# Concept question on External force

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.

Doc Al
Mentor
What does Newton's 1st law say?

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

Doc Al
Mentor
An object stays at rest, or if in motion, stays in motion at a constant velocity unless acted on by a net external force.
Good. Note the key word velocity. How does that differ from speed?

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

Doc Al
Mentor
Velocity has both magnitude and direction but speed doesn't have direction
Exactly! So see if you can try to answer the question now.

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

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A car going in a circle.

rcgldr
Homework Helper
A car going in a circle.
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.

arildno
Homework Helper
Gold Member
Dearly Missed
A car going in a circle.
Your answer is definitely correct! But, as the next post shows, other similar situations also exist.

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

arildno
Homework Helper
Gold Member
Dearly Missed
Ok so what is the net external force acting on the car?
Do you remember the expression for centripetal acceleration?

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

arildno
Homework Helper
Gold Member
Dearly Missed
Certainly.

rcgldr
Homework Helper
Do you remember the expression for centripetal acceleration?
That would be speed^2 / (radius of curvature), which gets complicated for paths other than a circle.

Yes. So centripetal force that pulls it toward the center is external?
The simple answer is that centripetal force = centripetal acceleration x mass of the car.