1. Circular motion - chapter 6
Physics

2. Describing circular motion
Uniform circular motion – object moving around a circle with a fixed radius at a constant speed
Is the object accelerating? Why or why not?
Velocity is always tangent to the circle
Displacement is the radius of the circle, r

3. Centripetal acceleration
Centripetal means CENTER SEEKING
Centripetal acceleration is in the same direction as the change in velocity, the center of the circle
Page 153 Figure 6-6
ac = v2 / r
ac = 4π2r / T2
T = period, the time it takes for the object to make one circle

4. Centripetal force Acceleration is always caused by a force
Centripetal acceleration is caused by centripetal force
They are both in the same direction – towards the center
Centripetal force is not a new force, it is a new direction for old forces
What causes the centripetal force?
A washer is tied to a string and swung over head in a circle.

5. Centripetal problems - 1
(Page 155 Example 2) A 13 g rubber stopper is attached to a 0.93 m string. The stopper is swung in a horizontal circle, making one revolution in s. Find the tension force exerted by the string on the stopper.
First find acceleration (a=4π2r/T2)
Then, find force (F=ma)
0.34 N

6. Centripetal Problems - 2
(Page 156 Example 13) A car racing on a flat track travels at 22 m/s around a curve with a 56 m radius.
Find the car’s centripetal acceleration.
8.6 m/s2

7. Centripetal problems – 3
(page 156, Problem 20) A bowling ball has a mass of 7.3 kg. If you move it around a circle with a radius of 0.75 m at a speed of 2.5 m/s, what force would you have to exert on it?
First find acceleration (ac=v2/r)
Then, find force (F=ma)
61 N

8. The imaginary force
Centrifugal force is supposedly the force that pushes you towards the outside of a circle
Ex:
When you are riding in a car and you take a sharp turn to the left, your body moves to the right.
When you take a bucket of water and spin it in a circle, the water stays in the bucket.
These are not a force! These are examples of inertia – an object’s resistance to change in motion

10. Gravitational Force Newton’s Law of Universal Gravitation
F = Gm1m2 / r2
G, universal gravitational constant = 6.67 x N*m2/kg2

11. Gravitational Force Problem
(page 191 Problem 55) Tom has a mass of 70.0 kg and Sally has a mass of 50.0 kg. Tom and Sally are standing 20.0 m apart on the dance floor. Sally looks up and sees Tom. She feels an attraction. If the attraction is gravitational, find its size.
5.84 x N