Newton’s Laws of Motion

Newton’s Laws of Motion

• Sir Isaac Newton (1643-1727) an English scientist and mathematician famous for his discovery of the law of gravity also discovered the three laws of motion.

• 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.

• Remember: Technology is limited by laws of nature such as these!!!

Newton’s Laws of Motion

Law 1: An object in motion tends to stay in motion and an object at rest tends to stay at rest unless acted upon by an unbalanced force.

Law 2: Force equals mass times acceleration (F = ma).

Law 3: For every action there is an equal and opposite reaction.

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.

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

Some Examples of the First Law

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.

• A force is viewed as a push or a pull, something that changes the motion of an object.

• Whenever there is an interaction between two objects, there is a force upon each of the objects.

• Forces can result from two kinds of interactions.

1. Contact Interaction 2. Non Contact Interaction

What is a force?

Applied Force• An applied force is a contact force which is

applied to an object by a person or another object

• If a person is pushing a box across the room, then there is an applied force acting upon the object.

• The applied force is the force exerted on the box by the person.

Normal Force• The normal force is the support force

exerted upon an object which is in contact with another stable object.

• For example, if a physics book is resting upon a table, then the table is exerting an upward force upon the book in order to support the weight of the book.

Friction• Friction is a contact force exerted by a

surface as an object moves across it or makes an effort to move across it.

• Friction often opposes the motion of an object.

• For example, if a book slides across the surface of a desk, then the desk exerts a friction force in the opposite direction of its motion.

• Air resistance is a type of friction.

Non Contact Forces

• Non contact forces are those types of forces which result even when the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite their physical separation.

• Non-Contact forces generate a force field – the area influenced by the push or pull of the non contact force.

Gravitational Force• Gravity is a non contact force• ALL objects attract each other with a force of

gravitational attraction. • The standard formula for gravity is:

Gravitational force = (G * m1 * m2) / (d2)

• where G is the gravitational constant, m1 and m2 are the masses of the two objects for which you are calculating the force, and d is the distance between the centers of gravity of the two masses.

Gravitational Force

• More massive objects will attract each other with a greater gravitational force. – as the mass of either object increases, the

force of gravitational attraction between them also increases.

• more separation distance will result in weaker gravitational forces. – as two objects are separated from each other,

the force of gravitational attraction between them also decreases.

Gravitational Force• The sun and planets exert a gravitational

pull on each other despite their large spatial separation.

• Even when your feet leave the earth and you are no longer in physical contact with the earth, there is a gravitational pull between you and the Earth.

• All objects upon earth experience a force of gravity which is directed "downward" towards the center of the earth.

Electric and Magentic Forces• Electric and magnetic forces are non contact

forces. • The protons in the nucleus of an atom and the

electrons outside the nucleus experience an electrical pull towards each other despite their small spatial separation.

• Two magnets can exert a magnetic pull on each other even when separated by a distance of a few centimeters.

• Electric and magnetic forces are stronger than gravity but only work over very small distances

Net Force• If multiple forces act on an object, they could

potentially add or cancel, depending on direction• The net force is the sum of all forces acting on an

object.

Force #1Force #2

Total Force Force #1

Force #2

Total Force = 0

UNBLANCED FORCE

BALANCED FORCE

What is meant by unbalanced force?

• If the forces on an object are equal and opposite, they are said to be balanced, and the object experiences no change in motion.

• If they are not equal and opposite, then the forces are unbalanced and the motion of the object changes.

When two parallel forces are acting on the cart in the same direction,

the net force is the two forces added together.

When two forces are opposite and of equal magnitude, the net

force is zero.

When two parallel forces are not of equal magnitude, the net force is

the difference in the direction of the larger force.

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.

• Mass is a measure of the amount of inertia an object has.

• 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).

Another Example of the First Law

There is the parlor trick of quickly pulling a tablecloth from under a setting of heavy dishes, leaving them on the table.

Example of the First Law

• This trick works because the inertia of the heavy objects tends to keep them in place.

• By quickly pulling the tablecloth, the force of friction is easily overcome.

• If the tablecloth was pulled slowly, the friction would be greater than the inertia, and the dishes would follow along. – (Note: The tablecloth must be pulled down at the

edge, otherwise the dishes may fly upward.)

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.

Applications of the First law• Blood rushes from your head to your feet while quickly

stopping when riding on a descending elevator. • The head of a hammer can be tightened onto the wooden

handle by banging the bottom of the handle against a hard surface.

• To dislodge ketchup from the bottom of a ketchup bottle, it is often turned upside down and thrusted downward at high speeds and then abruptly halted.

• Headrests are placed in cars to prevent whiplash injuries during rear-end collisions.

• While riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb or rock or other object which abruptly halts the motion of the skateboard.

Newton’s Second Law

Force equals mass times acceleration.

F = ma

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

According to Newton's Second law...

• Acceleration is produced when a force acts on a mass.

• The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object).

• Everyone unconsciously knows the Second Law -- heavier objects require more force to move the same distance as lighter objects.

What is Acceleration?

• Speed = distance/time (m/s, km/hr, mph)• Velocity: Same as speed but includes

direction of travel• Acceleration = (change in velocity)/time

(m/s2)

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What is Acceleration?• In physics, all of these are considered

examples of acceleration– Speeding up – Slowing down– Changing direction!

• Examples of acceleration:– A car speeding up or slowing down while

traveling in a straight line– A car rounding a curve at a constant speed– A car speeding up or slowing down while

rounding a curve

Four different ways (A-D) to accelerate a car.

Acceleration due to Gravity• Gravity pulls objects toward

Earth  • Causes objects to move

faster and faster, thus accelerate              

• Velocity increases by about 10 m/s, for every second of fall 

• thus, Acceleration of Gravity (g) ~10 m/s2                  

Mass vs. Weight Weight is the effect of gravity on an object.• Weight is a Force (F = ma) • Weight = Mass Acceleration of Gravity (g)

– measured in Newtons (N) in the metric system or pounds (lb) in the British system

Mass is the quantity of matter in an object or the measurement of the inertia– measured in kilograms (kg) in the metric system or

slugs in the British system

Mass vs. Weight

• The weight of an object changes with location, the mass of an object does not.

• For example, your mass is the same on earth and on the moon but you weigh less on the moon because, due to its smaller size, the moon has a weaker pull of gravity.

So, what does F = ma mean?

• Imagine a ball of a certain mass moving at a certain acceleration. This ball has a certain force (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.

• Now imagine we make the ball twice as big

Examples of the Second LawF = 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.

Newton’s Third Law

For every action there is an equal and opposite reaction.

Newton's Third Law of Motion• Whenever two objects interact, the force

exerted on one object is equal in size and opposite in direction to the force exerted on the other object.

FA due to B = FB due to A

• In other words…….For every force acting on an object, there is an equal force acting in the opposite direction.

What does this mean?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).

Consider the flying motion of birds• A bird flies by use of its wings. The wings of a

bird push air downwards. Since forces result from mutual interactions, the air must also be pushing the bird upwards.

• The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards).

• For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for birds to fly.