Circular motion

Constant acceleration

We've studied motion in a straight line by watching Felix step from his gondola, 40 km above the earth. You can skip to about 2 minutes if you want to see the highlights.

We've also studied motion in the curved trajectory of a projectile while hunting digital monkeys. Felix and the monkey fell in straight lines. The projectile followed a curved path. Were they subject to the same forces? What do these types of motion have in common?

Back to Newton's first and second laws

Do you remember what these laws say?

The first law says that an object in uniform motion (i.e. constant speed and direction) will maintain that speed and direction unless a net external force acts on the object. In that case, it will begin to accelerate in the direction of that force. So acceleration and force are bound together. You can't have one without the other.

    When Felix stands on the edge,

  • is he moving?
  • is the net force on him zero?
  • what are the forces acting on him?
  • what changes when he steps off?
    When Felix steps off,

  • is his motion changing?
  • is the net force on him zero?
  • is the force constant?
  • so what can you say about his acceleration?

An object moving in a circle

The situations we studied above were limited to situations where force was constant.

Now, imagine you are on a highway onramp or circular track, traveling at exactly 40 miles per hour. Would you say that you are accelerating? What does Newton's first and second laws say about whether you are accelerating?

Let's consider a more dramatic image of what is happening when we travel in a circle. Let's take a ride in a centrifuge.


This centrifuge was operated at Holloman AFB in New Mexico. Let's watch what happens to a person riding inside. Imagine yourself in the centrifuge.

  • Does increasing the speed of the rotor increase or decrease your acceleration?
  • Would increasing the radius of the rotor increase or decrease your acceleration?

What is the relationship between speed, radius, and acceleration? You can discover this for yourself by working through the centripetal forces at play lab.

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