M2

I have found the total loss in energy and the work done against resistive forces.
To find the work done by the skier, do i subtract total loss in energy from resistive forces of resistive forces from total loss in energy?

Hi there,

Subtract total loss in energy from work done against resistance, so:

Work done against resistance = Total loss of energy + Work done by Skier

becomes:

Work done against resistance - Total loss of energy = Work done by Skier.

For this, I got $16800 - 14025 = 2775J$

But isn't the Total change in energy = Total work done ?

Yes, for M2 I find it is best to think of this rule as saying:

Work against resistance = Total change (loss) in energy.

Some of this lost energy is Kinetic and Gravitational Potential energy. Energy is also lost from the work the skier is doing to slide down the hill. So:

Work against resistance = (KE lost + GPE lost) + Work done by Skier

This is an unusual M2 question, because most the time we treat the particle (skier) as not doing any work other than that needed to overcome resistance. Unless specified that the particle is doing this extra work, assume that it is not.

Ahh right but could you not also think of it as.
Total work done which would be Work done by the resistance and Work done by the skier which would be equal to the total change in energy?

Thanks!

So what you're suggesting is that:

Work Done against resistance + Work Done by the skier = Total change in Energy.

This is not true. If this were the case:

$16800 + W = 14025$ $\rightarrow$ $W = -2775J$

The negative sign for work done should be a red flag that you've got it the wrong way round. Negative work indicates that the skier is not doing work, but that the system (slope) is doing work on the skier. Since the questions asks for the work done by the skier, it has to be wrong.

But what about this exact same example?


Thanks!

I don't know where you got that question, but the question is at fault. This gives a negative value for work, suggesting that the skier hasn't actually done any work at all (the slope has 'worked' on the skier). I'm pretty sure it's wrong.

$1500*13 = (520+19110) + W$

$W = -130J$

Thanks, i got it from here, he explains it.
https://www.youtube.com/watch?v=klSKKiGruSc

I think he's made a mistake here. The commenters in the section below the video seem to think it's -130J as well.

Either he's wrong, or the Edexcel M2 textbook is wrong, since doing it the way the textbook says will give you -130J.

Edit: I've sent the question to my teacher to see if she gets my result or the video's result. I'll let you know what she says asap.

I think he's made a mistake here. The commenters in the section below the video seem to think it's -130J as well.

Either he's wrong, or the Edexcel M2 textbook is wrong, since doing it the way the textbook says will give you -130J.

Edit: I've sent the question to my teacher to see if she gets my result or the video's result. I'll let you know what she says asap.