# compression vs tension problem

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#1
If you see the attached images below:

My teacher tells me that C is positive because it is in the direction of motion. However, I don't see that from the diagram. Compression is force acting inwards on the shunt, which makes sense of course, but the arrows in the diagram (at least when resolving around the carriage)

is in the opposite direction to the acceleration (hence the motion of the system). So... is the diagram wrong? Why is C taken to be positive when resolving around the carriage part?

The only way the mathematics of this question would make sense to me (C is positive, in the direction of motion aswell) is if I reversed the direction of the arrowheads

The direction of compression (c) is now in same direction of motion and hence it makes sense now... but this doesn't show compression.

Any explanations?

thanks for any help I appreciate it greatly
Last edited by pondering-soul; 2 months ago
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2 months ago
#2
Looks like C is marked "incorrectly" in the original diagram as for the carriage, the equation of motion has the opposite sign (direction) for the compression and resistance. Tension pulls objects in, compression pushes objects away.

However, you could argue that the question asks for the (magnitude) compression in the shunt, rather than the force exerted on the train/carriage by the shunt. As these are equal and opposite, the original diagram could refer to the force(s) exerted on the shunt by the train and carriage, and so would be correct.

When you do a force diagram, you usually have a body of interest and you mark the forces exerted on that body. Usually, the forces exerted by that body on other things are not represented. Here, the diagram could be wrong or the direction(s) of C could refer to forces on the shunt. Its obviously confusing as b) takes the body to be the carriage, hence the direction of C is flipped. It's easy for mistakes to occur.
Last edited by mqb2766; 2 months ago
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