vectors application (forces?) qn (1 Viewer)

[Masaken]

how would you interpret the diagram and relate it to finding 17a (and b)? i'm so confused

the worked solutions for cambridge say you have to do T - 3g = 3g/2 but i don't even know how they got that equation in the first place

 

[011235]

This is really a physics question tbh (Cambridge moment)

Think about the net force on the green mass.



(Hint: , )

(nvm my sol was wong)

 

[Masaken]

This is really a physics question tbh (Cambridge moment)

Think about the net force on the green mass.



(Hint: $F_{net} = ma$, $F_{net} = F_g - T$)

Spoiler: sol

Assuming g=9.8 bc of 4.9 in Q.

yea i don't do physics (i'm literally about to beat up my year 10 self for not picking it just cos she was depressed about not being able to understand velocity-time graphs last year omg).

sorry but the answers says the answer is T = 44.1? not sure how they got that even with the worked solutions

 

[Vall]

I’ve tried to show my whole thought process here. Lmk if you want some more clarification/explanation.

from Newton’s third law (which sorry you’ll just need to memorise for 4u) F = ma where F is the total force (or net force) on an object.
if we consider the red block and apply f = ma:
F = m x 4.9 (we know acceleration of the red is 4.9 - the same as the green’s - because of how pulley systems work, the blocks are on the same piece of rope so travel at the same speed / experience same acceleration - coming from the rope’s tension force which acts upwards on each block).

Now we apply f = ma on the green block:
F’ = 3 x 4.9
= 14.7
[F’ isn’t the derivative just using to make clear the net force for each of the two blocks]

Now the physics bit. We can create another equation for the net force on each block by considering the actual forces acting on each block (in this case tension and gravity/weight).

For the red defining down as positive (I always define the direction it moves as positive but it doesn’t matter):
F = W - T
= mg - T
= 9.8m - T
= 4.9m ( from our first net force equation for red)
4.9m = T

For the green defining up as positive:
F’ = T - W
= T - 3 x 9.8
= T - 29.4
= 14.7 (from first green equation)
T = 44.1

oh lol I didn’t even need to consider the red block? <- need it for part b
P.s don’t label you’re two net forces F and F’ use subscripts (like one and two) and you can put a capital sigma in front of the f to denote sum or total

 

[Vall]

This is really a physics question tbh (Cambridge moment)

I mean look at 2020 q16 after this question our maths department added a bunch of pulley things to their teaching.

 

[Masaken]

I’ve tried to show my whole thought process here. Lmk if you want some more clarification/explanation.

from Newton’s third law (which sorry you’ll just need to memorise for 4u) F = ma where F is the total force (or net force) on an object.
if we consider the red block and apply f = ma:
F = m x 4.9 (we know acceleration of the red is 4.9 - the same as the green’s - because of how pulley systems work, the blocks are on the same piece of rope so travel at the same speed / experience same acceleration - coming from the rope’s tension force which acts upwards on each block).

Now we apply f = ma on the green block:
F’ = 3 x 4.9
= 14.7
[F’ isn’t the derivative just using to make clear the net force for each of the two blocks]

Now the physics bit. We can create another equation for the net force on each block by considering the actual forces acting on each block (in this case tension and gravity/weight).

For the red defining down as positive (I always define the direction it moves as positive but it doesn’t matter):
F = W - T
= mg - T
= 9.8m - T
= 4.9m ( from our first net force equation for red)
4.9m = T

For the green defining up as positive:
F’ = T - W
= T - 3 x 9.8
= T - 29.4
= 14.7 (from first green equation)
T = 44.1

oh lol I didn’t even need to consider the red block? <- need it for part b
P.s don’t label you’re two net forces F and F’ use subscripts (like one and two) and you can put a capital sigma in front of the f to denote sum or total

ohhhhh i see, i see (this was a much better method than whatever cambridge's worked solutions was trying to do smh). i get the process of thinking (tysm) but this might be a stupid question but i've never understood why to get the force you would need to do T - W? (like i understand that i *have* to but it became more of a thing i just memorised and never really understood why??? kind of???) is it alright if you could explain why?

 

[Masaken]

I mean look at 2020 q16 after this question our maths department added a bunch of pulley things to their teaching.
View attachment 36963

heartbreaking

 

[Vall]

heartbreaking

Cause the net force is like the sum of all the forces (that seems like a bad explanation).
like of some object had 2 newtons to the left and 5 newtons to the right, the net force would 3 newtons, right?
We get that from 5-2 (because force is a vector quantity we must take direction into account)
So for the pulley we do T - W because tension pulls upwards and gravity pulls downwards on the block.

 

[Masaken]

Cause the net force is like the sum of all the forces (that seems like a bad explanation).
like of some object had 2 newtons to the left and 5 newtons to the right, the net force would 3 newtons, right?
We get that from 5-2 (because force is a vector quantity we must take direction into account)
So for the pulley we do T - W because tension pulls upwards and gravity pulls downwards on the block.

OH that makes a lot of sense. thanks so much, i've been stuck on this question for an hour

 

[Vall]

OH that makes a lot of sense. thanks so much, i've been stuck on this question for an hour

Here’s some more pulley questions btw just be warned that some of the later ones get pretty hard (plus need some calculus that you mightn’t have learnt this early in yr12?)

https://drive.google.com/file/d/1hJDBmqK3NZ8eiRvvJQvUiDDzkSRJ1_xr/view?usp=drivesdk

 

[Masaken]

Here’s some more pulley questions btw just be warned that some of the later ones get pretty hard (plus need some calculus that you mightn’t have learnt this early in yr12?)

https://drive.google.com/file/d/1hJDBmqK3NZ8eiRvvJQvUiDDzkSRJ1_xr/view?usp=drivesdk

tysm, might not be able to do some of them now but definitely going to be helpful later

 

[Siwel]

This might be out of 4U syllabus but you can do sigma F_y = ma where up is positive then whenever there is a string that is always in tension and when something is in tension it is being pulled away (ie the vector arrow is away from the block) so that means the tension is positive and there is a weight force of the block downward (arrow downwards) so you get ma_block1=T-W