Force and Motion-II: continued

Friction and Drag;Circular Motion

Lecture 6

Monday:2 February 2004

UNIFORM CIRCULAR MOTION

CIRCULAR MOTION

• When an object travels in a circle, its velocity is constantly changing (in direction-at least).

• That means the object has a non-zero acceleration even if it moves at constant speed.

What is the direction of this acceleration?

Acceleration in Circular Motion

Centripetal acceleration:• Direction:

Toward the center of the circle

• Magnitude:

av

r

2

FORCES IN CIRCULAR MOTION

• An “unbalanced” force must cause the acceleration.

center. the towardisdirection theand

and

2

2

r

vmF

r

vaamF

Using Freebody Diagrams to Analyze Forces

Twirl a ball attached to a string in a vertical circle. What forces act on the ball and what are the direction of these forces?

Using Freebody Diagrams to Analyze Forces

What forces act on the ball and what are the direction of these forces?

Top

Bottom

T

T

W

W

a

a

Are we missing a force?

(the centripetal force)?

Is the acceleration =g?

Using Freebody Diagrams to Analyze Forces

What forces act on the ball and what are the direction of these forces?

Top

Bottom

T

T

W

W

a

aAre we missing a force?

(the centripetal force)?

Is the acceleration =g?

-T-W=m(-a) or

T+W=ma

T-W=ma

Any force can be a “centripetal” force.

-Gravitational Attraction

-Tension in a string

-Normal Force

-A push or pull…

Does the planet in orbit here have a gravitational force AND a centripetal force acting on it?

PERIOD IN CIRCULAR MOTION

vr2

T Period so

2revolution onefor time

2time

positionVelocity and

r2 nceCircumfera

Trr

v