Forces & Newton’s Laws of Motion

Sir Isaac Newton (1642-1727)English scientist and

mathematician famous for his discovery of the law of gravity and

the three laws of motion.

He published them in his book Philosophiae Naturalis Principia

Mathematica (mathematic principles of natural philosophy)

in 1687.

Today these laws are known as Newton’s Laws of Motion and

describe the motion of all objects on the scale we experience in

our everyday lives.

What is FORCE?

• Force- Ability to change motion(push or pull)

• Force is not a thing in itself, but rather an interaction between two objects.

• Forces always occur in pairs!!!

– Units of lb, N = kg.m/sec2

Newton’s First Law

An object at rest tends to stay at rest and an object in motion tends to stay

in motion unless acted upon by an unbalanced force.

“The Law of Inertia”

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.

It takes force to change the motion of an object. Objects do not accelerate unless a net force is applied.

• NET FORCE: Determined by combining ALL forces acting on an object.

– Zero net force = zero acceleration (BALANCED FORCES in EQUILIBRIUM)

– If there is a net force, there will be an acceleration (UNBALANCED FORCES will MOVE)• The acceleration depends directly

upon the “net force” and inversely upon the object’s mass.

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 state of 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).

More Examples from Real LifeA powerful locomotive begins to pull a long line of boxcars that were sitting at rest. Since the boxcars are so massive, they have a great deal of inertia and it takes a large force to change their motion. Once they are moving, it takes a large force to stop them.

On your way to school, a bug flies into your windshield. Since the bug is so small, it has very little inertia and exerts a very small force on your car (so small that you don’t even feel it).

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.

In outer space, away from gravity and any sources of friction, a rocket ship launched with a certain speed and direction would keep going in that same direction and at

that same speed forever.

Equilibrium

• Balance: When in motion, an object has a constant velocity if it is in equilibrium.

• There is NO acceleration.• Types of Equilibrium: Static & Dynamic

– STATIC: velocity is zero• Example: weighing yourself on a scale

– Scale pushes up, your weight pushes down

– DYNAMIC: velocity is nonzero and constant• Example: driving at constant velocity

– Force from road and friction resisting, normal up and weight down forces balance

Mass vs. Weight

MASS• The amount of matter in

an object• A constant property• A measure of inertia• Measured in kilograms

(kg) or grams (g)

WEIGHT• The force upon an object

due to gravity• Weight = mass x

acceleration due to gravity

• Fw = mg

• Measured in Newtons

Practice: The weight of a 10 kg brick is…A. 100 N B. 10 kg C. 10 N

LOCATION MASS WEIGHT

Earth 10 kg 100 N

Moon 10 kg 16.6 N

Space 10 kg 0 N

On the moon, the force of gravity is only 1/6 as strong as on the Earth. In space, you are practically weightless but your mass remains unchanged.Your mass does NOT depend on where you are.

Newton’s Second Law

Force equals mass times acceleration.

F = ma

Acceleration: a measurement of how quickly an object is changing speed.

What does F = ma mean?

Force is directly proportional to mass and acceleration. Imagine a ball of a certain mass moving at a certain acceleration. This ball has a certain force.

Now imagine we make the ball twice as big (double the mass) but keep the acceleration constant. F = ma says that this new ball has twice the force of the old ball.

Now imagine the original ball moving at twice the original acceleration. F = ma says that the ball will again have twice the force of the ball at the original acceleration.

More about F = ma

If you double the mass, you double the force. If you double the acceleration, you double the force.

What if you double the mass and the acceleration?

(2m)(2a) = 4F

Doubling the mass and the acceleration quadruples the force.

So . . . what if you decrease the mass by half? How much force would the object have now?

What does F = ma say?F = ma basically means that the force of an

object comes from its mass and its acceleration.

Something very small (low mass) that’s changing speed very quickly

(high acceleration), like a bullet, can still have a great force. Something

very small changing speed very slowly will have a very weak force.

Something very massive (high mass) that’s changing speed very slowly (low acceleration), like a glacier, can still have great force.

Free Fall• When the

acceleration is g we have Free Fall

• Objects are not encountering a significant air resistance

• Under these conditions, all objects will fall with the same rate of acceleration regardless of their mass.

Terminal Velocity• Air resistance increases with speed.• When falling, the force of air resistance becomes large

enough to balance the force of gravity.• At this instant in time, there is no net force – the object

stops accelerating (see D below); terminal velocity has been reached.

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.

Think about it . . .

What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force.

Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on

your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe

hurts).

ReviewNewton’s First Law: Objects in motion tend to stay in motion

and objects at rest tend to stay at rest 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.

Friction• The force that opposes the motion

between two surfaces that are in contact is FRICTION.

• The “evil monster” of all motion. Regardless of which direction something moves in, friction pulls it the other way.– Move something left, friction pulls

right. Move something up, friction

pulls down. It appears as if nature

has given us friction to stop us from

moving anything.

• Friction is actually a force that “appears” when there is relative motion between two objects.– Although two objects might look smooth,

microscopically, they’re very rough and jagged.

• Types of Friction:– Air friction- (air resistance)– Sliding friction (rub hands together)– Viscous friction- (oil in car engines & joint

fluid)– Rolling friction- (wheel on road, ball bearings)

• STATIC (starting) FRICTION: the force that opposes the start of the motion– Static means “stationary”

• KINETIC (sliding) FRICTION: the force between surfaces in relative motion

Pressure

• The amount of force applied over a given area.• Pressure = Force ÷ Area• P = FA ÷ A

• Measured in Pascals (Pa)• Earth’s atmosphere (air pressure) is pressing

against each square inch of you with a force of 1 kilogram per square centimeter (14.7 pounds per square inch).– The force on 1,000 square centimeters (a little larger

than a square foot) is about a ton!

How much pressure are you under?

What happens if air pressure changes? If you've ever been to the top of a tall mountain, you may have noticed that your ears pop and you need to breathe more often than when you're at sea level. As the number of molecules of air around you decreases, the air pressure decreases. This causes your ears to pop in order to balance the pressure between the outside and inside of your ear. Since you are breathing fewer molecules of oxygen, you need to breathe faster to bring the few molecules there are into your lungs to make up for the deficit.