EXAM I is coming up in two weeks!

Multi-choice (scantron!) type of exam

Some examples of possible questions will be available via the Exam link on our class web site (soon).

Just a little physics ….

MOTION

Kinematics: The description of how things move in space.

• Position• Velocity• acceleration

Dynamics: The prediction of why things move the way they do.

• Force• Newton’s laws of motion• Force of gravity

Kinematics: Position vs. time

Bug’s trajectory

Kinematics: Position vs. time

Bug’s trajectory

.

Bug was HERE at10:01 PM

Kinematics: speed and velocity

Bug’s trajectory

.

How fast was it going then?Bug was HERE at10:01 PM

Bug’s trajectory

.

Bug was HERE at10:01 PM

.

.

10:01 PM

1 msec later

sec001.0t

Kinematics: speed and velocity

Bug’s trajectory

.

Bug was HERE at10:01 PM

.

.

10:01 PM

1 msec later

sec001.0t

2/10 mmm0002.0d

Kinematics: speed and velocity

Bug’s trajectory

.

Bug was HERE at10:01 PM

.

.

10:01 PM

1 msec later

sec001.0t

2/10 mmm0002.0d

SPEED:

m/sec2.0

t

d

Kinematics: speed and velocity

Bug’s trajectory

.

Bug was HERE at10:01 PM

.

.

10:01 PM

SPEED:

m/sec2.0

t

d

0.2 m/sec

DIRECTION

Kinematics: speed and velocity

Bug’s trajectory

.

Bug was HERE at10:01 PM

0.2 m/sec

VELOCITY = SPEED & DIRECTION

Kinematics: speed and velocity

At 10:01 PM the bug was doing 0.2 m/sec in the SW

direction

Kinematics: velocity vs. time/position

..

.

.

.

.Velocity vectors:

(Length ~ speed)

Kinematics: Acceleration

.

.

10:01 PM

1 msec later

sec001.0t

Kinematics: Acceleration

.

sec001.0t

.

Acceleration =

Change in velocity(Speed and direction)

t

0.2 m/sec

0.5 m/sec

How is this done?

Kinematics: Acceleration

Change in velocity(Speed and direction) 0.2 m/sec 0.5 m/sec

How is this done?

?

Kinematics: Acceleration

Change in velocity(Speed and direction) 0.2 m/sec

0.5 m/sec

How is this done?

DV

Find length with ruler or trigonometry.

Say it’s 0.15 m/sec

direction

Acceleration =

Change in velocity(Speed and direction)

t

Acceleration =0.15 m/sec

sec001.0t

=

2m/sec150m/sec/sec150

=

t

V

EXAMPLE 1: LINEAR MOTION

mi/hr50

0t

mi/hr50mi/hr50

0t sec 2t

EXAMPLE 1: LINEAR MOTION

Acceleration =

mi/hr/sec0

mi/hr50

0t

mi/hr70

sec 4t

EXAMPLE 1: LINEAR MOTION

mi/hr50

sec 2t

Acceleration =

mi/hr/sec10

mi/hr70

sec 4t

mi/hr50mi/hr50

0t sec 2t

mi/hr110

sec 6t

EXAMPLE 1: LINEAR MOTION

Acceleration =

mi/hr/sec20

mi/hr50

0t

mi/hr110

sec 6t

mi/hr40

sec 8t

mi/hr70

sec 4t

EXAMPLE 1: LINEAR MOTION

mi/hr50

sec 2t

(De)acceleration =

mi/hr/sec30

mi/hr0

sec 10t

mi/hr50

0t

mi/hr110

sec 6t

mi/hr40

sec 8t

mi/hr70

sec 4t

EXAMPLE 1: LINEAR MOTION

mi/hr50

sec 2t

(De)acceleration =

mi/hr/sec20

EXAMPLE 1: CONSTANT ACCELERATION

Gravity!

EXAMPLE 1: CONSTANT ACCELERATION

Gravity!

2m/sec8.9

ft/sec/sec32OR

g

Speed (V) is constantbut velocity is NOT

EXAMPLE 3: UNIFORM CIRCULAR MOTION

Speed (V) is constantbut velocity is NOTV

Centripetal (center-seeking) acceleration R

VA

2

R

EXAMPLE 3: UNIFORM CIRCULAR MOTION

EXAMPLE 3: UNIFORM CIRCULAR MOTION

Speed (V) is constantbut velocity is NOT

V

Centripetal (center-seeking) acceleration R

VA

2

R

NEWTON!

His “laws of motion”

His law of gravitation+All of Kepler’s

results, and then some!!

1642-1747 AD

His “laws of motion”

1. An object that isn’t experiencing any net force moves with a constant velocity (At rest? → constant velocity of zero!)

What’s force, and what does it do?

2. F = M×A !What a (total or net) force does.

What mass is.

3. Action & reaction. How objects interact! (the physics)

NL#1 An object that isn’t experiencing any net force moves with a constant velocity

No force → velocity constant. So …

Force ~ changing velocity (acceleration)

(At rest? → constant velocity of zero!)

NL#2

Q: Why not just F = A ? A: Experience….

F = M A (mass times acceleration)

The standard ‘force’ machine

No pullNo force

Pulls with Standard force

Pull here

Applying our standard force to….

1 bowling ball produces twice the acceleration as …

2 bowling balls

The same force applied to different objects does NOT produce the same acceleration!!

INERTIA: The tendency of an object to resist acceleration

MASS: A measure of a specific object’s inertia! (HOW?)

volumedensity M

What it’s made of It’s size

INERTIA: The tendency of an object to resist acceleration

NL#2

MASS: A measure of a specific object’s inertia! (HOW?)

A

FM

Applied standard force

Measured acceleration

Derived mass – new units e.g. kg or Slugs

NL#3If object A exerts a force on an object B, object B must exert an equal but opposite force on object A.

spaceman,mass = 1

spaceship,mass = 10,000

spaceman pushes on ship with force F

spaceship accelerates to right at a rateA=F/10,000

At the same time ….

ship pushes back on spaceman with force F

A=F/1

Spaceman flies off at an acceleration 10,000× that of the ship.

Equal (opposite) forces,but very different accelerations

A=F/1

A=F/10,000

Newton's laws tell us how a force affects the motion of an object.

If we know the details of a force acting on an object, then we can use Newton’s Laws to

calculate how an object will move!

The POINT:

GRAVITY AND WEIGHT

https://www.youtube.com/watch?v=7Mrkpcn-_YU

Observational fact: Everything drops with the same acceleration (g) in a gravitational field (at a given point).

Feather and bowling ball:

WEIGHT

Force of gravity on a body; e.g. “lbs.”

Varies from place to place!

Earth Mars Jupiter180 lbs. 68 lbs. 426 lbs.

For instance, on the surface of …Your weight would be …

Near earth, at an elevation of …Your weight would be …

10 mi 100 mi 1000 mi179 lbs. 171 lbs. 114 lbs.

MASS

Intrinsic property of an object, it’s inertia to ANY force.

Same everywhere.

Earth 180 lbs.

For instance, IF on the surface of …Your weight is …

THEN … your mass is slugs 6.5ft/sec32

lbs1802

g

FM (82 kg)

EVERYWHERE

FLOOR

V = constant so …

A = 0 so Ftotal = 0

OUR SENSE OF WEIGHT

FLOOR

FORCE DUE TO GRAVITY = Mg:

20 lbs.

180 lbs.

FLOOR

20 lbs.

180 lbs.

FORCE DUE TO FLOOR:

20 lbs.

180 lbs.

FORCE DUE TO GRAVITY = Mg:

180 lbs.

180 lbs.

Our physical sense of weight comes from a

compressive feeling in our soft bodies

NOW ….

“WEIGHTLESSNESS”

FLOOR

REMOVE FLOOR ….

“WEIGHTLESSNESS”

20 lbs.

180 lbs.

FORCE DUE TO GRAVITY = Mg:

BOTH STILL HAVE WEIGHT!!

FREE FALL !!

“WEIGHTLESSNESS”

But since F = Mg ,

Both accelerate downward with acceleration g

2m/sec8.9

They maintain their relative positions as they drop together

FREE FALL !!

WEIGHTLESSNESS

https://www.youtube.com/watch?v=O9XtK6R1QAk

‘Weightless’ cats!!

Astronaut Marsha IvinsAstronaut ?