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M3 when to use energy or resolving forces

in m3 how do you decide whether to use energy or start resolving forces.

I was doing June 2010 question 3, and I started resolving forces (because it asked for lambda so I thought I needed tension) but this was wrong. so how do you know which to do?

I don;t understand what you mean by how do you know what to do. I mean it's kind of clear if they give you the CoF and the height in which the particle reached before start.

For me if I see they give you the NEW HEIGHT REACHED, and also if they are on about KE, then I will consider energy.

If energy doesnt work then you shuold use force. That's how I am thinking.

Solution to this question:

GPE = EPE - Workdone against friction

you need to workout EPE by workout the difference between the EPE at the start, becasue the natural length is .9m but it is 1.5m down. i.e lambda .6^2 / 1.8 - lambda .1^2/1.8

then you need to workout workdone against friction which is Reaction force (worked out by resolving force) X CoF X distance moved.

Then sub everything into the energy formula and get the answer.
(edited 10 years ago)
Reply 2
Original post by Jackfinance17
I don;t understand what you mean by how do you know what to do. I mean it's kind of clear if they give you the CoF and the height in which the particle reached before start.

For me if I see they give you the NEW HEIGHT REACHED, and also if they are on about KE, then I will consider energy.

If energy doesnt work then you shuold use force. That's how I am thinking.

Solution to this question:

GPE = EPE - Workdone against friction

you need to workout EPE by workout the difference between the EPE at the start, becasue the natural length is .9m but it is 1.5m down. i.e lambda .6^2 / 1.8 - lambda .1^2/1.8

then you need to workout workdone against friction which is Reaction force (worked out by resolving force) X CoF X distance moved.

Then sub everything into the energy formula and get the answer.


I know how to forces and energy I just have trouble working out which one to do before I've spent a lot of time doing the wrong one. thanks your post helps
Reply 3
The way I look at these is if it is in motion (unless it is SHM) then I'll use energy. If its a statics problem, resolving forces is always the way to go :wink:. As jack said, if you see things like "released from rest" or "projected with speed u" energy considerations should be popping into your head

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