Conservation of Energy: Why are Normal Forces Zero?

[t2r]

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]

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]

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.

 

[t2r]

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

Am I right ?

 

[Gaussian97]

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.