Need Help Understanding KE, PE and Mechanical Energy lost due to friction

In summary, the problem involves a 70 kg base runner sliding into second base with an initial speed of 4.0m/s and a coefficient of friction of 0.70. The questions ask for the amount of mechanical energy lost due to friction acting on the runner and the distance he slides. To answer the first question, the formula KEi - Wf = KEf is used, where KEi represents the initial kinetic energy and Wf represents the work done by friction. The answer is -560J, indicating that the runner loses 560J of energy due to friction. For the second question, the formula KEf-W=KEi is utilized, since there is no change in potential energy. The answer is not provided
  • #1
sarahjt1
9
0

Homework Statement



A 70 kg base runner begins his slide into second base when he is moving at a speed of 4.0m/s. The coefficient of friction between his clothes and the Earth is 0.70. He slides so that his speed is zero just as he reaches the base.
a) How much mechanical energy is lost due to friction acting on the runner?

b) How far does he slide?



So, I figured out b no problem but a) I have no idea where to even begin. I was using the conservation of energy equation KE + PE = KEi + PEi. But that obviously is not correct. How does mechanical energy EVER get calculated into these equations? I don't understand where it goes and what represents it...



Homework Equations



For part b) I used the KEf-W=KEi because PEi=PEf due to the fact that there is no y-direction... right?



The Attempt at a Solution

 
Physics news on Phys.org
  • #2
Welcome to PF!

Hi sarahjt1! Welcome to PF! :smile:

(Yes, you can ignore PE completely … in fact, I don't think the examiner will even expect you to mention it)
sarahjt1 said:
a) How much mechanical energy is lost …

a) I have no idea where to even begin. I was using the conservation of energy equation KE + PE = KEi + PEi. But that obviously is not correct. How does mechanical energy EVER get calculated into these equations? I don't understand where it goes and what represents it...

This isn't physics, it's just arithmetic …

(yeah, sometimes the problems are that easy! :wink:)

you know the mechanical energy before, and the mechanical energy after, so the mechanical energy lost is … ? :smile:
 
  • #3
I got -560J as my answer but I am not sure if that is right. If it is, I don't fully understand the rationale I used to get there. I guess it is more that I don't fully understand the concepts involved.

This is where my trouble lies... just in understanding the equations that are relevant to the questions being asked... :(
 
  • #4
Well for this question the runner as an initial kinetic energy of 560J. Friction does work on the runner which brings his velocity to zero and thus, his kinetic energy. So you would have...
[tex]KEi -Wf = KEf[/tex]
And you'll see that he losses 560j of energy.
 
  • #5
Hi sarahjt1! :wink:
sarahjt1 said:
I got -560J as my answer but I am not sure if that is right.

It is, except that it's plus 560J (the question asks for the energy lost, which of course is positive :wink:).
If it is, I don't fully understand the rationale I used to get there. I guess it is more that I don't fully understand the concepts involved.

Are you talking just about part a)?

You're just calculating the KE before and the KE after (using the standard 1/2 mv2 definition), and subtracting …

what is worrying you about that? :smile:​
 

Related to Need Help Understanding KE, PE and Mechanical Energy lost due to friction

1. What is kinetic energy?

Kinetic energy is the energy an object possesses due to its motion. It is dependent on the mass and velocity of the object and is represented by the equation KE = 1/2 * m * v^2, where m is the mass and v is the velocity.

2. What is potential energy?

Potential energy is the energy an object possesses due to its position or state. It can be stored in an object and converted into kinetic energy. Examples of potential energy include gravitational potential energy and elastic potential energy.

3. How is mechanical energy lost due to friction?

Friction is a force that opposes motion between two surfaces in contact. When an object moves against a surface, some of its mechanical energy is converted into heat energy due to the work done by friction. This results in a decrease in the object's kinetic energy and potential energy.

4. Can mechanical energy lost due to friction be recovered?

No, mechanical energy lost due to friction cannot be recovered. It is converted into heat energy, which is a form of energy that cannot be converted back into mechanical energy. This is due to the law of conservation of energy, which states that energy cannot be created or destroyed, only transferred or converted into different forms.

5. How can the effects of friction on mechanical energy be reduced?

The effects of friction on mechanical energy can be reduced by using lubricants, such as oil or grease, between surfaces to reduce the friction force. Additionally, using smoother surfaces and reducing the weight or load on the object can also help reduce the loss of mechanical energy due to friction.

Similar threads

Conservation of energy: Crane raising a mass
    • Introductory Physics Homework Help
Replies
17
Views
3K
Conservation of mechanical energy
    • Introductory Physics Homework Help
Replies
8
Views
1K
Draw a FBD, calculate tension in the string, determine mass
    • Introductory Physics Homework Help
Replies
8
Views
2K
General Equation about Energy lost due to Friction
    • Introductory Physics Homework Help
Replies
1
Views
5K
Starting height of marble rolling around a loop the loop
    • Introductory Physics Homework Help
Replies
12
Views
11K
Max KE of a squirrel jumping from the top of a tree
    • Introductory Physics Homework Help
Replies
6
Views
2K
Kinetic friction and lost mechanical energy
    • Introductory Physics Homework Help
Replies
12
Views
2K
Calculating Speed on an Incline Using the Law of Conservation of Energy
    • Introductory Physics Homework Help
Replies
5
Views
1K
Why do I not get the same result when I use change in PE?
    • Introductory Physics Homework Help
Replies
3
Views
2K
Mechanical energy lost when child jumps onto merry-go-round
    • Introductory Physics Homework Help
Replies
3
Views
2K

Hot Threads

Recent Insights

Back
Top