Unit 1, Chapter 3

Foundations of Physical Science

Unit One: Forces and Motion

3.1 Force, Mass and Acceleration

3.2 Weight, Gravity and Friction

3.3 Equilibrium, Action and Reaction

Chapter 3 Forces and Motion

Chapter 3 Learning Goals Explain the meaning of force. Show how force is required to change the motion of an object. Use a graph to identify the relationships between variables. Explain and discuss Newton's second law and the relationship

between force, mass and acceleration. Describe how changing the mass of the ca affects its

acceleration. Draw conclusions from experimental data. Demonstrate qualitatively how friction can affect motion. Explain Newton's third law of motion. Identify action-reaction pairs of forces. Recognize how Newton's third law of motion explains the physics

behind many common activities and useful objects.

Chapter 3 Vocabulary Terms air friction equilibrium force friction gravity inertia law of conservation

of momentum mass momentum

newton Newton's 1st law of motion Newton's 2nd law of motion Newton's 3rd law of motion pounds rolling friction sliding friction viscous friction weight

3.1 Sir Isaac Newton’s laws of

motionSir Isaac Newton (1642-1727), an

English physicist and mathematician, is one of the most brilliant scientists in history.

Before the age of 30, he formulated the basic laws of mechanics, discovered the universal law of gravitation, and invented calculus!

3.1 Force, Mass and Acceleration

1. Unless you apply force, things tend to keep on doing what they were doing in the first place.

2. Force causes an object to accelerate, while the object’s mass resists acceleration.

3. For every action, there is an equal and opposite reaction.

IN OTHER WORDS:

3.1 Force, Mass and AccelerationA force is what we call a push or a pull, or any action that has the ability to change motion.

There are two units of force that are commonly used: pounds and newtons.

Scientists prefer to use newtons.

3.1 The difference betweenforce and mass

Force is a push or pulling action that can change motion.

Force and mass have different units.

The metric unit of force, the newton, relates force and motion.

3.1 Mass Because mass is an amount of matter, mass is

independent of the force of gravity. The mass of an object is the same everywhere in the

universe. The only exception to this rule is when things go extremely fast, close to the speed of light.

3.1 The difference betweenforce and mass

Mass is the amount of “stuff” or matter in an object.

We can also define mass as the amount of matter an object has.

Mass is measured in kilograms.

3.1 The difference betweenweight and mass

Weight is a force that comes from gravity pulling on mass.

The weight depends on how strong gravity is.

Earth is bigger than Mars and has stronger gravity.

A kilogram weighs 9.8 newtons on Earth but only 3.8 newtons on Mars.

3.1 Newton's Second Law

a = F m

force (newtons, N)

mass (kg)

acceleration (m/sec2)

3.1 Force, Mass, and AccelerationKey Question:

What is the relationship between force, mass and acceleration?

*Read text section 3.1

BEFORE Investigation 3.1

3.2 Gravity What is gravity?

— Gravity is a force.— Gravity depends on mass.— Gravity accelerates objects.— Why is the total force exerted

by the bag of flour less on the moon?

3.2 Weight

Fw = mg

gravity (9.8 m/sec2)

mass (kg)

Weight force (N)

3.2 Weight and Galileo

A legend has it that, around 1587, Galileo dropped two balls from the Leaning Tower of Pisa to see which would fall faster.

3.2 Gravity

The attractive force from gravity between objects of ordinary mass is incredibly small.

3.2 Gravity

You feel weight because the mass of Earth is large enough to create significant gravity forces.

3.2 Newton's Law of Universal Gravitation

The force of attraction between two objects is directly related to the masses of the objects and inversely related to the square of the distance between them.

3.2 Newton's Law of Universal Gravitation

F = G m1m2

R2

gravity (9.8 m/sec2)

mass 1 (kg)

Force (N) mass 2 (kg)

distance (m)between m1 and m2

3.2 Friction

Friction is a force that always opposes motion.

What is left is often called the net force.

What is the net force acting on the car?

3.2 Weight, Gravity and FrictionKey Question:

How does increasing the mass of the car affect its acceleration?

*Read text section 3.2

BEFORE Investigation 3.2

3.3 Equilibrium, Action and ReactionKey Question:

What is Newton's third law of motion?

*Read text section 3.3

AFTER Investigation 3.3

Momentum— explains why the speed and the direction of

motion are related to the mass of the object.

3.3 Equilibrium, Action and Reaction

3.3 Momentum

P = mv

velocity (m/sec)

mass (kg)

Momentum (kg-m/sec)