# Concept question on External force

1. Sep 29, 2013

### crysland

Describe a situation in which the net external force is not zero, but its speed remains constant.

I don't know if this situation is possible to explain.

2. Sep 29, 2013

### Staff: Mentor

What does Newton's 1st law say?

3. Sep 29, 2013

### crysland

An object stays at rest, or if in motion, stays in motion at a constant velocity unless acted on by a net external force.

4. Sep 29, 2013

### Staff: Mentor

Good. Note the key word velocity. How does that differ from speed?

5. Sep 29, 2013

### crysland

Velocity has both magnitude and direction but speed doesn't have direction

6. Sep 29, 2013

### Staff: Mentor

Exactly! So see if you can try to answer the question now.

Give an example of something with constant speed but not constant velocity.

Last edited: Sep 29, 2013
7. Sep 29, 2013

### crysland

A car going in a circle.

8. Sep 29, 2013

### rcgldr

How about a car following any curved path while moving at constant speed? A section of road could be similar to a sine wave, ellipse, spiral, parabola, ..., and a car would be able to follow that path while moving at constant speed.

9. Sep 29, 2013

### arildno

But, as the next post shows, other similar situations also exist.

10. Sep 29, 2013

### crysland

Ok so what is the net external force acting on the car?

11. Sep 29, 2013

### arildno

Do you remember the expression for centripetal acceleration?

12. Sep 29, 2013

### crysland

Yes. So centripetal force that pulls it toward the center is external?

13. Sep 29, 2013

### arildno

Certainly.

14. Sep 29, 2013

### rcgldr

That would be speed^2 / (radius of curvature), which gets complicated for paths other than a circle.

The simple answer is that centripetal force = centripetal acceleration x mass of the car.