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# Shm help watch

1. Can someone explain why it is t- mg
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2. (Original post by Super199)
Can someone explain why it is t- mg
Centripetal force needs to act inwards (Towards the pivot point of the rope) for the system to carry on moving in the direction it's going to go. So the tension in the string needs to be bigger than the weight force so that the resultant force, which is the centripetal force, is acting inwards. Thus

T - mg = mv2 / x
3. (Original post by Super199)
Can someone explain why it is t- mg
Say T is the tension of the string, the tension has to be strong enough to withstand the force of the rope which depends on the mass m and the gravity acceleration g. Otherwise the rope would break during swinging.

Another point for the tension T which has to be considered is the movement of the pendulum (or the bob) which depends on the velocity v, the mass m of it plus the length x of the string. After this is considered, you got the formular for T in total:

T = mv²/x + mg

For the bottom of the swing, an equilibrium exists. That is why you convert the formula above:

T - mg = mv²/x

And now you got your formula for the bottom of the swing.

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