Even when it is said the system is in equilibrium and the object is stationary, in so many a level physics questions, the tension is not equal to weight. we know the system has to be in equilibrium becuase we are applying the Law of moments. Or, is the tension equal to the sum of the weight of different components in the system, that we are not given or able to calculate in the questions?

Original post by Nat4695

Even when it is said the system is in equilibrium and the object is stationary, in so many a level physics questions, the tension is not equal to weight. we know the system has to be in equilibrium becuase we are applying the Law of moments. Or, is the tension equal to the sum of the weight of different components in the system, that we are not given or able to calculate in the questions?

It depends on the question, but if the string was attached to the COM of the rod, then the tension would be equal to the weight. If you had a hinge at one end of the rod and a string at the other, then taking moments about the rod, the tension must be equal to half the weight to remain in equilibrium. If the string was close to the hinge, then the tension would be greater than the weight.

(edited 4 months ago)

Do you mean the same as pivot when mentioning a hinge? Why is tension half the weight in the first scenario? Is it just to do with the moment created by the tension? as in the tension would be greater than the weight when it is closer to the hinge since perpendicular distance is smaller?

Original post by Nat4695

Do you mean the same as pivot when mentioning a hinge? Why is tension half the weight in the first scenario? Is it just to do with the moment created by the tension? as in the tension would be greater than the weight when it is closer to the hinge since perpendicular distance is smaller?

In a sense, a pivot or hinge is doing a similar thing and the tension not being equal to the weight (in general) is the mechanical advantage (usually) which is the Archimedes quote "give me a lever long enough and Ill move the world"

https://en.wikiquote.org/wiki/Archimedes

Tension is just a force so for the first scenario with a hinge at one end and a force at the other (tension) with the COM in the centre, then taking moments

2*tension = weight

due to the distances. If you put a long, light "rod" extension on the end so the COM remained where it was but the force was applied a long distance away, then the tension would be small (Archimedes). Obviously the hinge would have an upwards vertical component equal to the remainder of the weight (resolving vertically).

If the applied force was close to the hinge, then (taking moments) it must be >> weight so the hinge would have a downwards vertical component (resolving vertically).

If you think about the energy or work done (force*distance moved), then in the first case the force or tension moves a long distance (arc length) compared to the COM so the appllied force must be less than the weight for energy to be conserved. In the second scenario, the force must be > weight as it moves a smaller distance or arc length (smaller radius) compared to the COM. In both cases the COM and force/tension move through the same angular displacement, so the arc length (work done / conservation) is proportional to the radius or perpendicular distance (moments), so its effectively the same thing.

(edited 4 months ago)

- A level physics mechanics question
- CYLINDER SLIDING DOWN A SLOPE physics question
- Further Mechanics 1 question (Elastic String and Springs)
- A level maths mechanic question
- Further Maths Mechanics Forces Question HELP
- Tension in a Simple Pendulum
- AQA A Level Physics Paper 1 7408/1 - 26 May 2022 [Exam Chat]
- RODS, RINGS AND STRINGS isaac physics
- Physics AS LEVEL help!!!!
- A1/A2 Mechanics - Pulleys MadasMaths Question
- don't understand force exerted by A on B
- A level physics waves question
- Tension question
- Normal Reaction Forces and Weight
- Circular motion
- Careless Astronauts
- Maths Further ( Mechanics)
- Physics question confusion
- Projectile on a string
- AQA A-level Further Mathematics Paper 3 (7367/3) - 7th June 2024 [Exam Chat]