Need help with connected particles M1

Hi, could someone please explain to me why these two questions are worked out so differently although they are both about connected particles? In part d of question 7, thrust is taken into account. However in the other question (final part of the question) the tension is not only considered. Both question have the force removed but why does one have a thrust but the other doesn't . I heard that's because the string is slack now but I am really confused as to why it is like that. Thanks.

The question involving friction does not involve friction however F is removed in both questions. Please help !

There is a difference between a light horizontal string and a light horizontal rod. A string can only 'pull' but a rod can 'push' and 'pull'.

i.e: if I put a rod between two balls and press the balls towards each other, the rod inbetween them wouldn't let me do that, it'd keep them away, it is thrusting them away. But if I put a string between the two balls and pressed them towards each other, the string would go limp immediately and wouldn't be able to resist my force, there is no thrust. This is why the rod has thrust as well as tension whereas a string has only tension.

1) So there is a thrust (instead of tension )in the rod during deceleration right?

2) The question involving the string ( the second picture), why is there no tension after F is removed? It doesn't mention that the strings are are now slack so how do you know you can ignore tension ?

3) What does it mean when string is taut and when string is slack in connected particles questions?

I can't rep you anymore, so please bear with me. Thanks.

Yes, for all of these, just try and use common sense and intuition. If it's deceleration, that means the particles are being 'pressed together' so the rod thrusts them away.



Again, more common sense. You have two marbles (perhaps you should sit down with some marbles at home and do some experiments for yourself?) and you tie a string between them. Now put the marbles on the ground and don't touch them. The string just lies there limply. When you pull the one of the marble, then the string becomes taught and tries pulling the marble backwards. This pulling of the marble is equivalent to providing the force $\mathbf{F}$ to the particle, so once that force is removed, i.e: you stop 'pulling' the marble, the string goes back to being limp and useless. i.e: No tension.



Again, more common sense and physics intuition. What do you think a string being taut looks like? It means that the string has some tension in it whereas a slack string is just 'limp' and 'useless', i.e: it doesn't have any tension.



No worries.

Okay, this makes much more sense now. I don't do physics and my common sense when it comes to this is quite appalling really. And I also don't have any marbles (too bad) so I don't think I can do an experiment to test this. Anyway, thanks!

You don't need marbles, really; literally just take two pieces of paper and scrunch them up into a ball. Now take a string and glue it to each of the paper balls. Pull them apart or leave them be and see how the string behaves.

Doing Physics's wouldn't have helped, FWIW, the mechanics there is appalling.

In either case, it's good that you're coming on here to ask questions and clarify stuff. Well done on that. Good luck for your exam!