# Homework Help: Easy conceptual energy problem?

1. Oct 26, 2013

### mot

"Easy" conceptual energy problem?

1. The problem statement, all variables and given/known data
Here's the diagram

http://nkkkjkjjjjjjjjjjj.tumblr.com/image/65137695911
A bead slides on a wire, which is in a vertical plane, as shown in the diagram. Gravity acts in the -y direction. The bead starts at A, moving to the right with an initial velocity v. The wire is frictionless between A and D and between F and G, but there is friction between D and F. (For each statement select True, False, Greater than, Less than, Equal to, or Not enough information to tell.
1. The bead's kinetic energy at B is ... its kinetic energy at F
2. Between A and C, there is positive work done on the bead
3. Between D and F, the speed decreases, then increases
4. The speed at C is ... the speed at E

2. Relevant equations
E_g=mgy
E_k=1/2mv^2

3. The attempt at a solution
The bead's kinetic energy at B is ... its kinetic energy at F: For this is said greater than because, although they are at the same height, all the energy is not conserved because it experiences friction between D and F.
Between A and C, there is positive work done on the bead
I said True because the dispacement is in the same direction as the force of gravity
Between D and F, the speed decreases, then increases
I said true for this one. At D it experiences friction and slows down while moving horizontally, then it starts to go down, so a component of gravity acts to accelerate it. Even if it is still slowing down, it's still speeding up relative to its speed at E.
The speed at C is ... the speed at E
I put not enough info...We don't know how much friction slowed the bead down between D& E compared to how much it "sped up" on that decline between C & D.

But these answers are wrong...can someone help? Thanks so much!

2. Oct 26, 2013

### nil1996

Your solutions look correct to me.
Welcome to PF

Last edited: Oct 26, 2013
3. Oct 26, 2013

### Staff: Mentor

Your answer to 3 looks wrong. You don't know whether gravity will win out over friction. So, it could still be slowing from E to F. Not enough info.

4. Oct 26, 2013

### mot

That was it! It makes sense too. The "deceleration" from E to F should be less than D to E, but it could still be slowing relative to the speed before D, or it could be speeding up depending on the angle.
Thanks! :)

5. Oct 26, 2013

### nil1996

agree

6. Oct 26, 2013

### Staff: Mentor

Also note that the slope between E and F never becomes vertical, so there is always going to be a normal force. This allows a frictional force to always be present over the entire path between E and F.

Chet

7. Oct 23, 2014

### Ruobing Shao

the same diagram but different question, mine is
True False Greater than Less than Equal to Not enough information to tell The bead's potential energy at B is ... its potential energy at F
True False Greater than Less than Equal to Not enough information to tell Between A and B, the speed increases
True False Greater than Less than Equal to Not enough information to tell The speed at C is ... the speed at E
True False Greater than Less than Equal to Not enough information to tell Between D and E, there is positive work done on the bead,
i choose these answers but its incorrect i don't know which one do i get wrong

8. Oct 23, 2014

### Ruobing Shao

the same diagram but different question, mine is
The bead's potential energy at B is ... its potential energy at F
Between A and B, the speed increases
The speed at C is ... the speed at E
Between D and E, there is positive work done on the bead,
the first one i choose greater than , second i choose true,third i choose not enough inform, the last one i choose false
i choose these answers but its incorrect i don't know which one do i get wrong

9. Oct 24, 2014

### Staff: Mentor

Chet

10. Oct 25, 2014

### Ruobing Shao

i think EP B is GREATER than F is because when its the same height F experience the friction which means that the Ep is not all transferred at position F
and i think the speed is increasing from A to B due to the conservation of energy mgh(i) + 1/2mv(i)^2= mgh(f)+1/2mv(f)^2,so as the height decrease the speed will increase.speed between C and E is not enough inform to tell since E undergoes friction but c has the larger height, and the last one i think it should be negative,because they have the same height and when undergoing the friction its a process of losing energy which will cause negative work , this is my understanding i am not sure whether it's correct or not

11. Oct 25, 2014

### Staff: Mentor

This is the one that's wrong. Now that you know it, can you figure out why?

Chet

12. Oct 25, 2014

### Ruobing Shao

sorry i don't get why, wait is that because potential energy is no relation to friction since kinetic energy will be the one matters?

13. Oct 25, 2014

### Ruobing Shao

i get it , it should be equal thank you so much