1. Unit 3: Circular motion, Work & Energy
Horizontal Circular Motion

2. Horizontal Circular Motion
Circular motion is one of the most common motions in the universe
Circular motion is just a special case of two-dimensional motion
When moving in a circle, an objects speed may be constant but its velocity is always changing because its direction is constantly changing
*v1 and v2 are different because they have different directions

3. Calculating Circular Motion
We can use Newton’s Laws to describe circular motion
The speed of an object moving in uniform circular motion is given by the equation:
This is a special case of: v = d/t

4. Calculating Circular Motion
Since an object moving in circular motion has a changing velocity it also has an acceleration – which can be calculated using the following:
Since, centripetal means “centre seeking” the direction of the acceleration is always towards the middle

5. Centripetal Force
According to Newton’s Laws of Motion, since there is an acceleration towards the centre, there must also be a force acting towards the centre
Centripetal Force (Fc)can be supplied by
a string (tension)
a frictional force
a gravitational force
Car rounding a corner Fc = Ff
What direction would the car go if the roadway was frictionless?

6. Calculating Centripetal Force
Two ways:
When acceleration is known:
When time period is known:

7. Sample Problem
A kg rubber stopper is attached to a 0.93m length of string. The stopper is swung in a horizontal circle making 1 revolution in 1.18s.
Find the speed of the stopper
Find the centripetal acceleration of the stopper
Find the centripetal force

8. Time Period Vs. Frequency
Period- (T) The time it takes for one complete cycle (revolution)
Frequency- (f) the number of revolutions per time period
Ex. RPM - Frequency is usually measured in Hertz (Hz) (RPS)

9. Sample Problem
A disc is moving at 200RPM. What is its period in s? (Hint: first find rotations per second)

10. Homework
p. 99 #1-11