Newton's Laws of Motion: Inertia, Forces, and Action-Reaction Pairs

Newton's 1st Law of Motion

An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Law of Inertia

Sometimes referred to as the Law of Inertia, it describes the tendency of objects to keep on doing what they're doing.

Unbalanced Force

The force that does not become completely balanced (or canceled) by the other individual forces.

Inertia

The resistance an object has to a change in its state of motion.

Newton's Second Law

The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

Acceleration Formula

a = Fnet / m

Net Force (Fnet)

The total (resultant) force; vector sum of all forces acting on an object.

Net Force Equation

Fnet = F1 + F2 + F3 +...

Direction of Net Force

The direction of the net force is in the same direction as the acceleration.

Practice Problem 1

Determine the accelerations that result when a 12-N net force is applied to a 3-kg object and then to a 6-kg object.

Practice Problem 2

A net force of 15 N is exerted on an encyclopedia to cause it to accelerate at a rate of 5 m/s². Determine the mass of the encyclopedia.

Practice Problem 3

An object feels two forces: one of strength 8 N pulling to the left and one of strength 20 N pulling to the right. If the object's mass is 4 kg, what is its acceleration?

Force

A push or a pull that acts upon an object as a result of its interaction with another object.

Newton's Third Law

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

Action-Reaction Force Pairs

Forces always come in pairs - equal and opposite action-reaction force pairs.

Size of Forces

The size of the forces on the first object equals the size of the force on the second object.

Direction of Forces

The direction of the force on the first object is opposite to the direction of the force on the second object.

Example of Action-Reaction

A bird flies by use of its wings. The wings of a bird push air downwards, and the air pushes the bird upwards.

Firefly and Bus Problem

A firefly strikes the windshield of a bus, illustrating Newton's third law of motion.

Rifle Recoil Problem

A rifle recoils when fired due to action-reaction force pairs; the bullet pushes backwards upon the rifle.

Acceleration of Recoiling Rifle

The acceleration of the recoiling rifle is smaller than the acceleration of the bullet.

Identifying Action-Reaction Force Pairs

Involves identifying the two interacting objects and making two statements describing who is pushing on whom and in what direction.

Baseball Bat and Baseball Example

The baseball forces the bat to the left; the bat forces the ball to the right.

Baseball Pushes Glove

Describing the other force in the action-reaction force pair: Bowling ball pushes pin leftwards.

Enclosed Air Particles

Enclosed air particles push balloon wall outwards.

Earth and Person Interaction

Earth pulls on person (gravity) and person pulls on Earth.

Tricky Wording

Finish the action-reaction statement: Earth pulls on person (gravity)... Person pulls on Earth.