Mechanics 1: Tension or Thrust

I know its a vague question but in what circumstances is the force in a rod tension or thrust? And just to confirm the force in a rope is always tension?

Reply 1

steve2005

Original post by thegreenchildren

I know its a vague question but in what circumstances is the force in a rod tension or thrust? And just to confirm the force in a rope is always tension?

Tension is pulling, thrust is pushing.

Reply 2

Noble.

A rod is in tension if it's being "pulled" if you've got two forces on it and both are facing away from the centre of the rod, then the rod will be under tension. When in thrust (or compression) it's essentially being "squashed" and the forces will point towards the centre of the rod. In regards to string/rope, if you try compressing it, there's no force to counteract, the string just becomes slack, so the force will always be tension.

Reply 3

thegreenchildren

Original post by Noble.

So which force would there be in a rod connecting two trucks, moving at a constant speed? or accelerating/decelerating?

Reply 4

Noble.

Original post by thegreenchildren

So which force would there be in a rod connecting two trucks, moving at a constant speed? or accelerating/decelerating?

Imagine the rod was a spring (which could stretch/compress) would the length of the spring be longer or shorter, compared to the spring's natural length, in each of the following cases

a) Moving at constant speed
b) Accelerating
c) Decelerating

If you can answer those, that should tell you whether the rod is in tension or compression/thrust.

Reply 5

thegreenchildren

Original post by Noble.

ok so would it be its normal length when at constant speed, longer when accelerating and shorter when decelerating?

Reply 6

Noble.

Original post by thegreenchildren

ok so would it be its normal length when at constant speed, longer when accelerating and shorter when decelerating?

You're right about accelerating and decelerating. The situation regarding constant speed is dependent on a number of things.

Firstly, if it's a car pulling a trailer/truck and there is no friction on the trailer, then the spring would be normal length. If there is friction, the car is still going to be pulling the trailer to counteract the frictional forces, so the spring would be longer.

So knowing this, what can you say about whether the rod is in tension or compression?

Reply 7

thegreenchildren

Original post by Noble.

So when it is accelerating it is in thrust because the car is pulling away from the trailer. When deccelerating it is in tension because the trailer is pushing towards the car. When its at constant speed with friction it is also tension and when there is no friction its tension (don't know about this one). ?

Reply 8

Noble.

Original post by thegreenchildren

You've mixed them up. When something is in thrust, it's in compression (i.e. it's being squashed/pulled). So when the car is accelerating it's pulling the trailer through the rod - so it's in tension.

When decelerating, the car is braking, but this isn't automatically braking the trailer, the trailer is decelerating because the rod is attached to the car. As you said, if it was a spring this would be compressing the spring, so this is thrust/compression.

When it's at constant speed is friction, you're right it's tension, because the car is having to pull the trailer.

If there is no friction, then at constant speed there is no resultant force - it's neither in tension or thrust.

Reply 9

thegreenchildren

Original post by Noble.

Thanks for your help, I understand much more now and will keep reading this until I get it completely. :smile:

Reply 10

GPODT

Original post by Noble.

Original post by steve2005

Tension is pulling, thrust is pushing.

So the red arrows are thrust, and the yellow arrows are tension? Also, is there any difference in the way one should treat thrust and tension when resolving?

Reply 11

Noble.

Original post by GPODT

So the red arrows are thrust, and the yellow arrows are tension? Also, is there any difference in the way one should treat thrust and tension when resolving?

Yep, your diagram is correct.

When it comes to resolving, as long as you set one direction to be positive (and obviously the other will be negative) it makes no difference.

Reply 12

GPODT

Original post by Noble.

Yep, your diagram is correct.

When it comes to resolving, as long as you set one direction to be positive (and obviously the other will be negative) it makes no difference.

Thanks. When they use the word ''rod'' is there anything we should be considering? For example, when they say inextensible you have to consider the fact that it doesn't stretch. Also can a rod have tension? Or only thrust?

Reply 13

Noble.

Original post by GPODT

Inextensible is put there so you don't consider energy being put into the rod in compressing or stretching it. A rod can have both thrust and tension. As an example, it's under tension when the car is accelerating but under thrust when braking.

Reply 14

steve2005

Original post by thegreenchildren

Thanks for your help, I understand much more now and will keep reading this until I get it completely. :smile:

I don't think you should be concerned about what the forces are called, all you have to do is put the direction of the forces on your diagram and then resolve in the appropriate direction.

A force has magnitude and direction that's it.

Reply 15

Noble.

Original post by steve2005

As much as you are right, it massively helps if you actually understand the physical properties behind forces, and can intuitively determine the situation and generally what's happening.

Reply 16

steve2005

Original post by Noble.

As much as you are right, it massively helps if you actually understand the physical properties behind forces, and can intuitively determine the situation and generally what's happening.

Thanks for the condensing contribution.

Reply 17

GPODT

Original post by Noble.

Thanks a lot! Sorry to bother you but I just have 2 quick questions,

1) I always thought tension acts away from the object? So in the diagram I drew, why aren't the tension arrows pointing away from the objects? IE shouldn't the red arrows be representing tension instead of thrust, since they are pointing away from each object?

2) If the car is moving at a constant speed (so acceleration is = 0), will the rod have thrust or tension?

(edited 11 years ago)

Reply 18

Noble.

Original post by GPODT

1) No. Sometimes it can be difficult to imagine with a rod, since it's inextensible. But imagine that rod was a spring which you could stretch and compress. Then the situation described by the red arrows would compress the spring, it is being pushed from both sides towards the centre of the spring/rod - and compression is thrust. Tension would be a situation where the spring/rod is being stretched and clearly in the situation with the red forces, it isn't going to be stretched.

2) If the car is moving at constant speed the forces on the rod depend on whether friction is involved or not. If there is no friction, the rod will be neither in thrust or tension, because both the car and the trailer will be moving at a constant same speed and there's no forces acting on the two bodies whatsoever. If there is friction, however, then the car is having to pull the trailer along via the rod, if it was a spring this would be stretching the spring - so it's tension.