Introduction to Newton’s Laws

Are heavier objects harder to stop when moving than light objects?

Why do you move to the side of a car when the car turns a corner?

Back in the day…Aristotle tried to answer the question of why objects fall to Earth? By explaining that objects would seek their “natural position.”

-This also explain why Earth didn’t move.It was already in its natural resting place at the center of the universe.

This was, until Copernicus

came along and explained that the Earth orbited the sun.

This was later backed up by Galileo and Newton when they explained forces and motion in the universe.

Comparison between Aristotle and GalileoComparison between Aristotle and Galileo

ARISTOTLEARISTOTLE• Things at rest were at Things at rest were at

their natural resting their natural resting place.place.

• Objects that are Objects that are moving away from moving away from their natural resting their natural resting place need to be place need to be continuously forced to continuously forced to keep them movingkeep them moving

GalileoGalileo• Objects at rest stay at Objects at rest stay at

rest until something rest until something forces them to do forces them to do otherwise.otherwise.

• Objects in motion stay Objects in motion stay in motion unless in motion unless something forces it to something forces it to do otherwisedo otherwise

Newton’s LawsWhat are Newton’s 3 Laws of motion?

• 11stst Law Law – – An object at rest will stay at rest, An object at rest will stay at rest, and an object in motion will stay in motion and an object in motion will stay in motion at constant velocity, unless acted upon by at constant velocity, unless acted upon by an unbalanced force.an unbalanced force. ( (Law of InertiaLaw of Inertia))

• 22ndnd Law – Law – Force equals mass times Force equals mass times acceleration.(acceleration.(F = maF = ma))

• 33rdrd Law – Law – For every action there is an For every action there is an equal and opposite reaction.equal and opposite reaction. ( (All forces All forces come in pairscome in pairs))

Newton’s 1st LawLaw of Inertia

An object at rest will stay at rest, and an An object at rest will stay at rest, and an object in motion will stay in motion at constant object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.velocity, unless acted upon by an unbalanced force.

What does this all mean?

VolumeVolume is the amount space an object takes up. is the amount space an object takes up. • NotNot what gives an object inertia. what gives an object inertia.

What gives an object Inertia?

WeightWeight is the affect that gravity has on an object. is the affect that gravity has on an object.

•NotNot what gives an object inertia. what gives an object inertia.

MassMass is the amount of matter an object possesses. is the amount of matter an object possesses.

(the amount of “stuff” something is made of)(the amount of “stuff” something is made of)•This This isis what gives an object inertia. what gives an object inertia.

InertiaInertia is an objects ability to resist a change in motion.

Mass is directly related to Inertia. The more mass you have, the more inertia you have.

Some Inertia Demos

Newtons’s 1Newtons’s 1stst Law and You Law and You

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

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

If Newton’s 1st law is true:

Why then, do we observe every day Why then, do we observe every day

objects in motion slowing down and objects in motion slowing down and

becoming motionless seemingly becoming motionless seemingly

without any outside influence?without any outside influence?

Forces!

Friction is a type of force. It is a force that opposes motion.

•Acts in the direction opposite motion

A force is a “push or pull.”

-force is a vector quantity (it has a size and direction)

In the absence of friction, an object you In the absence of friction, an object you

push (like a book on a table top) would continue in push (like a book on a table top) would continue in

motion with the same speed and direction - forever!motion with the same speed and direction - forever!

(Or at least to the end of the table top.) (Or at least to the end of the table top.)

Up until this point, we have only been dealing with objects at rest or moving with a constant velocity.

Can we describe the motion of objects that don’t move at a constant velocity?

Yes!!!

Things that aren’t moving at a constant velocity are accelerating.

Acceleration ReviewAcceleration is how much velocity changes in a certain amount of time.

Acceleration is a vector quantity.

The units for acceleration are meters per second per second, or m/s2

Newton’s 2nd Law

F = maThe net force of an object is equal to The net force of an object is equal to the product of its mass and the product of its mass and accelerationacceleration

Newton’s 2nd LawIf F = ma then the units for force are kgm/s2

We call kgm/s2 Newtons (N)

How much force is needed to accelerate a 1400 kilogram car 2 meters per second/per second?

Another form of F = ma

We can write this as

What does this mean in English?

What is a net force?

The sum of all forces acting on an object in a given direction.

3 N 2 N 1 N

Unbalanced forces vs balanced forces

Forces are unbalanced when the net force on an object is not at equilibrium (or at zero).

Forces are balanced when the net force on a object is at equilibrium.

3 N 2 N 1 N

=

3 N 3 N

= No net force

So why does everything fall with the acceleration due to gravity?

Fm

Fm

a= a =

Big F over big mLittle F over little m

g=

9.8 m/s2

g =

9.8 m/s2

Newton’s 3rd LawFor every action there is an equal and opposite For every action there is an equal and opposite

reactionreaction

All forces come in pairsAll forces come in pairs

OR

Whenever an object exerts a force on a second object, the second object will exert an equal force in the opposite direction on the first object…

If “A” pushes on “B” then “B” also pushes on “A”

• Consider the propulsion of Consider the propulsion of a fish through the water. A a fish through the water. A fish uses its fins to push fish uses its fins to push water backwards. In turn, water backwards. In turn, the water the water reactsreacts by pushing by pushing the fish forwards, propelling the fish forwards, propelling the fish through the water.the fish through the water.

• The size of the force on The size of the force on the water equals the size of the water equals the size of the force on the fish; the the force on the fish; the direction of the force on the direction of the force on the water (backwards) is water (backwards) is opposite the direction of the opposite the direction of the force on the fish (forwards).force on the fish (forwards).

Can you think of other examples of Newton’s 3rd Law?

Horse and CartA horse can pull a cart with a force of 3120N. According to Newton’s 3rd, what is the force that the cart pulls back on the horse with?

If this is true, how can the horse ever move the cart if the forces cancel each other out?

Tug of WarHow much force does the bird on the right pull with if the bird on the left pulls on the worm with a force of 2 N and it wins the tug of war? What is the tension in the worm?

Free Body Diagram(Force Diagrams)

Lets draw a picture of the force of a man pushing on a Physics Box

15N

Free Body DiagramsWhen we draw forces acting on an object (in the form of a free body diagram), we should draw all the forces so that they are pulling from the “center of mass.”

15N

Other types of forcesNormal force is the force that is always perpendicular to the surface of what ever an object is sitting on. It generally opposes the downward force of gravity.

Weight is an object’s force due to gravity. They are interchangeable.

Gravitational force: Force due to gravity is the force that an objects exerts because of gravity. On Earth, all objects gravitational forces act downward toward the center of the planet.

Tension: only observed when an object is under the influence of a “pull.”

Friction: a force that opposes motion (retarding force)

When you sit in your When you sit in your chair, your body exerts chair, your body exerts a downward force on a downward force on the chair (the chair (your weightyour weight) ) and the chair exerts an and the chair exerts an upward force on your upward force on your body (known as the body (known as the Normal ForceNormal Force).).

Question:For each question, make sure to draw an accurate free body diagram!

What is the net force on a 15 kg box that is sitting on a table?

What is the net force on a 15 kg box that is being pulled to the left with 40 N of force and 28 N to the right?

Question:For each question, make sure to draw an accurate free body diagram!

What is the net force on a 15 kg box on a frictionless surface that is moving at a constant velocity of 4 m/s to the right?

What is the net force on a 15 kg box on a frictionless surface that is accelerating to the right at a rate of 3 m/s2?

Can you do this one?What is the acceleration of a 15 kg box sitting on a frictionless surface if you pull it from a rope to the right at an angle of 30 degrees from the horizontal?

For this one we need TRIGONOMETRY!

Draw a free body diagram of a 20 kg box that is being pushed to the right with a force of 50 N and there is a 30 N frictional force. What is the net force? What is the acceleration?

Question:

Types of frictionStatic Friction: The frictional force that exists between two surfaces when they are stationary

Kinetic Friction: The frictional force that exists between two surfaces when the objects are moving across each other (like a book sliding on a table)

Both of these types of friction can be calculated by using the coefficient of friction (either the coefficient of static friction or the coefficient of kinetic friction.)

Coefficient of FrictionA value that is different for any two surfaces.

- It is a constant that can be used to help calculate the force due to friction if you know the normal force of an object.

Ff = FN

Mu (like the pokemon)

is usually in between zero and one

Coefficient of FrictionThe coefficient of static friction tends to be greater than that of kinetic friction

Another way to think about what is, is how “sticky” the interaction between two surfaces is.

- The greater the , the greater the friction.

QuestionsIf the coefficient of friction between your tires and the road is 0.2, and the car has a mass of 1700 kg, what is the frictional force between the tires and the road?

QuestionsA car with a mass of 700 kg is breaking hard. If the car is traveling at an initial velocity of 15 m/s, and takes 3 seconds to stop, what is the coefficient of friction between the tires and the road? (draw a free body diagram)

Here’s a doozyIf this 15 kg box is sliding down the hill at a constant velocity, what is the frictional force between the box and the incline?

30oF

Fg

F

Ff

FN

Thought questionWhy is it impossible to make a piece of string completely horizontal and straight if you hang a mass from the center?