Newton’s Laws of Motion

1st - Inertia.2nd - F = ma3rd - Action/Reaction

Newton’s First Law

An object at rest will remain at rest and an object that is moving at constant velocity will continue

moving at constant velocity unless acted upon by an unbalanced force.

What does this mean?

Basically, an object will “keep doing what it was doing” unless acted on by an unbalanced force.

If the object was sitting still, it will remain stationary. If it was moving at a constant velocity, it will keep moving at a constant velocity.

It takes force to change the motion of an object.

Some Examples from Real Life

Two teams are playing tug of war. They are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion.

A soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its motion.

Newton’s First Law is also called the Law of Inertia

Inertia: the tendency of an object to resist changes in its motion

The First Law states that all objects have inertia. The more mass an object has, the more inertia it has (and the harder it is to change its motion).

So, which has more inertia? A bowling ball or a baseball?

Newton’s 1st Law and You

Don’t let this be you. Wear seat belts.

Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 km/hour.

If objects in motion tend to stay in motion, why don’t moving objects keep moving

forever?

Things don’t keep moving forever because there’s almost always an unbalanced force

acting upon it.

A book sliding across a table slows down and stops because of the force of friction.

If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

Newton’s Second Law

Force equals mass times acceleration.

F = ma

What does F = ma say?F = ma means that the force acting on an object

is equal to the product of its mass and acceleration.

Something very small (low mass) that has high acceleration, like a bullet, still has a great force acting on it.

Something very massive (high mass) that’s accelerating very little, like a glacier, has a great force acting on it.

2nd Law (F = m x a)

How much force is needed to accelerate a 1400 kg car 2 m/s2?

• Write the formulaF = m x a

• Fill in given numbers and unitsF = 1400 kg x 2 m/s2

• Solve for the unknown

2800 kg-m/s2 or 2800 N

mFa

If mass remains constant, doubling the acceleration, doubles the force. If force remains constant, doubling the mass, halves the acceleration.

Newton’s Third Law

For every action there is an equal and opposite reaction.

What does this mean?

For every force acting on an object, there is an equal force acting in the opposite direction.

Right now, gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing

up against you with equal force.

This is why you are not moving. There is a balanced force acting on you– gravity pulling

down, your seat pushing up.

Action: Your foot pushes on skateboard

Reaction: The skateboard pushes on your foot.

Remember:•There are always 2 forces

•They are always equal

•They act on different objects

Action and Reaction forces act on DIFFERENT OBJECTS!

ReviewNewton’s First Law:

An object at rest will remain rest and an object that is moving at a constant velocity will continue moving at a constant velocity unless acted upon by an unbalanced force.

Newton’s Second Law:

Force equals mass times acceleration (F = ma).

Newton’s Third Law:

For every action there is an equal and opposite reaction.

Other Motion Concepts

Gravity

Freefall

Weight

Momentum

GRAVITY - attraction force between all masses

Newton’s universal law of gravitation: Every object in the universe exerts a gravitational attraction to all other objects in the universe

Gravity depends on the mass of the objects and the distance between the objects

What is Gravity?

The greater the mass, the greater the forceforce

The greater the distancedistance, the lessless the force

Acceleration on Earth due to gravity = 9.8 m/s/s or 9.8 m/s2

What is Gravity?

The rate of falling increases by 9.8 m/s every second.

Free FallA ball thrown horizontally will fall at the same rate

as a ball dropped directly.

• In air…– A stone falls faster

than a feather• Air resistance affects

stone less

• In a vacuum– A stone and a

feather will fall at the same speed.

Air Resistance Effects Free Fall

Free Fall– A person in free fall

reaches a terminal velocity of around 54 m/s

– With a parachute, terminal velocity is only 6.3 m/s

• Allows a safe landing

Terminal Velocity

Weight vs. mass

• Weight and mass are not the same.

• Mass is a fundamental property of matter measured in kilograms (kg).

• Weight is a force measured in newtons (N).

• Weight depends on mass and gravity.

Weight depends on mass and gravity

A 10-kilogram rock has the same mass no matter where it is in the universe. On Earth, the10 kg. rock weighs 98 N.. On the moon, the same rock only weighs 16 N.

Weight is a measure of the gravitational force between two objects

The greater the mass the greater the force (weight)

Measured in units called Newtons (N)

Weightlessness – free from the effects of gravity

Momentum: The quantity of motion A property of moving objects Calculated by: P = mv (p = momentum, m = mass, v = velocity) Law of conservation of momentumLaw of conservation of momentum: the total

amount of momentum of a group of objects does not change unless outside forces act on the objects