# A Physics Question. No calculations Involved here. General Theory Type

Just two general theory types of questions:

1. Is an object that undergoes circular motion accelerating? Is a force needed for this? Where does the force come from when something is moving in a circular motion? What is the name of this force?

2. For a car going around a flat curve, what provides the force that allows it to turn?

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enigma
Staff Emeritus
Gold Member

Velocity is speed and direction.

Is an object undergoing circular motion changing its speed or direction?

If it is, does that mean it's accelerating?

Is there any way for something to accelerate without a force?

The name of the force is in your book. I guarantee you.

For the second one, what do you think is making the car turn?

Hint: what happens if the car tries to turn on a sheet of ice?

g4orce
u answered his question like a teacher....without actually telling him anything.

Yeah, I got answer 1. I found that out like 5 minutes after posting it.

Yes, it does undergo acceleration, because there is a constant change in direction.

With this change in direction, there is a force (called centripetal force) that is directed towards the center of the object moving in a circular fashion.

I'm just a bit stumped on number 2 now :|

"1. Is an object that undergoes circular motion accelerating? Is a force needed for this? Where does the force come from when something is moving in a circular motion? What is the name of this force?
"

Yes, it is accelerating, because the velocity vector is changing (if the magnitude changes it is a different kind of acceleration).
In a rotating refrence frame, the force comes from change in angular momentum, mathematically different from linear momentum. In "Robot wars," the robot Hypno-Disc has a massive rotating disc that spins very fast, and obliterates the competing robots. The reason is the great amount of angular momentum stored in the massive disc, hence a great amount of kinetic energy, the anti-derivative of momentum.

"2. For a car going around a flat curve, what provides the force that allows it to turn?"

The friction between the tires and the road.

enigma
Staff Emeritus
Gold Member
Originally posted by g4orce
u answered his question like a teacher....without actually telling him anything. That's because it's homework help, not homework answers

Yes, it does undergo acceleration, because there is a constant change in direction.
Umm, no, actually it doesn't, the object should be travelling at a constant speed in spite of the constang change in direction.

The only force that is acting on the object is the centripetal force, which acts directly perpendicularon the object, hence it is pulling the object in, but the object itself is travelling at a certain velocity, so therefore the object is forced to travel in a circular path. Note that if there is an acceleration, there would be a vector force pointing against the velocity vector of the object, and since centripetal force is acting perpendicularly on the object, it doesn't experiences any opposing forces.

enigma
Staff Emeritus
Gold Member
Acceleration includes changes in direction with constant speed.

Any time either direction or speed change, it is because of acceleration.

g4orce
Originally posted by Hyperreality
Umm, no, actually it doesn't, the object should be travelling at a constant speed in spite of the constang change in direction.
There is a change in Direction, causing a change in Velocity (the magnitude remains constant) thus resulting in Acceleration. This is a fundamental principle of Circular Motion; I can't believe you would say otherwise or not know it for that matter.

It’s like saying the earth is flat.

russ_watters
Mentor
Acceleration includes changes in direction with constant speed.
Clarification (for hyper)

The definition of acceleration is change in VELOCITY.

Velocity is speed AND direcion.

Therefore, a change in direction is a change in velocity and is an acceleration.

Since velocity has two components, so does acceleration. Change in speed is LINEAR acceleration and change in direction is ANGULAR acceleration.

Ooops , sorry, guess I've got speed and velocity mixed up.