AP Physics 1 Help? Centripetal motion/Kinematics/Friction problem

In summary, the conversation discusses a physics problem involving a track with a quarter circle section and a rough horizontal section. The problem involves determining the speed of a block at different points on the track and the force and work of friction acting on the block. The conversation points out an error in the solution for part 1 and suggests using a different approach involving the block's energy. The final answer for part 1 is confirmed to be correct.
  • #1
tortilla
6
0
Homework Statement
A track consists of frictionless arc XY which is a quarter circle of radius R and a rough horizontal secion YZ. Block A of mass M is released from rest at point X slides down the curved section of the track to point Y. Block A continues moving along the horizontal track to the right, sliding past point P which is a distance l from point Y. The coefficient of kinetic friction between the blocks and the horizontal part of the track is mu.

Express your answers in terms of M, l, mu, R, and g.

1. Determine the speed of block A at point Y.

2. Determine the force due to friction acting upon block A as it slides horizontally.

3. Determine the work done by friction on block A as it passes point P.

4. Determine the speed of block A as it passes point P.
Relevant Equations
Fc = (Mv^2)/R
Ff = Fn*mu
w = Ff*d
I think I have solved the first three, and only really need help on question four.

For number one, I used Fc = (Mv^2)/R and just rearranged it for velocity so I ended up with v = sqrt(ac * R)

For number 2 I used Ff = Fn*mu and got Mg*mu = Ff

For number 3 I used w = Ff*d and got w = -Mg*mu*l

For number four I have no idea.
 

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  • #2
The potential energy should equal the kinetic energy minus the work of friction. That should be what you use to find the velocity.
 
Last edited:
  • #3
tortilla said:
Homework Statement:: A track consists of frictionless arc XY which is a quarter circle of radius R and a rough horizontal secion YZ. Block A of mass M is released from rest at point X slides down the curved section of the track to point Y. Block A continues moving along the horizontal track to the right, sliding past point P which is a distance l from point Y. The coefficient of kinetic friction between the blocks and the horizontal part of the track is mu.

Express your answers in terms of M, l, mu, R, and g.

1. Determine the speed of block A at point Y.

2. Determine the force due to friction acting upon block A as it slides horizontally.

3. Determine the work done by friction on block A as it passes point P.

4. Determine the speed of block A as it passes point P.
Homework Equations:: Fc = (Mv^2)/R
Ff = Fn*mu
w = Ff*d

I think I have solved the first three, and only really need help on question four.

For number one, I used Fc = (Mv^2)/R and just rearranged it for velocity so I ended up with v = sqrt(ac * R)

For number four I have no idea.

Your answer to number 1 is not correct. What value are you going to use for ##a_c##? You've used an equation for the centripetal accleration associated with constant speed circular motion, which is not relevant here.

If you work out how to solve part 1, you may see how to solve part 4.
 
  • #4
PeroK said:
Your answer to number 1 is not correct. What value are you going to use for ##a_c##? You've used an equation for the centripetal accleration associated with constant speed circular motion, which is not relevant here.

If you work out how to solve part 1, you may see how to solve part 4.

Thanks so much for pointing that out! should I be using v=(2piR)/t? I don't know the time it was on the track
 
  • #5
tortilla said:
Thanks so much for pointing that out! should I be using v=(2piR)/t? I don't know the time it was on the track

This problem will be difficult using forces and acceleration. Can you think of another approach?
 
  • #6
I'm really not sure. Should I be looking at the angular speed? just v/R?
 
  • #7
tortilla said:
I'm really not sure. Should I be looking at the angular speed? just v/R?

No. Something else. You used it to solve part 3.
 
  • #8
PeroK said:
No. Something else. You used it to solve part 3.
Because it isn't uniform the work is not zero?
 
  • #9
PeroK said:
No. Something else. You used it to solve part 3.
since at point X it is at rest all the energy is gravitational potential, right? So if I set mgR equal to .5*m*v^2 and get v = sqrt(2gR) is that correct for part 1?
 
  • #10
tortilla said:
since at point X it is at rest all the energy is gravitational potential, right? So if I set mgR equal to .5*m*v^2 and get v = sqrt(2gR) is that correct for part 1?
Yes.
 
  • #11
haruspex said:
Yes.
Thank you so so much
 

1. What is centripetal motion and how does it relate to AP Physics 1?

Centripetal motion is the movement of an object in a circular path with a constant speed. In AP Physics 1, students will learn about the forces that are needed to maintain this motion, as well as the mathematical equations that describe it.

2. What are the key concepts of kinematics in AP Physics 1?

Kinematics is the study of motion without considering the forces that cause it. In AP Physics 1, students will learn about displacement, velocity, and acceleration, and how to use equations to analyze and describe these variables.

3. How do I approach solving friction problems in AP Physics 1?

When solving friction problems in AP Physics 1, it is important to first identify the forces at play, including the normal force and the force of friction. Then, use the appropriate equations and diagrams to determine the net force and the resulting motion.

4. What are some common mistakes to avoid when working with centripetal motion in AP Physics 1?

One common mistake when working with centripetal motion is forgetting to consider the direction of the centripetal force, which is always directed towards the center of the circular motion. It is also important to use the correct equation for the given scenario, whether it is centripetal acceleration or centripetal force.

5. How can I strengthen my understanding of these topics and improve my performance on AP Physics 1 exams?

Practice is key when it comes to mastering centripetal motion, kinematics, and friction problems in AP Physics 1. Be sure to work through a variety of problems and seek help from your teacher or peers if you are struggling. It can also be helpful to make connections between these topics and real-world situations to solidify your understanding.

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