Chapter 11Section 1: Laws of Motion

Section 2: Gravity

Section 3: Newton’s Third Law

Key Term

Inertia

Newton’s First Law

Every motion you observe or experience is related to force.

Newton’s First Law of motion states that an object at rest remains at rest and an object in motion maintains its velocity unless it experiences an unbalanced force.

Objects tend to maintain their state of motion. This tendency is known as inertia. All objects resist changes in motion, so all objects have inertia.

How does the mass of an object relate to inertia?

Small mass = less inertia

Large mass = more inertia

Newton’s first law is sometimes called the law of inertia.

This law describes what happens when the net force acting on an object is zero

Newton’s Second Law

Newton’s second law describes the effect of an unbalanced force on the motion of an object

Force equal mass times acceleration

This law states that the unbalanced force acting on an object equals the object’s mass times its acceleration.

Force = mass x acceleration F = ma

Force is measured in newtons. This is the SI unit for force (N)

1N = 1kg x 1m/s²

Key Terms

Free Fall

Terminal Velocity

Projectile Motion

Law of Universal Gravitation

Sir Isaac Newton came up with a law that helps explain why objects fall towards the Earth.

This law states that all objects in the universe attract each other through gravitational force.

This equation says that the gravitational force increases as one or both masses increase and the force decreases as the distance between the masses increase.

All matter is affected by gravity no matter how big or how small

Gravitational force increases as mass increases

Gravitational force decrease as distance increases

Gravitational force is weaker than other types of forces, even though it holds the planets, stars, and other galaxies together

Free Fall and Weight

When gravity is the only force acting on an object, the object is said to be in free fall.

Free fall is the motion of a body when only the force of gravity is acting on the body.

Where is the acceleration of free fall directed?

Towards the center of the Earth.

Free fall acceleration near Earth’s surface is constant

Without air resistance, all objects on Earth accelerate at 9.8m/s²

Weight is equal to mass times free fall acceleration

The force on an object due to gravity is called weight

Why do astronauts appear to be weightless in space?

Because the astronauts and their surroundings all accelerate at the same rate.

Weight is different from mass

Mass is a measure of the amount of matter in an object

Weight is the gravitational force an object experiences because of mass

Mass remains the same everywhere, but weight changes as the gravitational force acting on an object changes.

Free Fall and Motion

Why is it incorrect to say that skydivers are free falling before the open their parachute?

Because air resistance is a force, free fall can occur only where there is no air

Projectile Motion and Gravity

The orbit of a space shuttle around Earth is an example of projectile motion

Projectile motion is the curved path that an object follows with thrown, launched, or otherwise projected near the surface of Earth

Projectile motion has some horizontal motion and vertical motion.

Key Terms

Momentum

Action and Reaction Forces

Newton’s third law says that for every action force, there is an equal and opposite reaction force.

If you kick a soccer ball the action force is your foot hitting the ball and the reaction force is the ball hitting your foot.

Forces always occur in pairs

Do action and reaction force occur when there is no motion?

Yes. When you sit on a chair, your weight pushes down on the chair (action force), and the chair pushing back up (reaction force)

Action and reaction forces occur at the same time

Equal forces don’t always have equal effects

Momentum

Momentum is a property of all moving objects.

Momentum is a quantity defined as the product of the mass and velocity of an object

Momentum = mass times velocity

(p = mv)

The more mass an object has, the greater its momentum. Also the more velocity an object has, the greater its momentum

If an object is not moving, its momentum is zero

Like velocity momentum has direction. The direction is the same as its velocity.

Force is related to change in momentum

If the time period of the momentum’s change becomes longer, the force need to change the momentum is less.