High School Physics Question -- Skateboarder Jumps & Conservation of Momentum

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Homework Help Overview

The discussion revolves around a physics problem involving conservation of momentum, where a skateboarder jumps off her skateboard, resulting in different velocities for both the skateboard and the skateboarder. The problem seeks to determine the mass of the skateboarder based on the given velocities and mass of the skateboard.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • Participants discuss the implications of the skateboarder jumping off and the resulting speeds of both the skateboard and the skateboarder. Some express confusion regarding the feasibility of the scenario, suggesting that it might imply a creation of free energy or questioning the assumptions about friction and external forces.

Discussion Status

The discussion is ongoing, with participants raising concerns about the problem's setup and the assumptions involved. Some have suggested that there may be missing information or that the question itself might be poorly constructed, indicating a lack of consensus on how to approach the problem.

Contextual Notes

Participants note the assumption that there is no friction between the skateboard and the ground, which may not align with real-world physics. There is also a suggestion that the skateboard might be expected to move in the opposite direction, indicating potential misunderstandings in the problem's framing.

Kate2018
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Homework Statement


Lee is riding on her 6 kg skateboard with a constant speed of 2 m/s. She jumps off of her skateboard and continues forward with a velocity of 4 m/s relative to the ground. This causes the skateboard to go flying forward with a speed of 18.5 m/s relative to the ground. What is Lee’s mass?

Homework Equations


m1v1+m2v2= (m1+m2)v3
m1v1+m2v2=m1v3+m2v4

The Attempt at a Solution


Info
m1=?
v1= 2m/s
m2= 6kg
v2=2m/s
v3=4m/s
v4= 18.5m/s
 
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Kate2018 said:

Homework Statement


Lee is riding on her 6 kg skateboard with a constant speed of 2 m/s. She jumps off of her skateboard and continues forward with a velocity of 4 m/s relative to the ground. This causes the skateboard to go flying forward with a speed of 18.5 m/s relative to the ground. What is Lee’s mass?

Homework Equations


m1v1+m2v2= (m1+m2)v3
m1v1+m2v2=m1v3+m2v4

The Attempt at a Solution


Info
m1=?
v1= 2m/s
m2= 6kg
v2=2m/s
v3=4m/s
v4= 18.5m/s
So the skateboarder jumps off faster forward and the skateboard goes forward faster too. That would be a great way to create free energy... I guess I'm missing something...
 
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berkeman said:
So the skateboarder jumps off faster forward and the skateboard goes forward faster too. That would be a great way to create free energy... I guess I'm missing something...
Kate2018 said:

Homework Statement


Lee is riding on her 6 kg skateboard with a constant speed of 2 m/s. She jumps off of her skateboard and continues forward with a velocity of 4 m/s relative to the ground. This causes the skateboard to go flying forward with a speed of 18.5 m/s relative to the ground. What is Lee’s mass?

Homework Equations


m1v1+m2v2= (m1+m2)v3
m1v1+m2v2=m1v3+m2v4

The Attempt at a Solution


Info
m1=?
v1= 2m/s
m2= 6kg
v2=2m/s
v3=4m/s
v4= 18.5m/s
Go and check your problem details again something is missing i guess.
 
Kate2018 said:

Homework Statement


Lee is riding on her 6 kg skateboard with a constant speed of 2 m/s. She jumps off of her skateboard and continues forward with a velocity of 4 m/s relative to the ground. This causes the skateboard to go flying forward with a speed of 18.5 m/s relative to the ground. What is Lee’s mass?

Homework Equations


m1v1+m2v2= (m1+m2)v3
m1v1+m2v2=m1v3+m2v4

The Attempt at a Solution


Info
m1=?
v1= 2m/s
m2= 6kg
v2=2m/s
v3=4m/s
v4= 18.5m/s

I suspect the skateboard is supposed to go flying backwards.

That said, this is a poor question. You have to assume that there is no friction possible between the skateboard and the ground. In general, when you jump the Earth is involved. So, it's not good to develop the idea that conservation of momentum applies to jumping.
 
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