Mechanics Newton’s 3rd law

For the 3 kg particle, T = 3 a (not T = 3g which is what you have effectively used) The 3 g is vertical so not included in a horizontal equation (whoops, sorry if you rerad my first version)
So combine this with 7g - T = 7a

For the diagram:
- For the 3 kg mass, the weight is directed towards the Earth so you need the weight 3g downwards,
- For the 7 kg mass, the acceleration is downwards.

A good strategy here is to resolve each mass separately then use simultaneous equations to remove T (tension of the string) and to find a (acceleration).

how do i remove the tension, the tension has to be equal on both sides so the string doesn't rip but is it 7g or 3?

For the 3Kg mass (I don't see where that 3g force comes from - it doesn't exist):
$T=3a$

Now resolve vertically for the 7Kg mass, to get a second equation.

Solve for a (Hint use the first equation to remove T from the second).

I got it thanks

ive been stuck in this for a while and an't have a clue what i did wrong

For the diagram, the 60 kg mass is accelerating downwards by 2 m/s^2 and the x kg mass is accelerating upwards.

For your first line of calculation, best to resolve downwards (as there are lack of forces acting sideways). Also, the weight is not 60 but 60g.

Your second line of calcuation does not make sense.
- Resolve upwards or downwards for this mass,
- The weight is not x, it is xg.
Resolving upwards you should have: T - xg = -xa, where a is the acceleration.

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Does this help?

what have i done wrong here?

Do these questions tell you what value to use for g e.g. 10 or 9.8 or something else?

Also, you've posted this in two different threads - can you just use one as it's very confusing!!

what have i done wrong here?