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# work-energy principle watch

1. Just going through the edexcel M2 spec and found a mention of this but can't find it in the heinemann book. can someone please explain.

MB
2. Work done (eg work done against friction) = Loss in Kinetic Energy - Gain in Potential Energy
3. (Original post by Indelible)
Work done (eg work done against friction) = Loss in Kinetic Energy - Gain in Potential Energy
Thanks - does this always include P.E. and should I check my results with newton equations of motion if I have time? can someone put up an example.

MB
4. This is a VERY hard question: 16 marks in total, if you can do this your ready for anything!

Particle P is projected from the point A up the line of greatest slope of a rough plane which is inclined at an angle beta (written as B from now on) to the horizontal where sinB=4/5. P is projected with speed 5.6ms^-1 and the coefficient of friction between P and the plane is 4/7

Given that P comes to rest at the point B

a) use the Work-Energy principle to show that the distance AB is 1.4m (10 marks)

The particle then slides back down the plane

b) Find correct to 2 significant figures, the speed of P when it returns to A (6 marks)

5. ok so draw a label with R, (mu)R, mg, and the slope angled at B and distance AB = d

Resolve perp to the plane: R = mgcosB (cosB = 3/5 using 345 triangle)
so R = mg(3/5)
(mu)R = (12/35)mg
Work done against friction = loss in KE - gain in PE
change in height (h) = dsinB
so:
(12/35)mgd = [0.5m(5.6)^2] - [mgdsinB] = 15.68 - (4/5)gd
(40/35)gd = 0.5(5.6)^2
d = 1.4

part b coming soon
6. b) work done against friction = loss of KE (PE returns to normal due to no change in height)

(12/35)mg(2d) = 0.5m(5.6^2 - v^2)
1.92g =5.6^2 - v^2
v^2 = 12.544
v= 3.5ms^-1 (2sf)

quite straight forward but remember to double the distance because it moves up and then back down the slope
7. Thanks, I think i've got it. I got the first part right already -I'll practice in the morning.

MB
8. this is more like an M1 quaestion.
9. (Original post by musicboy)
Just going through the edexcel M2 spec and found a mention of this but can't find it in the heinemann book. can someone please explain.

MB
In case you want to take heat into consideration:

delta U = Q - W

Where delta U is the change in energy of the system, Q is the heat flowing into the system, and W is the work done by the system.
10. yeah, thanks for the questions, i got the first part right, although i think that the marks scheme is giving too many marks for it, although im not complaining.

and the second part was kool. although i did it slightly different to you.

i had the two forces, the friction and accelerative force due to gravity, and putting them into F=ma cancels the m's out so you get acceleration in terms of g and then use suvat to work out the speed after travelling 1.4m.
11. (Original post by lexazver203)
this is more like an M1 quaestion.
It would be M1 if you solved it with equations of motion (as you can) but it specifies work-energy and is thus M2.

MB
12. (Original post by musicboy)
It would be M1 if you solved it with equations of motion (as you can) but it specifies work-energy and is thus M2.

MB
But he still used a principle of resolving R perpendicullar to the plane, which is M1
13. OK - finally got my head round it - you are a real comrade. have some rep!

MB
14. so? it also uses multiplication which is primary school work.
All maths is a progresion using things youve learnt before.
The question states you must use the work-energy principle so its M2

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