Newton's Laws of Motion

Newton’s First Law of Motion

An object at rest wants to stay at rest. An object in motion wants to stay in motion, until acted on by an unbalanced force.

Inertia

Resistance to change in motion

Relationship of Inertia and Mass

The more massive object has a greater inertia

Newton’s Second Law of Motion

F=ma; Mass resists acceleration. Force causes acceleration.

If you were to increase a car’s acceleration, what would you change to it?

Decrease the amount of mass

If you were to decrease a car’s acceleration, what would you change to it?

Increase the amount of mass

Acceleration is measured in…

m/s²

Mass is measured in

kilograms (kg)

Weight is measured in

Newtons (N)

Derive the equation for Newton’s second law that is written to solve for acceleration. Use the variables a, F, and m.

a=F/m

A book sits on the shelf. Which of Newton’s Laws of Motion best describes why the book doesn’t fall?

The Third Law. If the shelf did not push upward on the book with equal force, the books would fall because it is an unbalanced force.

What is Newton’s Third Law of Motion?

If an object exerts a force on another object, then the second object exerts a force of equal strength and in the opposite direction on the first object.

An action force is always paired with…

A reaction force

When you walk, you push backward on the ground with your feet. What is the reaction force?

The ground pushes forward on your feet with an equal and opposite force.

Unbalanced force

Net force does not equal zero. It causes motion.

Balanced force

Net force = 0. It causes equilibrium.

Why aren’t action-reaction forces in Newton’s third law of motion balanced?

In Newton's third law, action-reaction forces are not balanced because they act on different objects. Balanced forces must act on the same object.

When you push against a wall (action), the wall pushes back with an equal force (reaction). Why aren’t the forces balanced?

These forces are equal in magnitude and opposite in direction, but since they act on different bodies, they do not cancel each other out.

If a soccer player kicks a ball, why does the ball accelerate and not the player?

While the ball has a reaction force on the player, it has a lower mass so we can see it accelerate. (Newton Laws of Motion 2 & 3)

Forces come in…

pairs

F=ma can be written also…

1N=1kg * m/s²

Friction

a contact force that two surfaces exert on each other when they rub against each other.

Gravity pulls down 1kg by how many Newtons?

9.8N=1kg

Gravity

a force that pulls objects towards each other as a result of their masses

Acceleration

the rate at which velocity changes in speed, direction, or both