urgent mechanics question!

how do you apply F=Ma on the rough plane which B rests on, the mark scheme shows it involving two tensions minus friction, but wouldn't the tension be constant throughout the string? thank you!

(edited 10 months ago)

Screen Shot 2023-06-19 at 18.58.00.png127.8KB

Screen Shot 2023-06-19 at 19.04.06.png38.1KB

Reply 1

davros

Original post by lavely

how do you apply F=Ma on the rough plane which B rests on, the mark scheme shows it involving two tensions minus friction, but wouldn't the tension be constant throughout the string? thank you!

Edit: didnt see 2nd attachment!

(edited 10 months ago)

Reply 2

mqb2766

Original post by lavely

how do you apply F=Ma on the rough plane which B rests on, the mark scheme shows it involving two tensions minus friction, but wouldn't the tension be constant throughout the string? thank you!

THere are two strings, so two different tensions.

Reply 3

lavely

thank you! But if they are all connected to each other, would it not be like they are acting together as one string? I think I'm still a bit confused in imagining the situation..

Original post by mqb2766

THere are two strings, so two different tensions.

Reply 4

mqb2766

Original post by lavely

thank you! But if they are all connected to each other, would it not be like they are acting together as one string? I think I'm still a bit confused in imagining the situation..

Imagine if B & C are cannonballs and A is a marble. They must have the same acceleration so
tension in BC >> tension in AB
So no, they dont act as a single string.

(edited 10 months ago)

Reply 5

old_engineer

Original post by lavely

how do you apply F=Ma on the rough plane which B rests on, the mark scheme shows it involving two tensions minus friction, but wouldn't the tension be constant throughout the string? thank you!

Just as an aside, you can dramatically simplify part (a) of this problem by first reasoning that the maximum possible acceleration will correspond to mu = 0, and then treating the system of masses as one combined mass of 9kg being pulled to the right by one external force and to the left by a second eternal force. That way you don't need to deal with the individual tensions at all.