# Motion of objects

1. Jan 26, 2014

### chemistry1

Hi, I have begun physics and we are seeing systems of reference as an introduction.Now I have a small exercise which I'm not sure of, I would need you to check if it's correct or not.

There are two cyclists A and B. Suppose that you take place on a sidewalk and look at them pass. They move to the right and to a constant speed.

What trajectory do you associate to the cyclists ? Rectilinear (so, one "x" axis.)

What answer would the cyclist A give if we asked her to determine the trajectory of cyclist B ? Rectilinear also.

Represent the trajectory of the reflector placed in the front wheel of cyclist B, such that you would see her from the sidewalk.

The curves are halfs of a circle and I considered that the reflector began at half the height of the wheel.( so there's the highest point, middle point and the case where the reflector "touches" the ground.)

THank you !

2. Jan 26, 2014

### Staff: Mentor

The basic idea is right, but the curves are not half-circles. Think of the moment where the reflector touches the ground. What is its speed forwards at that point? Compare this to the speed the reflector has at the top, and at the front and back of the wheel.

3. Jan 26, 2014

### chemistry1

I've been suggested that it's a cycloid. Btw are the other two correct ? thank you

4. Jan 26, 2014

### Staff: Mentor

The linear things? Yes.

It is a cycloid, right.

5. Jan 26, 2014

### chemistry1

To be frank, I did not expect it at all.... it makes me quite ashamed ^^

6. Jan 26, 2014

### haruspex

The in-wheel reflectors I've seen are not that close to the periphery of the wheel. The curve would be a curtate cycloid. http://en.wikipedia.org/wiki/Cycloid#Related_curves

7. Jan 26, 2014

### chemistry1

Yeah, it makes more sense. Thanks for the correction!