M1 Past Paper Help Diagrams

I can't understand how R acts upwards and not downwards. I imagined it in real-life to get my viewpoint of R acting downwards. Imagine a pen held by one hand and imagine a hole of a scissor put in the pen. If you exert a force with a vertical and horizontal component on the scissor, you notice that there's friction making it more difficult for the scissor hole to move horizontally across the pen, so I understand the direction of friction. But there's some "resistance" to scissor going vertically up due to R acting down on it, right?

THANKS

You have to consider the weight of the ring.

Oh I kinda get it now.

If the W is high, then I can imagine R being upwards.

If W is low, then I can imagine R being downwards (my pen and scissor example in the OP).

How do I know which one is happening?

Thanks

R is the reaction force of the ring pushing upwards on the rod. If there were no upwards reaction, wouldn't the rod simply fall?

The rod is fixed, so no.

For this example, resolve vertically, the weight is 0.25g and the upward component of the tension is 1.2sin40.

0.25g>1.2sin40 means there must be another force acting upwards, which is the reaction. If the weight were less than the upward component of the tension then R would be down.

Ah okay. Makes sense. What if the tension force of 1.2N was unknown?

Thanks

You wouldn't be able to tell. The question would probably give you some other kind of information so you could figure it out.

Ok. I put R down in my working out and then got a -ve value for R. Then I thought what I did was nonsense, so I crossed it out. Now I realised that actually I should've just said R was the modulus of the -ve value I got

If you get a negative value for a force, it means you've drawn it in the opposite direction than what it acts in.

If you get a negative value for a force, it means you've drawn it in the opposite direction than what it acts in.

In that case I'm guessing you have something like this in your working

R=-... [R down in diagram]

So, R=+ve... Upwards

Thanks

The Examsolutions guy somehow put R downwards straight away without doing that calculation above you posted. How did he know straight away? He didn't even mention

Yeah you'd just make a comment like that to show you know what it means.


Just a quick look tells you that it should be up, as 0.25g is over twice as big as 1.2cos40. It's just through experience really.