# Bike wheel and a step

1. Nov 7, 2006

### plutonium

The drawing shows a bike wheel resting against a small step whose height is h=0.120m. The weight and radius of the wheel are W = 25.0N and r = 0.340m. A horizontal force F is applied ot eh axle of the weehl. As the magnitude of F increases, there comes a time when the wheel just begins to rise up and loses contact with the ground. What is the magnitude of the force when this happens?

I do not even know where to start. I tried to think of torques, but Fg acts on the center of the wheel, whose distance is 0 from the radius. I also tried to think of a normal force exerted by the step, but that just got really weird. Finally, I thought about friction from the floor below the step, but it didin't work out as well.

And the other problem was the force exerted by the horizontal force, which is also 0 from the radius since it acts directly on the axle.

And as an extention, can I prove that the height of the step must be between 0 and the radius of the wheel?

2. Nov 7, 2006

### Staff: Mentor

Here's a hint: When the wheel begins to rise up, about what point is it pivoting? (It's not the center of the wheel.)

3. Nov 7, 2008

### rcrx

Glad I found this. I'm doing the exact same problem out of the Cutnell Physics 7th Ed text. I get that there is a pivot point, but what the heck do I do? This problem is wracking my brains. Can anyone suggest what to start with? Once I get a hint I can probably piece it together. Much appreciated, thanks!

4. Nov 9, 2008

### Staff: Mentor

Hint: What torque must the horizontal force overcome?