apr-15general science chapter 21 chapter 2 moving objects motion speed instantaneous speed ...

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Chapter 2 Moving Objects Motion Speed Instantaneous

Speed Constant Speed Average Speed Velocity Acceleration

Force Net Force Balanced Forces Inertia Friction Gravity Weight Mass

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Section 1 Motion

Motion Speed Instantaneous Speed Constant Speed Average Speed Velocity Acceleration

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Motion You don’t have to see something move to

know that motion has taken place. You only have to know that its position has

changed. Best described as a change in position.

You need a reference point to know if position has changed.

Depends on the observer’s frame of reference.

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Speed

How fast something moves The rate of change in position The distance traveled in a given

amount of time The rate of motion Rate means change over time

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Instantaneous speed

The rate of motion at a given instant Speedometer

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Constant speed

Speed that does not vary Cruise control

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Average speed

Total distance traveled divided by total time of travel

Used when the object in motions makes changes in speed, including starts and stops.

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Calculating speed

If you know distance and time, you can calculate speed.

t

dv =

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Example

You skate a distance of 100 m in 67 s. What is your average speed?

Known: d = 100 m t = 67 s

Unknown v = ?

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Example continued

t

dv =

s 67

m 100=v

s

m 5.1=v

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Graphing speed

When distance is graphed on the y-axis and time is graphed on the x-axis, we call this a distance-time graph.

The slope of a distance-time graph is the speed.

What kind of graph would you use to graph distance vs time?

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Section 1: Discussion Questions

What is motion? What two quantities do you need to

know to calculate average speed? What does a flat line on a segment of

a distance-time graph tell you about the average speed for that time period?

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Velocity vs. speed

Velocity describes both speed and direction.

Speed describes only rate of change in position.

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REVIEW #1

What is the difference between instantaneous and average speed?

What is the difference between speed and velocity?

What is the equation used to solve for average speed?

What SI units are used for speed? What is motion?

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Acceleration The rate of change of velocity. Acceleration occurs if either the

speed or direction changes, or if both change.

“Slowing down” is a change in speed, so when something slows down, it is accelerating.

When speed decreases, acceleration is opposite in direction to velocity.

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Acceleration

The size of acceleration depends on both the change in velocity and the time interval over which the change takes place

If the change in velocity is large, the acceleration will be large.

If the time interval is short, then acceleration will be large.

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Calculating acceleration

The triangle is the Greek letter delta, and stands for “change in”

t

v

t

vva if Δ

=−

=

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Units on acceleration

Meters per second per second or Meters per second squared Written as m/s2

second

ondmeters/sec

second

1

second

meters⋅=

( )2second

meters=

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Example A car increases its velocity from 10

m/s to 32 m/s in 3 seconds. What is the acceleration of the car?

Known: vi = 10 m/s vf = 32 m/s t = 3 s

Unknown: a=?

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Example continued

t

vva if −

=

s 3

m/s 10m/s 32 −=a

s 3

m/s 22=a

2s

m 3.7=a

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Discussion Questions, Part 2

What is acceleration? What are three ways an object can

accelerate? What equation is used to determine

acceleration? What is the SI unit of acceleration? Is the SI unit of acceleration a derived

unit?

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Section 2 Forces

Force Balanced Forces Net Force Inertia Friction

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Force

A push or a pull one object exerts on another

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Balanced forces

Forces that are equal in size and opposite in direction are called balanced forces.

Tug-of-war

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Net Force

When forces are unbalanced, there is a net force.

A net force always causes a change in velocity.

Could be a change in speed, a change in direction, or both.

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Inertia

The tendency of an object to resist any change in its motion.

If it is moving, it tends to keep moving at the same velocity unless a force acts on it.

If it is not moving, it tends to remain at rest unless a force acts on it.

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Mass and inertia

The more mass an object has, the greater its inertia is.

Objects with greater inertia take more force to change their velocity.

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Newton’s first law An object moving at a constant

velocity keeps moving at that velocity unless a net force acts on it.

An object at rest stays at rest unless a net force acts on it.

An object at rest/in motion will stay at rest/in motion unless a net force acts on it.

Also called the law of inertia.

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Seatbelts

When a car crashes, a force stops the car and any passengers in the car.

A passenger who isn’t wearing her seatbelt isn’t acted on by the force of the seatbelt, so she keeps moving at the same velocity.

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Friction

The force that opposes motion between two surfaces that are touching each other.

Enables us to walk, hold things, etc.

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Friction

The amount of friction depends on two things The kinds of surfaces The force pressing the surfaces together

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Discuss

What is a force? What are balanced forces? What is inertia? What is friction?

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Section 3 Gravity

Gravity Weight Mass Scales

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Gravity

Every object in the universe exerts a force on every other object That force is gravity

Often, the force is too small to notice Example – the force of gravity between

two people

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Gravitational force The amount of gravitational force between

two objects depends on two things Their masses and the distance between them.

The mass of a person is small, so the gravitational force between two people is weak.

The mass of the Earth is large, so the gravitational force between the Earth and a person is strong.

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Gravitational force

Gets weaker as the distance increases

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Weight

The measure of the force of gravity on an object.

We usually use the term weight when we talk about the gravitational force between the Earth and an object near its surface.

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Weight

Related to mass Objects with greater mass have

greater weight. Weight is determined by the

gravitational pull on the object.

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Weight and mass

Weight is measured in Newtons (N). 1 kg of mass at the Earth’s surface

has a weight of 9.8 N

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Weight on Earth vs. Weight on Moon

Since the Earth has more mass than the moon, it exerts a greater gravitational force than the moon.

So, things weigh less on the moon than they do on Earth.

Do they have less mass on the moon?

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Scales

Use the principal of balanced forces to measure how much something weighs.

Scales use springs to balance the force of an object’s weight.

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Discuss

What is the force of gravity? What is weight? Weight is measured in what SI units? What instrument is used to measure

weight?

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Chapter Review

Do you know the difference between: Speed and velocity Average speed and instantaneous speed Motion and speed Balanced forces and net forces Mass and Weight A balance and a scale