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

AI Thread Summary
The discussion revolves around calculating mechanical energy lost due to friction for a base runner sliding into second base. The runner's initial kinetic energy is determined to be 560J, and the energy lost due to friction is calculated using the equation KEi - Wf = KEf, where Wf represents the work done by friction. Participants clarify that potential energy can be ignored in this scenario since there is no vertical movement. The confusion primarily lies in understanding how to apply the conservation of energy principles correctly. Ultimately, the energy lost due to friction is confirmed to be 560J.
sarahjt1
Messages
9
Reaction score
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
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:
 
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... :(
 
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...
KEi -Wf = KEf
And you'll see that he losses 560j of energy.
 
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:​
 

Thread 'Errors and significant figures'

I multiplied the values first without the error limit. Got 19.38. rounded it off to 2 significant figures since the given data has 2 significant figures. So = 19. For error I used the above formula. It comes out about 1.48. Now my question is. Should I write the answer as 19±1.5 (rounding 1.48 to 2 significant figures) OR should I write it as 19±1. So in short, should the error have same number of significant figures as the mean value or should it have the same number of decimal places as...
View full post »
Thread 'A cylinder connected to a hanging mass'

Thread 'A cylinder connected to a hanging mass'

Let's declare that for the cylinder, mass = M = 10 kg Radius = R = 4 m For the wall and the floor, Friction coeff = ##\mu## = 0.5 For the hanging mass, mass = m = 11 kg First, we divide the force according to their respective plane (x and y thing, correct me if I'm wrong) and according to which, cylinder or the hanging mass, they're working on. Force on the hanging mass $$mg - T = ma$$ Force(Cylinder) on y $$N_f + f_w - Mg = 0$$ Force(Cylinder) on x $$T + f_f - N_w = Ma$$ There's also...
View full post »

Similar threads

General Equation about Energy lost due to Friction
Replies
1
Views
5K
Conservation of energy: Crane raising a mass
Replies
17
Views
3K
Draw a FBD, calculate tension in the string, determine mass
Replies
8
Views
3K
Kinetic friction and lost mechanical energy
Replies
12
Views
3K
Conservation of mechanical energy
Replies
8
Views
2K
Starting height of marble rolling around a loop the loop
Replies
12
Views
12K
Why do I not get the same result when I use change in PE?
Replies
3
Views
2K
Conservation of Momentum Problem - Mechanical and Kinetic Energies
Replies
2
Views
912
Max KE of a squirrel jumping from the top of a tree
Replies
6
Views
2K
How far does the block travel up the incline?
Replies
1
Views
4K

Hot Threads

Recent Insights

Back
Top