the sled and the girl moving as one system, so shouldn't their total weight (girl plus sled) be accounted for?

Also: I am having a hard time on questions that ask what happens if the person moves forward (standing on a sled) and a frictionless surface. What happens as the person moves forwards or backwards? The direction that they move is the opposite direction that the sled will move? How does this work?

Question 5: Once the sled has stopped, the child walks to the front of the sled in an attempt to touch the log. How will she move relative to the stationary log.
A: She will move forward, but not as fast as she would on dry ground.

Thanks for the help.

 

[Biebs]

Oops... Mods could you please move this to the MCAT Study Question Q & A Sub-forum. I guess I accidentally posted it here. Sorry.

 

[GeronimoMD]

The passage states: "A young girl decides to play on her sled, which is resting on a frozen and frictionless pond. The 15kg girl runs and then slides across the ice at a speed of 2 m/s until she collides with the 20-kg sled. Now stuck to the sled, she continues to slide until she and the sled hit a 50-kg log that is embedded in the ice." --There is a picture of the girl running _______ sled ______ log.

Question 1 asks: What is the approximate speed of the center of mass of the child/sled system, after the collision.

I set this up to be an inelastic collision, because I assume that the upon impact, the girl will just stop.

so: m1v1 i + m2v2 i = (m1 + m2) v
solve for v.

So I set it up like this:

v= m1v1 i / (m1+m2)
v= (35)(2) / (15+20)
v= 2

However the answer is 1 m/s. I see that they did not factor in the sleds weight when finding the initial momentum. Why is this? Aren't the sled and the girl moving as one system, so shouldn't their total weight (girl plus sled) be accounted for?

Also: I am having a hard time on questions that ask what happens if the person moves forward (standing on a sled) and a frictionless surface. What happens as the person moves forwards or backwards? The direction that they move is the opposite direction that the sled will move? How does this work?

Question 5: Once the sled has stopped, the child walks to the front of the sled in an attempt to touch the log. How will she move relative to the stationary log.
A: She will move forward, but not as fast as she would on dry ground.

Thanks for the help.

Isn't M1 = 15kg???

Therefore,

v= m1v1 + m2v2 / (m1+m2)
v= (15)(2) + (20)(0) / (15+20)
v= 30/35
v = 0.85 ~ 1m/s