Dynamics zWhy things move yAccelerations caused by Forces.

Dynamics

Why things move Accelerations caused by Forces

Forces

Non Contact Contact

Pushes PullsGravitational Electromagnetic Nuclear

Normal Friction Tension

M T f N

WElectric Magnetic

perpendicular to surface

Labeling Forces

What forces are acting on the yellow ball?

F

Labeling Forces

What forces are acting on the yellow ball?

Fby, on

Labeling Forces

What forces are acting on the yellow ball?

Wby, on

Labeling Forces

What forces are acting on the yellow ball?

WEarth, Ball

Labeling Forces

Fby, onANYTHING LESS IS AN INCOMPLETE ANSWER!!!

Sir IsaacNewton

(1642-1727)

Newton’s Laws of Motion

1st Law:•no net force no acceleration•no acceleration no change in

motion•Objects at rest will remain at rest

and objects in motion will remain in motion at constant speed, in a straight line, unless acted on by a net force

Newton’s Laws of Motion

1st Law:•no net force no acceleration•no acceleration no change in

motion•Objects at rest will remain at rest

and objects in motion will remain in motion at constant speed, in a straight line, unless acted on by a net force

Mass

Measure of how much ‘stuff’

Measure of how hard it is to speed up or slow down the object

Mass and weight are very different

POP!

More net force - more acceleration(constant mass)

F

a

F

a

a Fnet

More mass - less acceleration(constant net force)

F

a

a

F

a M1

Putting it all together

a MFnet

Constant *

Newton is defined such that constant = 1.

a MFnet

aMFnet

Newton’s 2nd Law

Newton’s Laws

1st

Objects at rest will stay at rest and objects in motion will stay in motion, at a constant speed, in a straight line, unless acted on by a net force.

2nd

Objects with a non-zero net force accelerate in the direction of the net force via Fnet = ma

THE BRICK OF DEATH!!!

I

Physics

•What THING is pushing on the rocket?

•Is this a contact or non-contact force?

Poor Gary:

Gary has a major problem. After studying Newton’s Third Law in his physics class, he has convinced himself of its validity. Unfortunately, he has convinced himself while sitting in a closed room with a single door. He reasons to himself: “If I push on the door to go out, Newton’s 3rd Law tells me that the door is going to push back with an equal force. That force is going to cancel the force that I push with. Even if I push as hard as I can, that door is going to push back just as hard! Oh no! I’m stuck in this room! Why did I ever take physics!”

The famous bug problem:

• A bug splats on the windshield of a car moving at 100 mph. Which feels the greater force, the car or the bug?

• Which has the worst day?

WEP

NSP

person

NFS

WES

NPS

scale

Apparent Weight

True Weight

Newton’s 3rd Law and the Scale

Apparent weight

• measured as the reading of the scale

• equal to the normal force of the scale on your feet

• equal to weight for a=0

Elevator

WEP

NSP

v = constant

a = 0

Apparent Weight

True Weight

Elevator

WEP

NSP

a is upward

a

Apparent Weight

True Weight

Elevator

WEP

NSP

a is downward

a

Apparent Weight

True Weight

Elevator

WEP

NSP

How large should the normal force be?a =9.8 m/s/s down

g

Elevator

WEP

a =9.8 m/s/s down

How large should the normal force be?

g Weightless??Apparently

The weightlessness of space!

What’s his mass?

80 kg

What’s his weight?

648 N !

What ?

Apollo 17

Earth as seen from the Moon

What is the correct orbit of the space shuttle?

What is the correct orbit of the space shuttle?

a = 8.1 m/s2

a = 8.1 m/s2

a = 8.1 m/s2

a = 8.1 m/s2

a = 8.1 m/s2

a = 8.1 m/s2

Orbit

“…there is an art to flying, or rather a knack. The knack lies in learning how to throw yourself at the ground and miss.”

– Douglas AdamsLife, The Universe, and Everything

Circular motion

Which direction is the instantaneous velocity?

Uniform Circular motion

Which direction is the instantaneous velocity?

Which direction is the acceleration?

Uniform Circular motion

WE,B

TS,B

Freebody

Which direction is the instantaneous velocity?

Which direction is the acceleration?

Uniform Circular motion

Which direction is the instantaneous velocity?

Which direction is the acceleration?

WE,B

TS,B

Freebody

Uniform Circular motion

Which direction is the instantaneous velocity?

Which direction is the acceleration?

Freebody

Aren’t we missing something?

Ty

WE,BTx

THE CENTRIFUGAL FORCE!

Centrifugal. adj. Moving or directed away from the center.

Ty

WE,BTx

THE CENTRIFUGAL FORCE!CENSORED

Centrifugal. adj. Moving or directed away from the center.

Ty

WE,BTx

CENSORED

Forces

Non Contact Contact

Pushes PullsGravitational Electromagnetic Nuclear

Normal Friction Tension

M T f N

WElectric Magnetic

perpendicular to surface

Ty

WE,BTx

THE CENTRIFUGAL FORCE!CENSORED

Centrifugal. adj. Moving or directed away from the center.

The centri*&%#l force DOES NOT EXIST!For uniform circular motion the net force

must be towards the center of the circle.Therefore: The acceleration is ‘center

seeking’

CENSORED

CENSORED

Ty

WE,BTx

THE CENTRIFUGAL FORCE!CENSORED

Centrifugal. adj. Moving or directed away from the center.

The centri*&%#l force DOES NOT EXIST!For uniform circular motion the net force

must be towards the center of the circle.Therefore: The acceleration is

centripetal

CENSORED

CENSORED

The swinging bucket of death

A

B

CI

Physics

Draw freebody diagrams for the water at points A, B, and C.

Centripetal Acceleration

For U.C.M. there must be a centripetal net force

Centripetal Acceleration

For U.C.M. there must be a centripetal net force centripetal acceleration