# Conservation of energy: Why is the work of the normal forces equal to 0?

Hello everyone,
someone could explain me please, why the work of the normals forces are 0 ?

He used with conservation energy equations.
How should I refer to the displacement point ?

Thx everyone !

kuruman
Homework Helper
Gold Member
Because the work done by a force is ##W= Fd\cos\theta##. The normal force is perpendicular to the displacement by definition in which case ##\theta =90^o## and ##\cos(90^o)=0##.

Gaussian97
Homework Helper
Well, the work is, by definition ##dW=\vec{F}\cdot d\vec{r}##, and a normal force is, again by definition a force which fulfils ##\vec{F}\cdot d\vec{r}=0##. So the work is zero almost by definition again.

The direction of the Normal forces is changed, and also the Δx is not downwards because the angular velocity.

Am I right ?

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Gaussian97
Homework Helper
The direction of the Normal forces is changed, and also the Δx is not downwards because the angular velocity.

Am I right ?
Nop, the direction of the Normal forces is the same before and after, and also the ##\Delta x## is downwards in one case and horizontal in the other. From where you can see that ##\vec{F}\cdot d\vec{r}=0## as we said before.

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