Happy Thursday 11-12-16

• Grab your calculator• Get ready to take notes• Get ready for your warm up• No quiz tomorrow• Next test: after Thanksgiving break

Warm up:

• Jeffrey pulls to the right on an 18 kg box with a force of 15 N while Eugene pulls to the left with a force of 22 N.

• A. What is FNET ?• B. What is the acceleration of the box?• C. Calculate the friction force if m =.15

3

Uniform Circular Motion

http://www.physicsclassroom.com/mmedia/circmot/circmotTOC.html

Question:

• Which moves faster on a merry-go-round: a horse on the inside or a horse near the outer rail?

• The horse on the outside • It must move faster than the horse on the

inside in order to make one full rotation

Uniform Circular Motion• motion of an object

in a circle with a constant or uniform speed

Velocity of circular motion

• When an object spins in a circle, the distance it travels is = to the circumference o the circle, or

• The time it takes for one full revolution of the circle = Period

• formula for the speed of a circle:

Centripetal Acceleration

• Centripetal Acceleration (ac) acceleration is toward the center of the circle,

• Acceleration = change in speed OR direction

• Since an object moving in a circle is constantly changing direction, it is constantly accelerating!

Centripetal Acceleration Formula

2

c

va

r

a c= centripetal accelerationV2 = velocity squaredr= radius

Centripetal Acceleration practice problems:

• A car turns a curve at a speed of 20 m/s. If the radius of curvature of the turn is 100 m, what is the centripetal acceleration of the car?

• 4 m/s2

• A ball on a string swings in a horizontal circle of radius 2.0 m. If the speed of the ball is 15 m/s, what is the centripetal acceleration?

• 112.5 m/s2

2

c

va

r

Centripetal force examples

• Gravitron• gravitron 2• how to create artificial gravity• Centripetal force examples• Demos• spinning tube trick

Centripetal Force

Centripetal Force – A force that causes an object to move in a circle.

Centripetal force points toward the CENTER of the circle.

12

Applying Newton’s 2nd Law:

F ma

Fmv

r

2

Centripetal Force

Always points toward center of circle. (Always changing direction!)

13

Direction of Centripetal Force, Acceleration and Velocity

Without a centripetal force, an object in

motion continues along a straight-line path.

Without a centripetal force, an object in motion

continues along a straight-line path.

14

Direction of Centripetal Force, Acceleration and Velocity

15

What if velocity decreases?

16

What if mass decreases?

17

What if radius decreases?

The diagram below shows the top view of a 65 kg student at point A on an amusement park ride. The ride spins the student in a horizontal circle of radius 2.5 meters, at a constant speed of 8.6 m/s.

The floor is lowered and the student remains stuck against the wall without falling to the floor.

a. 12,000 N c. 220 N

b. 1900 N d. 30 N

The magnitude of the centripetal force acting on the student at point A is approximately ___.