# Moments/torque in relation to laws of motion

1. Jun 4, 2013

### Kinhew93

I've just started learning about torque, and understand that tau=Fd, but wondered how this relates to F=ma.

For example if there is a rod with a pivot in the middle (say a nail), when one end is pushed down, why does the other end move up? Where is the force that causes the end to move up coming from?

thanks :)

2. Jun 4, 2013

### Staff: Mentor

The force that moves the other end is the attractive forces holding the atoms/molecules of the rod together. The force applied on one end is transferred to the other via these bonds. Hence, if you strike the end of the rod hard enough you will apply too much acceleration for the bonds to hold and it will break, while the other end barely moves at all.

3. Jun 5, 2013

### CWatters

The "equivalent" expressions would be...

Force = mass * acceleration

Torque = moment of inertia * angular acceleration

Take a look at the forces in a beam. The top of the beam is in tension and the bottom of the beam is in compression. If these forces are unbalanced by the application of a new force the beam will accelerate (eg it will start to rotate or if already rotating it will rotate faster).

and as Drakkith said..if the forces are too large the beam will fail.

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