# Energy and Velocity

1. Apr 19, 2012

### shemo1993

1.

2.

3. my attempt to solve the question

Last edited: Apr 19, 2012
2. Apr 19, 2012

### Staff: Mentor

Welcome to the Physics Forums shemo1993!

The guidelines here are that you must show work before we can help you.

Let us see what you got.

3. Apr 19, 2012

### shemo1993

Hi
In this picture my attempt to solve the question

4. Apr 19, 2012

### PhanthomJay

It is given, from conservation of energy principles, when there are no other forces acting besides gravity which do work, that the GPE at the top for all 3 cases is the same, and that therfore the KE at the bottom will also be the same for each case. So for case 1, why did you use the height as (5 + 5)??

Note that KE is a scalar quantity and the v in that equation for KE is the objects speed. It's velocity has direction associated with it.

When you add friction, besides creating heat, what does that say about the speed of the object when it reachs the bottom, as compared to the frictionless case?

5. Apr 19, 2012

### shemo1993

First of all, thank you so much
For case1 I used the height as (5 + 5), because there are two distances incline and ramps and each one of them has length 5 meters. So what I have done is correct?
About the note, I know the speed is a scalar quantity and the velocity is a vector quantity, but when the direction does not change the speed will be equal to velocity. And I think the direction does not change.
about the speed of the object when it reachs the bottom I think it will be less! And in case1 and 2 will be less than case 3

6. Apr 19, 2012

### PhanthomJay

I don't know what you mean by 'two distances incline and ramps' for case 1. For case 1, the block is just dropped 5 m to the ground, in free fall, like as if you were dropping it from the roof of a 5 m tall building.
well the magnitude of the velocity is the same as its speed, but at the bottom, in case 1, the velocity direction is down, in case 2 the direction is parallel to the sloped incline, and in case 3 , it is more or less horizontal. So for case 2, the final velocity is 9.8 m/s parallel to the slope, and for case 3 the final velocity is 9.8 m/s more or less in a horizontal direction, and for case 1 , the velocity is ___??___?
Yes it will be less than the frictionless case.....

7. Apr 19, 2012

### shemo1993

Now I understood the first case
Therefore, the final answers will be as well?

8. Apr 19, 2012

### PhanthomJay

Yes, the speed is the same at the bottom (9.8 m/s) for all three cases. With friction present, some of the energy 'lost' due to the work done by frictiion shows up as heat, but not all of the lost energy is heat, some shows up as sound energy, or other forms .

9. Apr 20, 2012

### shemo1993

Thank you very much for help me

Allah bless you

shemo1993