Understanding Work and Energy: A Short Guide

[idllotsaroms]

Homework Statement

http://postimage.org/image/sezpli4pv/
Having some difficulty understanding some concepts of Work and Energy.

Homework Equations

W = F * D
KE = 1/2 mv^2
PE = mgh

The Attempt at a Solution

A) Speed is at B is less than the speed at D, because B has more PE than D
B) False, because there is the force of gravity acting always
C) True, Same location in respect to the graph
D) Less, because force of gravity < 0
E) Less, because B has more PE than H
F) Less, acceleration due to gravity

 

[haruspex]

A) Speed is at B is less than the speed at D, because B has more PE than D

Looks to me that D is slightly higher than B. Are you saying B is higher? But it's such a small difference I wonder if they're meant to be the same height. (I think you looked at the wrong two points.)

B) False, because there is the force of gravity acting always

Acceleration is the result of the net force. If you're standing still, gravity is acting on you but it's countered by the floor, so you don't accelerate. But your answer is right for a different reason. What is happening to the velocity at B?

C) True, Same location in respect to the graph

At (A) above you said it was faster at D. But anyway, this question is asking about velocity, not speed. There's a difference - what is it?

D) Less, because force of gravity < 0

Same misunderstanding as your answer (B).

E) Less, because B has more PE than H

Yes.

F) Less, acceleration due to gravity

It's asking about acceleration in the x direction.

 

[idllotsaroms]

looks to me that d is slightly higher than b. Are you saying b is higher? But it's such a small difference i wonder if they're meant to be the same height. (i think you looked at the wrong two points.)

Yes, sorry they are on the same level, so speed at B is equal to the speed at D

Acceleration is the result of the net force. If you're standing still, gravity is acting on you but it's countered by the floor, so you don't accelerate. But your answer is right for a different reason. What is happening to the velocity at B?

Is it because the bead is moving in a circular motion at that point (semi circle)?

At (A) above you said it was faster at D. But anyway, this question is asking about velocity, not speed. There's a difference - what is it?

Since they are at the same level, they have the same potential energy, the same kinetic energy, and therefore the same speed So, the velocity at B is equal to D.

Same misunderstanding as your answer (B).

At this point would acceleration be greater than 0 because the dip is a semi-circle with centripetal force aimed towards the middle?

It's asking about acceleration in the x direction.

Sorry!, the acceleration in the X-direction at H would be equal to zero then, because there is no external horizontal force.

A) Speed is at B is equal to the speed at D, because they are at the same height
B) False, because there is some circular motion downward which is less than 0
C) False, because of directionality (different directions)
D) Greater than, because centripetal force towards the center which in this case is upward (+)
E) Less, because B has more PE than H
F) Equal to 0, because there are no external forces acting in the horizontal plane.

 

[haruspex]

Yes, sorry they are on the same level, so speed at B is equal to the speed at D

Yes.

Is it because the bead is moving in a circular motion at that point (semi circle)?

Yes. The direction of movement is changing from up to down, so there's a downward acceleration.

Since they are at the same level, they have the same potential energy, the same kinetic energy, and therefore the same speed So, the velocity at B is equal to D.

No. Velocity is a vector. Speed is just its magnitude; it also has direction. If the direction is different the velocity is different.

At this point would acceleration be greater than 0 because the dip is a semi-circle with centripetal force aimed towards the middle?

Yes.

the acceleration in the X-direction at H would be equal to zero then, because there is no external horizontal force.

Yes.

 

[idllotsaroms]

Thank you for the help Haruspex, I appreciate it!