The Student Room Group

Mechanics 1 question on connected particles

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My question is about part b), why do we have to consider them as separate particles and not consider the strings as a single particle


The requirements to consider something as a single particle was said as follows "If all parts of the system are moving in the same straight line then you can treat the whole system as a particle"

And I mean obviously the two masses are going in a straight line so why not?

The example next to this one(not shown in the picture) had the tension considered as a single particle because its in a "straight line" so why are they not considering this one as a single particle?
(edited 1 year ago)
The string is light (zero mass) and inextensible (not elastic). So as the problem solving box says, you can consider the complete system (two masses) without representing the string/tension or consider the forces applied to each mass individually (modelling the tension each time as a force).

If you treat it as a single system, the (internal) tension applied to each mass is equal and opposite by Newton 3, so there is zero resultant force due to tension on the system. The string represents an internal force pair (equal and opposite) and does not affect the motion of the overall system.

If you model the 5kg mass say, then there is a 40N force to the right, a frictional force (10N) opposing motion to the left and the string applies a (tension) force to the left. Youre not concerned with the way the 40N force is applied (or how friction works), so why be concerned about the string (it has zero mass) if it simply represents a force?
(edited 1 year ago)
Original post by mqb2766
The string is light (zero mass) and inextensible (not elastic). So as the problem solving box says, you can consider the complete system (two masses) without representing the string/tension or consider the forces applied to each mass individually (modelling the tension each time as a force).

If you treat it as a single system, the (internal) tension applied to each mass is equal and opposite by Newton 3, so there is zero resultant force due to tension on the system. The string represents an internal force pair (equal and opposite) and does not affect the motion of the overall system.

If you model the 5kg mass say, then there is a 40N force to the right, a frictional force (10N) opposing motion to the left and the string applies a (tension) force to the left. Youre not concerned with the way the 40N force is applied (or how friction works), so why be concerned about the string (it has zero mass) if it simply represents a force?


Thanks I got it now!
I did not consider Newtons 3rd law so got confused.
Original post by Goldenknight
Thanks I got it now!
I did not consider Newtons 3rd law so got confused.


When you consider the overall system, the tension force is equal and opposite at each end (pulls each end towards the center) so you can consider it as effectively the two masses glued together and newton 3 is the tension/glue/... Its why you generally draw two arrow heads (in opposite direction) when you draw the string.
Original post by mqb2766
When you consider the overall system, the tension force is equal and opposite at each end (pulls each end towards the center) so you can consider it as effectively the two masses glued together and newton 3 is the tension/glue/... Its why you generally draw two arrow heads (in opposite direction) when you draw the string.


Oh yeah that is actually a great way to think about it! Being glued together.

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