Forces in a cirle
when you resolve a force for a body in equilibrium, you just do f-mg=0 and work out the force from there.
However, when an object is in motion, how do you resolve the forces?
I'm doing a question now that says a bob is at the bottom of the pendulum (vmax), how would you calculate the tension in the string?
i got t - mg = ???
However, when an object is in motion, how do you resolve the forces?
I'm doing a question now that says a bob is at the bottom of the pendulum (vmax), how would you calculate the tension in the string?
i got t - mg = ???
The force up minus the force down is indeed T - mg
This must provide the resultant force upwards towards the centre (centripetal force) for the bob.
You need to know the speed of the bob and the length of the pendulum.
This must provide the resultant force upwards towards the centre (centripetal force) for the bob.
You need to know the speed of the bob and the length of the pendulum.
(edited 10 years ago)
Original post by physicso
when you resolve a force for a body in equilibrium, you just do f-mg=0 and work out the force from there.
However, when an object is in motion, how do you resolve the forces?
I'm doing a question now that says a bob is at the bottom of the pendulum (vmax), how would you calculate the tension in the string?
i got t - mg = ???
However, when an object is in motion, how do you resolve the forces?
I'm doing a question now that says a bob is at the bottom of the pendulum (vmax), how would you calculate the tension in the string?
i got t - mg = ???
It depends on the type of motion. When resolving forces you are implicitly using newtons 2nd law F=ma. If a is zero then there is no net force on the object so they all must add to zero, which implies that the object is moving at a constant velocity, i.e moving at a constant speed and not changing direction.
If the object is accelerating then a is non zero, for circular motion the net force is given by mv^2/r which is a centripetal force that accelerates a body towards the centre of the circle.
With a pendulum think about the forces acting on the bob, you have the restoring force which acts towards the equilibrium position of the pendulum, causing it to accelerate towards that point. However we are assuming that the length of the pendulum doesn't change, so the force acting along the axis of the pendulum must be zero, i.e the tension must be the same as the component of gravity acting along the axis of the pendulum. You need to thus work out the angle that the pendulum is at the vertical at vmax, you should be able to do this using knowledge of SHM and pendulums and from this you can work out the component of gravity as required which will be equal to the tension.
All of this is using the small angle approximation, none of this is valid if the angle is large.
(edited 10 years ago)
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