Racing to the Finish: A Boy Paddles from Point 1 to Point 7

[physicdummy]

Homework Statement

A boy paddles his bike as fast as he can from point 1 to point 2. At point 2, he releases his feet from the pedal and coasts to point 7.

http://i649.photobucket.com/albums/uu220/mapo_tofu/picture-3.png


a)At which point did he travel fastest?
b)State the energy conversion from point 1 to 2.


________ energy of the boy pedaling --> ________ energy of the bike at point2

Homework Equations

Law of conservation of energy

The Attempt at a Solution

a ) If neglecting friction, then the total energy possessed by the bike should be the same throughout. Hence, the ke at 2-3 and 5-7 are the same and so the speed is the same throughout.

If considering friction, then ke is greatest at 2 so the speed is fastest at 2.


b) kinetic energy of the boy pedalling --> potential energy of the bike

Not sure of the potential energy part... is there a specific name for this
form of potential energy?

 

[cepheid]

a ) If neglecting friction, then the total energy possessed by the bike should be the same throughout. Hence, the ke at 2-3 and 5-7 are the same and so the speed is the same throughout.

This is simply not true. There is a net input of energy to the bike from the rider. Have you ridden a bike before? If you pedal hard enough, you accelerate.

 

[physicdummy]

This is simply not true. There is a net input of energy to the bike from the rider. Have you ridden a bike before? If you pedal hard enough, you accelerate.

So is the bike fastest at point 2 then? Is potential energy present? As you can tell from my name, I'm a dummy when it comes to physics, so I'd appreciate it if you can shed some light on this!

 

[HallsofIvy]

The potential energy of the bike is a constant. Assuming no friction, pedelling will increase velocity and so that energy will go into increasing the kinetic energy. Including friction, some of the energy of pedelling will go into overcoming friction. Yes, without friction, the bike will be fastest at point 2, where the boy stops pedelling and, neglecting friction, will keep that speed. With friction, the bike will still be fastest at point 2 and then will slow down.