GCSE Physics

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Equilibrium has nothing to do with it.

Important thing to remember is that 3rd law forces act on 2 different bodies.

If A exerts a force on B - where A and B can be anything and any kind of force - then B exerts an equal and opposite force on A.

Reply 2

generalebriety

hoiyiu2512

I am not familiar with problems related to mechanics :frown:

also, i don't really understand Newton's Third Law
I always fail to identify the action and reaction pairs

That's because they don't always come in pairs. For example, if a book is resting on a desk that's tilted slightly, there are three forces acting - a resistance (friction), its weight, and a normal reaction. The friction and the normal reaction both act against the weight to stop it moving - if there was only a normal reaction it'd slide down the table.

Reply 3

teachercol

3rd law forces ALWAYS come in pairs.

You are getting this wrong - 3rd law forces NEVER act on the same body ...

Reply 4

generalebriety

teachercol

3rd law forces ALWAYS come in pairs.

You are getting this wrong - 3rd law forces NEVER act on the same body ...

Oh, I see, I misunderstood. I think I was thinking of the 1st law. Fair enough. Meh, I'm not a physicist. :redface:

Reply 5

Theorist

teachercol

3rd law forces ALWAYS come in pairs.

You are getting this wrong - 3rd law forces NEVER act on the same body ...

In this case, what would be the Third Law pair for the reaction force and friction force on the ball? I think it's got something to do with e.m. repulsion, but I don't know specifically.

I know this sounds silly but: isn't the reaction force acting on the ball almost induced by virtue of the ball's weight? I mean, yes, it's not an N3 pair with the weight, but surely if the Earth was not pulling the ball down, there'd be no reaction force?

Reply 6

teachercol

The 3rd law pair for the gravitational force of the earth pulling the ball down would be the graviational force of the ball pulling the earth up.

The 3rd law pair for the friction force of the slope on the ball is the friction force of the ball on the slope

If the ball isnt in equilibrium - it is just bouncing say then the 'reaction' forceupwards would not be equal to the weight it would be bigger!

Thats why use of the terms 'action' and 'reaction' is sooooo misleading.

Reply 7

Theorist

teachercol

The 3rd law pair for the friction force of the slope on the ball is the friction force of the ball on the slopeLike when you ride your bike. I see.

But what you haven't answered is what the N3 pair is for the reaction force, which I'm assuming as being separate to the friction force. Is that even right to assume this?

Reply 8

**mentalheroin**

the weight on the slope is the opposite force to the reaction force. wasn;t it a book?

Reply 9

Theorist

Oh I see.

So you're saying the vertical component of the weight induces the reaction force, and the horizontal component induces the friction force?

Thanks for that. :smile:

Reply 10

**mentalheroin**

kind of. the component of the weight at 90 degrees to the slope is equal to the reaction force, and the component of the weight parallel to the slope is equal to the friction but only if it is limiting(if book cant move)

Reply 11

Theorist

Yes, provided they're in limiting equilibrium.

Reply 12

teachercol

Dharma

Like when you ride your bike. I see.

But what you haven't answered is what the N3 pair is for the reaction force, which I'm assuming as being separate to the friction force. Is that even right to assume this?

The 3rd law pair for the upward reaction force of the slope on the object is a downward reactionforce of the object on the slope.

It is not any kind of weight! The reaction force and the weight MAY be equal if the object is in equilibrium.

3rdlaw pairs are ALWAYS equal whether we are in an equilibriul situation or not.