Astronomy Lectures 09/10

Describe position and time

specify where and when a particle is

Speed

Rate at which an object moves (changes position)

Velocity

speed and direction

Velocity can change if

1. the speed changes

2. the direction changes

3. both speed and direction changes

accerleration

the rate of change of velocity with time

= change of velocity/time

Newton’s first and third law

1. every body continues in a state of rest or in a state of uniform motion in a straight line unless it is compelled to change that state by a force acting on it

2. to every action there is an equal and opposite reaction

Newton’s first law of motion

an object remains at rest of at constant velocity unless a net force acts to change its speed or direction

Newton’s third law of motoin

to every action there is an equal and opposite reaction

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

Earth and I exert equal and opposite forces on each other

Newton’s law of gravity

1. every mass attracts every other mass

2. attraction is directly proportional to the product of their masses (the more massive, the more attractive!)

3. attraction is inversely proportional to the square of the distance between their centers

the acceleration of gravity (g)

Galileo showed that *g* is the *same* for all falling objects regardless of their mass

the motion of an object solely under the influence of gravity is called what?

free fall

What is mass?

• amount of matter in an object

• an intrinsic property of an object

• the same no matter what forces are acting on an object

What is weight?

* the force of gravity exerted on an object
* directly proportional to the mass

What is the difference between mass and weight?

• Mass is a measure of the amount of matter in an object. It is a scalar quantity that is measured in units such as kilograms or grams. Mass is an intrinsic property of an object and does not change with its location in the universe or with the gravitational field it is in

• Weight is a measure of the force exerted on an object due to gravity. It is a vector quantity that is measured in units such as newtons or pounds

What is your weight on the moon?

Your weight is less, but the mass is the same

What is orbital motion?

An object orbiting earth is actually falling (being accelerated) toward earth’s center

what is apparent weight?

the weight that an object appears to have when it is experiencing acceleration or deceleration.

Why are astronauts weightless in space?

• due to a constant state of free-fall around the earth

• the apparent weight of an object in free-fall is zero (no sensation of weight)

What is escape velocity?

the minimum velocity that an object must have in order to escape the gravitational field of a planet, moon, or other celestial body and not fall back to the surface

What is hypothesis?

educated guess

What is the order of scientific method?

1. observation

2. hypothesis

3. experiment

4. outcome

hallmarks of science #1

modern science seeks explanation for observed phenomena that rely solely on natural causes

hallmarks of science #2

science progresses through the creation and testing of models of nature that explain the observations as simply as possible

hallmarks of science #3

a scientific model must make testable predictions about natural phenomena that would force us to revise or abandon the model if the predictions don’t agree with observations

a scientific theory must do what?

• explain a wide variety of observations with a few simple principles

• must be supported by a large compelling body of evidence

• must NOT have failed any crucial test of its validity