Acinod's Question Thread

[acinod]

Why is the direction of the frictional force on the driving wheel of a bicycle in the direction the bicycle is traveling?

The weight and the normal force that act on you when you stand are not action/reaction pairs. Explain why.

By using the law of conservation of momentum, explain how you can swim through water.

[xZero]

Imagine the wheel spinning to the right in free air, if i move the wheel so it touches an object, the wheel will push the object to the left, which means that the force of the object on wheel is to the right. If you replace 'object' with road, you'll see that the friction force on wheel is in the direction of the bicycle is traveling at.

Weight force is earth on you, normal force is ground on you. Its on the same system so its not a action/reaction pair.

Lets say that your stationary in the water, the momentum of you and water is 0. If you push the water to the left by... 5ms^-1, you will have to be traveling at 5ms^-1 to the right to counter the change in momentum of the water.

[acinod]

Thanks xZero!

Weight force is earth on you, normal force is ground on you. Its on the same system so its not a action/reaction pair.

If the normal force is ground on me, what would be the reaction force to this? Me on ground? How does this work and where does the force come from?

[schnappy]

You push on the ground, the ground pushes up on you.
-Yes ultimately gravity is what is pulling you down, but it's not the reaction of the gravity pulling you down.
The Earth pulls you down, you pull the Earth up.

There are 2 objects intereacting, saying that grav pulls you down then the chair you're standing on is the reaction of the earth pulling you down is an easy and common mistake to make.