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# M2 Mechanics - resolving forces Watch

1. When do you consider the resolved forces and when you don't?
Like whenever I need to do a question involving a particle moving up/down an inclined plane, I tend to resolve all the forces before hand.
For example, in June 2008, to do part b) you shouldn't consider 3.5gsin20 (horiz). If I was to do the question I would've done:
R(<--); F=3.5gsin20-(miu)R=0

But the mark scheme says to simply do Fmax=(miu)R.

I might be doing a really stupid mistake but I can't think. Thanks.
2. (Original post by RK1998)
R(<--); F=3.5gsin20-(miu)R=0
The problem here is that you've set F=0 effectively, but F does not equal zero because of the the acceleration.
3. (Original post by ghostwalker)
The problem here is that you've set F=0 effectively, but F does not equal zero because of the the acceleration.
Oh you're right. I completely ignored the fact that velocity isn't constant :/

But anw, would the same apply for other questions?
Like you should not consider the resolved forces if acceleration doesn't equal to zero, right?

Thank you
4. (Original post by RK1998)
Oh you're right. I completely ignored the fact that velocity isn't constant :/

But anw, would the same apply for other questions?
Like you should not consider the resolved forces if acceleration doesn't equal to zero, right?

Thank you
You can consider resolved forces, as long as you use F=ma, rather than F=0.

F=0 is in fact a special case of F=ma, where there is no acceleration.

Acceleration here is the component of the acceleration in the direction that you're resolving forces.
5. (Original post by ghostwalker)
You can consider resolved forces, as long as you use F=ma, rather than F=0.

F=0 is in fact a special case of F=ma, where there is no acceleration.

Acceleration here is the component of the acceleration in the direction that you're resolving forces.
Thank you That makes so much more sense now!
Well, in this case, acceleration was unknown so not considering the components was more realistic, or that's what I understood :3
6. (Original post by RK1998)
Thank you That makes so much more sense now!
Well, in this case, acceleration was unknown so not considering the components was more realistic, or that's what I understood :3
Since the package is moving, friction will be maximum, so as the markscheme says.

On reflection you're not told whether there is air resistance or not, so you don't know what forces are acting. So trying to work in terms of forces and acceleration is not the way to do it. Edit: See below.
7. (Original post by ghostwalker)
Since the package is moving, friction will be maximum, so as the markscheme says.

On reflection you're not told whether there is air resistance or not, so you don't know what forces are acting. So trying to work in terms of forces and acceleration is not the way to do it.
Yeah..
Thank you so much
8. (Original post by RK1998)
Yeah..
Thank you so much
Correcting myself.

You're going to have to assume no air resistance etc. otherwise there's no way to work out the frictional force, which you need to find the Coeff of friction.

I don't know how the markscheme has done it, but there are serveral methods. Given the first part of the question, then considering work done against friction is the easiest way to go.

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