Column Statistics for: test1Count:108Average:33.3Median:33.0Maximum:49.0Minimum:12.0Standard Deviation:8.37

Test 1 Result: Section 1

1

Test 1 Result: Section 2

Column Statistics for: Test1Count:117Average:35.0Median:36.0Maximum:49.0Minimum:12.0Standard Deviation:8.79

2

AST 103Ch.4 Making Sense of the Universe:

Motion & GravityProf. Ken Nagamine

UNLV

3

Sec 4.1: Describing Motion

• Our goals for learning:

• How do we describe motion?

• How is mass different from weight?

4

How do we describe motion?Precise definitions to describe motion: Speed: Rate at which object moves

example: speed of 10 m/s

Velocity: Speed and direction example: 10 m/s, due east

Acceleration: Any change in velocity units of speed/time (m/s2)

5

Momentum and Force

• momentum = mass × velocity

• “applying a force” ==> acceleration (change in velocity) ==> change in momentum

6

Angular Momentum

• Rotational momentum of a spinning or orbiting object is known as angular momentum

M

VR

angular momentum = mass x velocity x radius

7

Why do objects fall?

8

Which object will hit the ground first?

A. the piece of paper

B. the eraser

C. Both will hit at the same time.

D.We can’t predict which will hit first.

Quiz

9

The Acceleration of Gravity (g)

• Galileo showed that g is the same for all falling objects, regardless of their mass.

Apollo 15 demonstration

(search by “Galileo experiment on moon)http://www.youtube.com/watch?v=4mTsrRZEMwA

10

Acceleration due to Gravity

• All falling objects accelerate at the same rate on Earth (not counting friction of air resistance).

• On Earth, acceleration g~10 m/s2 : speed increases 10 m/s with each second of falling.

11

Is there a difference between mass and weight?

A. Yes, there is a difference.

B. No, they are the same thing.

Quiz

12

How is mass different from weight?

• Mass – the amount of matter in an object

• Weight – the force that acts upon an object

13

`Gedanken Experiment’ of Einstein

? ? ?

???14

How is mass different from weight?

? ?

??15

How is mass different from weight?

?

?16

How is mass different from weight?

17

Einstein realized that acceleration is equivalent to gravity.

Weight can change according to the environment, but mass cannot.

18

There is gravity in space

Weightlessness is due to a constant state of free-fall

Why are astronauts weightless in space?

Another good example:

19

On the Moon,

A. My weight is the same, my mass is less.

B. My weight is less, my mass is the same.

C. My weight is more, my mass is the same.

D. My weight is more, my mass is less.

Quiz

20

What have we learned?

• How do we describe motion?

• Speed = distance / time

• Speed & direction => velocity

• Change in velocity => acceleration

• Momentum = mass x velocity

• Force causes change in momentum, producing acceleration

21

What have we learned?

How is mass different from weight?

Mass = quantity of matter

Weight = force acting on mass

Objects are weightless in free-fall

22

S. 4.2: Newton’s Laws of Motion

• How did Newton change our view of the universe?

• What are Newton’s three laws of motion?

23

• Realized the same physical laws that operate on Earth also operate in the heavens

• one universe

• Discovered laws of motion and gravity

• Much more: experiments with light; first reflecting telescope, calculus, ....Sir Isaac Newton

(1642-1727)

How did Newton change our view of the universe?

24

• An object moves at constant velocity unless a net force acts to change its speed or direction.

Newton’s first law of motion

In layman’s terms....“a scooter will coast with a same speed until you brake or kick!”

25

Newton’s second law of motion

• Force = mass × acceleration

• The bigger the force, the greater the resulting acceleration on a given mass.

26

Newton’s third law of motion:

• For every force, there is always an equal

and opposite reaction force.

27

Newton’s law of universal gravitation

• The force of gravitational attraction exerted by one object on another depends on

• the mass of the first object (M)

• the mass of the second object (m)

• the distance between the objects (r)

or, in math terms, F = GMm r2

28

© 2008 Pearson Education Inc, publishing as Pearson Addison-Wesley

What determines the strength of gravity? The Universal Law of Gravitation:1. Every mass attracts every other mass.2. Attraction is directly proportional to the product of

their masses.3. Attraction is inversely proportional to the square of

the distance between their centers.

29

Is the force Earth exerts on you larger, smaller, or the same as the force you exert on Earth?

A. Earth exerts a larger force on me.

B. I exert a larger force on Earth.

C. Earth and I exert the same amount of force on each other.

Quiz

30

m

M

31

m

M

moon

Earth

m

M

32

Lecture-Tutorial (LT):Newton’s Laws and Gravity (p.29-31)

• Work with a partner!

• Read the instructions and questions carefully.

• Discuss the concepts and your answers with one another.

• Come to a consensus answer you both agree on.

• If you get stuck or are not sure of your answer, ask another group.

• If you get really stuck or don’t understand what the LT is asking, ask for a help.

33