I. Law of InertiaII. F=ma

III. Action-Reaction

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. 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.

Background

“If I have ever made any valuable discoveries, it has been owing more to patient attention, than to

any other talent.”-Sir Isaac Newton

While most people know what Newton's laws say, many people do not know what they mean (or simply do not believe what they mean).

11stst Law of Motion Law of Motion (Law of Inertia) (Law of Inertia)

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

1st LawInertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless. These pumpkins will not move unless acted on

by an unbalanced 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.

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.

1st Law Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction), it would never stop!

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).

1st Law

Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

Why then, do we observe every day objects in motion slowing down and becoming motionless seemingly without an outside force?

It’s a force we sometimes cannot see – friction.

Objects on earth, unlike the frictionless space the moon travels through, are under the influence of friction.

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.

Newton’s Laws of MotionNewton’s Laws of Motion

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

22ndnd Law Law – Force equals mass times acceleration.

33rdrd Law Law – For every action there is an equal and opposite reaction.

Newton's 3 Laws of Motion

There are four main types of friction:Sliding friction: ice skatingRolling friction: bowlingFluid friction (air or liquid): air or water

resistanceStatic friction: initial friction when moving an

object

What is this unbalanced force that acts on an What is this unbalanced force that acts on an object in motion?object in motion?

Slide a book across a table and watch it slide to a rest position. The book comes to a rest because of the presence of a force - that force being the force of friction - which brings the book to a rest position.

In the absence of a force of friction, the book would continue in motion with the same speed and direction - forever! (Or at least to the end of the table top.)

Newtons’s 1st 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 m/hour.m/hour.

11-1 Review1. Explain the difference between balanced and unbalanced forces.

2. What is the relationship between force and motion described by Newton’s First Law.

3. What is inertia? How is the inertia of an object related to its mass?

4. Once a baseball has been hit in the air, what forces are acting on it? How can you tell that any forces are acting on the ball?

5. A ball is at rest on the floor of a car moving at a constant velocity. What will happen to the ball if the car swerves suddenly to the left?

6. What can the changes in an object’s position tell you about the forces acting on the object? Describe an example of everyday life that shows how forces affect the position of an object.

11-2 Notes

Newton’s Second LAW

http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l1a.html

2nd Law

The net force of an object is equal to the product of its mass and acceleration, or F=ma.

2nd Law

2nd Law

When mass is in kilograms and acceleration is in m/s/s, the unit of force is in newtons (N).

One newton is equal to the force required to accelerate one kilogram of mass at one meter/second/second.

Newton=kgm/s/s

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

2nd Law (F = m x a)How much force is needed to accelerate a

1400 kilogram car 2 meters per second/per second?

Write the formulaF = m x aFill in given numbers and unitsF = 1400 kg x 2 meters per second/secondSolve for the unknown2800 kg-meters/second/second or

2800 N

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

remains constant, doubling the mass, halves the acceleration.

Greater Force=Faster Acceleration(IF Mass is constant)

Greater Mass= Slower Acceleration

(IF Force is constant)

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

Falling Objects ONLY

Acceleration due to gravity = 9.8m/s/s

Newton’s 2nd Law proves that different masses accelerate to the earth at the same rate, but with different forces.

• We know that objects with different masses accelerate to the ground at the same rate.

• However, because of the 2nd Law we know that they don’t hit the ground with the same force.

F = maF = ma

98 N = 10 kg x 9.8 m/s/s98 N = 10 kg x 9.8 m/s/s

F = maF = ma

9.8 N = 1 kg x 9.8 9.8 N = 1 kg x 9.8 m/s/sm/s/s

Check Your Understanding

1. What acceleration will result when a 12 N net force applied to a 3 kg object? A 6 kg object?

 2. A net force of 16 N causes a mass to

accelerate at a rate of 5 m/s2. Determine the mass.

3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?

4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?

Check Your Understanding 1. What acceleration will result when a 12 N net force applied to a 3

kg object? 12 N = 3 kg x 4 m/s/s

  2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2.

Determine the mass. 16 N = 3.2 kg x 5 m/s/s

  3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?

66 kg-m/sec/sec or 66 N

4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?

 9800 kg-m/sec/sec or 9800 N

1. A linebacker runs towards a Quarterback who is standing in the pocket looking for a receiver. The linebacker tackles the stationary quarterback in the pocket. What is the linebackers acceleration if we know the Linebackers weight is 312kg and the force of the impact on the QB is 342 Newtons.

2. A 10.0 kg log was pulled along the ground (assuming with no friction) by a force of 200 N. What is the acceleration?

3. What is the force of a 6kg baby sitting in your lap?

5. A 15.0 kg mass pulled along a frictionless surface by a horizontal force of 100 N will have what acceleration?

6. An object sits on a frictionless surface. There is a 16 N force applied to an object parallel to the surface and its acceleration is at 2m/s/s. What is its mass?

7. What is the force due to gravity of a hamburger sitting on a table? Assume that each bun weighs 1/4 kg and the beef weighs 1/2 kg.

8. A constant force is applied to an object, causing the object to accelerate at 8.0m/s^2. What will the acceleration be if the objects mass is doubled?

9. What is the force needed to pull a car that weighs 67,500 kg at an acceleration of 5m/s^2?

10. Suzan pulled a wagon that weighed 5 kg along the street at a force of 100 N, what is the acceleration?

11. Jose pushes a boulder that weighs 60 kg and exerts a force of 60 N. What is the acceleration of the boulder?

12. A hockey puck is sent flying over the ice. If the air resistance and friction of the hockey puck are ignored, what is the force required to keep that puck moving at a constant velocity?

13. A 32.1 kg log is rasied off the ground with a rope. The acceleration of the gravity is 9.8m/s^2. If the upward acceleration of the log is 3.9m/s^2, find the force excerted by the rope on the log. Please use units of newtons.

14. If you have a box that is pulled at an acceleration of 10 m/s^2 with a force of 98 N. What is the box’s mass?

3rd Law

For every action, there is an equal and opposite reaction.

3rd LawAccording to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.

3rd Law

There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces.

Newton’s 3rd Law in NatureEx: Water and

Swimming

3rd Law

Flying gracefully Flying gracefully through the air, through the air, birds depend on birds depend on Newton’s third Newton’s third law of motion. As law of motion. As the birds push the birds push down on the air down on the air with their wings, with their wings, the air pushes the air pushes their wings up their wings up and gives them and gives them lift.lift.

Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. In turn, the air reacts by 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).

Action-reaction force pairs make it possible for birds to fly.

Other examples of Newton’s Third Law

The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).

3rd LawConsider the motion

of a car on the way to school. A car is equipped with wheels which spin forward. As the wheels spin forward, they grip the road and push the road backwards.

3rd LawThe reaction of a rocket The reaction of a rocket is an application of the is an application of the third law of motion. third law of motion. Various fuels are burned Various fuels are burned in the engine, producing in the engine, producing hot gases. hot gases.

The hot gases push The hot gases push against the inside tube of against the inside tube of the rocket and escape out the rocket and escape out the bottom of the tube. the bottom of the tube. As the gases move As the gases move downward, the rocket downward, the rocket moves in the opposite moves in the opposite direction.direction.

11-4 Notes - MomentumA moving object has

a property that is called momentum.

Momentum is a measure of mass in motionIt is the product of

mass and velocity.

Momentum = P

Mass = mVelocity = v

FORMULA:p= mv

UNITS:Momentum –

o kg x m/sVelocity – m/sMass - kg

Momentum is also like a vector. It has both size and direction.

CollisionMomentum can

be transferred from one object to another.

A collision is a situation in which two objects in close contact exchange energy and momentum.

Conservation of MomentumMomentum is also

conserved!So momentum is not lost

in a situation it is simply conserved or transferred to the other object.

Conservation of momentum states that total momentum of a system of objects does not change as long as no outside forces are acting on that system.

QuestionsLets think about

some questions!!1. If speed goes

up, what does momentum do ??

2. A car and a tank are traveling at the same speed. Which has more momentum and why?

Questions1. A car has a

velocity of 15 m/s and a mass of 10 kg. What is the momentum?

2. Chris has a plane that travels at 20 m/s and has a mass of 20000 kg. What is the momentum?

3. Todd’s boat has a mass of 100 kg and has a velocity of 10 m/s. What is the momentum?

4. Bill’s truck has a mass of 100 kg and a momentum of 100 kg x m/s. What is the velocity?

Questions5. Tony can run

at 5 m/s with a momentum of 15 kg x m/s. What is the mass?

6. Charles has a momentum of 15 kg x m/s and a velocity of 5 m/s. What is his mass?

7. Chris has a truck with a mass of 500 kg. It also has a momentum of 50 kg x m/s. What is the velocity?

8.