Momentum Qs: Help With Questions 5 & 6

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In summary, the conversation discusses two questions related to physics problems about a space pod and a rotating surface. In question 5, the participants discuss the reason why the space pod does not move after they take their seats, and in question 6, they talk about the concept of kinetic and static friction on a rotating surface. The participants provide explanations and calculations to understand the physics behind these concepts.
  • #1
xplosiv
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Homework Statement


http://img16.imageshack.us/img16/5889/physics.jpg [Broken]

The Attempt at a Solution



Well for question 5, I'm having a bit of trouble recognising the reason why the space-pod doesn't move after they take their seats. I understand that the centre of mass doesn't change but I can't see the reason.

As for question 6, I'm wondering what bearing the 'same frictional force' has on the situation. When I first looked at it I thought that the kinematics equations could be used but now I'm thinking I have to do something with that 'same' friction force.

Any help greatly appreciated.
 
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  • #2
5) When Simma and Stan are swapping, they can do so only by pushing the floor of the space pod. After they take their new seats, there in no action on space-pod by them. So the space-pod does not move.
Initially all of them are at rest.( Including space pod ). When they are swapping, they are pushing the floor of the space pod in the opposite direction. Assuming that they take the same time to swap the seats, calculate the change in momentum of Simma and Stan, and equate it to the change in momentum of the space pod.
In problem 6, after pushing each other, they can come to rest due to the friction force.
 
  • #3
Great thanks a lot rl.bhat, I'll try have a crack at those properly now.

Oh, I also have another quick conceptual question. Why is it when you are on a rotating surface that it goes from kinetic friction to static friction in the centre? I hope you understand what I mean. I'm saying as you walk and the radius gets smaller why does it become static friction? I'm interested to know the actual physics behind it. Is it something to do with the magnitude of centripetal force or the tangential velocity?
 
  • #4
When you keep any object on a rotating surface, its tendency is to fly away radially. If you want to prevent it from sliding, there must be sufficient kinetic friction between surface and the object.
 
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  • #5
I'll show you the question and why I'm asking such a conceptual question. It's a bit of a weird one. I get how to find the radius of static friction but I don't understand the concept. I know it's best it walks from outside to inside.

http://img257.imageshack.us/img257/2746/phs2.jpg [Broken]
 
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  • #6
when a car is negotiating a curve, the passenger is thrown away radially.
Similarly the robot will be thrown away radially with a force mv^2/r. To prevent this motion there should be equal amount of kinetic friction toward the center.
 
  • #7
So when this slipping stops, static friction takes effect? What I'm trying to find out is the reason it slips on the outside and stays in the inside. Thanks for your help, and for being so patient.
 

1. What is momentum?

Momentum is a measurement of an object's motion, taking into account both its mass and velocity. It is calculated as mass multiplied by velocity.

2. How is momentum different from speed?

Speed only measures how fast an object is moving, while momentum takes into account the object's mass as well. This means that two objects traveling at the same speed can have different momentums if they have different masses.

3. What are the units of momentum?

The SI unit for momentum is kilogram-meters per second (kg*m/s). However, it can also be expressed in other units such as grams-centimeters per second (g*cm/s) or pounds-feet per second (lb*ft/s).

4. How is momentum conserved?

The law of conservation of momentum states that the total momentum in a closed system remains constant. This means that in any interaction between objects, the total momentum before the interaction is equal to the total momentum after the interaction.

5. How is momentum used in real life?

Momentum plays a crucial role in various real-life applications such as sports, transportation, and engineering. For example, understanding momentum can help in designing safer cars and improving the performance of athletes in sports like track and field.

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