Students are often confused by the term centrifugal force. Centrifugal force is not an actual force. It is called a fictitious or apparent force because we perceive it due to our frame of reference, not because of an actual force being applied. In order to keep an object moving on a circular path, it is necessary to apply a force toward the center of the circle; this is called the centripetal force. If you are in a car going through a curve to the left, the road is pushing the car to the left, which is toward the center of the curve. As you car seat moves inwards to your left, you feel as though you are being pressed outward to the right. In fact, it is the car's door pushing you, not you pressing the car door. The force you feel is simply inertia's opposition to the centripetal force. If the car goes around a curve of radius R with a speed v and it has a mass m, then the centripetal force is mv^2/R and points toward the center of the circle.

## Centripetal Force

Measure the mass of the object, and label it m. For instance, a liter of water in a bucket weighs one kilogram.

Measure the radius of the curve, and label it R. Suppose a string holding the bucket is one meter long.

Determine the speed of the object, and label it v. The velocity of an object changes as it goes around a circle. Its speed is equal to how great its velocity is at a given moment. Imagine you swing the bucket so it goes around once per second. The velocity will be the distance around the circle, which is 2_pi_R divided by one second: v = 2_pi_(1 meter) / (1 second) = 6.28 meters per second.

Calculate f_centripetal = mv^2/R: f_centripetal = (1 kilogram)*(6.28 meters / second)^2 / (1 meter) = 39.4 Newtons. We can imagine the water being pushed against the bottom of the bucket by centrifugal force, But really, the bucket is pushing against the water to keep it moving in a circle. Without this centripetal force, the water would fly out in a straight line tangent to the circular path.

#### Tip

Remember not to confuse centripetal and centrifugal. Remember, the centripetal force is a real force; the centrifugal force is an apparent force.