Newton’s laws

NEWTON HIMSELF

Newton

Newton

Life

• Scholar– Brilliant mind– Became Head of Oxford Colleges in England– Royal Society of England– Master everything in his age

• Teacher– Help to develop whole branch of Mathematics– Newtonian physics

• Alchemist

LIMITATIONS OF HIS LAWS

Newton’s laws of motion

• Clearly laid down the rules that govern why things move in our world

• Rules can not fully explain motion on or smaller than the atomic level or movement close to the speed of light

Review

• Net force

• Balanced Force

• Unbalanced forces

NEWTON’S FIRST LAWLaw of Inertia

Rolling Ball

A bowling ball will move with nearly constant velocity

• No significant friction from the ground or air resistance to help change the bowling ball’s motion

• Velocity of ball changes only when it hits the pins or the back wall of lane

Newton’s first law

• An object at rest stays at rest

or an object in motion stays in motion

UNLESS

acted upon by a net outside force

How does the 1st law relate to you as you walk down the hall?

Newton’s first law

• Definition

Why should a baby not be transported on a car in a person’s

lap?

Would it be ok if you put the seat belt around you and the baby?

If Fluffy the Kitty was sleeping (without a seatbelt) in the rear

window well of a sedan…

Describe Fluffy’s motion if the car was involved in a head-on

collision?

How does one get whiplash?

• How does whiplash relate to 1st law?

1st law of motion

• An object at ______, stays at ______

• An object in motion, stays at a _______ rate

Unless

A net outside ________ acts upon it.

Trebuchet

NEWTON’S SECOND LAWThe equation law

Newton’s 2nd law

• If there is a net force on an object, then the object will accelerate

• The amount of acceleration depends on the following relationship:

A = Fnet

m

2nd law

• A = Fnet / m or rearranged Fnet = ma

• Triangle set up to solve for different variables

• Units: acc m/s2

Fnet Nmass kg

2nd law can be represented using a triangle

Fnet = m a

Fnet

M A t

When Solving problems using 2nd law

• Fnet means net force or the sum of all forces on the object

• Force is measured in N (Newtons)

• Mass is measured in kg

• Acceleration is measured in m/s2

The greater amount of net force applied, the larger the

acceleration of a specific object

The greater the mass, more force is needed to produce the same acceleration

• An object will accelerate at a higher rate as more force is applied

• An object will accelerate if more mass is added to it

2nd law

• An object will change its motion only if a net force is applied to it (fnet ≠ 0)

If you apply the same force to roll each a long the floor, how will their

motion compare?

NEWTON’S 3RD LAW

Newton’s third Law

3rd Law

• For every _____________

There is an __________ but opposite

__________.

Newton’s Third Law

• For every action force, there is an equal reaction force applies in the opposite direction

What are Action-Reaction forces

• For any force applied to an object, the object will respond with a like force back

• Forces are equal in size• Forces that are applied to

different objects

• Examples in Picture?

Action and Reaction Forces

• Action Force: the force of most interest, initiates the action

• Reaction force: force applies in response to action force

Action and reaction forces

• Equal and opposite

• Applied to different objects

• Applied at same time

• May have different effects

If all forces are equal and opposite, how does anything

move?

Answer

• Though equal in size, the forces act on different objects

Force of ground on box

Force of box on ground

The effect of the action reaction forces may differ on an object.

• By first law, objects accelerate if there is a net force on them

• What other piece of information is required to determine the amount of acceleration?

If I throw a punch, why does my hand hurt?

Consider the guys hands pressing down on desk. What are the action /reaction

forces?

Action/Reaction force related to the hand

Action/Reaction force of the falling object

FINDING REAL LIFE EXAMPLES OF 3RD LAW

Examples of 3rd law

Actions that can be related to 3rd law in the classroom

APPLICATION OF NEWTON’S 2ND LAW

Determining the weight of an object

• Weight = force due to gravity

• F = ma (by 2nd law)

• F = mass (in Kg) times (-9.8)

• Determine mass by measurement on scale or conversion

Determining mass

• Convert from pounds to kg by:

=(weight in lbs) / 2.2

2nd Law problem set up

#1 Draw free body diagram

#2 Calculate as many of the forces on the FBD as possible

#3 What are the givens?

#4 what do you want?

#5 what equations will connect the givens to what you want

#6 solve and check

Using Newton’s laws

• use 2nd law and free body diagrams as tools to determine force, acceleration and mass values

• Combine with con acceleration equations to solve for all of the old variables

Finding Net force

• Net force is the sum of all forces on an object

• At the start only one direction will have unbalanced forces

• All forces are added, but forces in one direction are positive, and forces in opposite direction are negative

Net force on a helium balloon

• Assuming the balloon goes up, how would the net force on the balloon be determined?

• How would you determine the acceleration of the balloon?

Force of helium

Weight

• Fnet = Fhelium – Weight

• By 2nd law Fnet = ma

Example

• Bob is holding his very favorite stuffed dog (mass 0.35 kg).

• Mary decides that she wants it and tries to pull it away from Bob.

• If Sarah pulls horizontally to the left with 100 N of force and Bob pulls horizontally to the right with 120 N, what is the horizontal acceleration of the dog?

Helicopter problem

• A 4500 kg helicopter accelerates upward at 2.0 m/s2. What is the lift force exerted by the air on the propellers?

Elevator Problem

Elevator problem

• A elevator car from the disney ride “Tower of Terror” is lifted by a cable up to the top of the shaft. If the 1200 kg car is carrying 12 people with an average mass of 64.3 kg and is lifted by the cable with a force of 54000N, what is the acceleration of the car?

• How do you determine if the car is moving with constant acceleration?

info

• How many people =

• Average mass of each person =• Mass of car =• Acceleration upwards =

• Applied force needed to move the object upwards

DETERMINE VALUES USING CONSTANT ACCELERATION EQUATIONS AND 2ND LAW

Combination problem

• A 4500kg car moves down a level highway at 20 m/s. If 1300 N of force pushes it forward and a 650 N resistance (friction and air resistance combined) pushes backward on object, find the following…

– The acceleration of object– How fast object travels after 1 minute– How far it travels after 1 minute.

Sliding into base

Free Fall

• Any object that moves under the influence of only the force due to gravity

• Thrown, Dropped objects

• Acceleration = 10 m/s2 or (9.8)

• What is the velocity and acceleration value at the following points in time of a heavy object being thrown upwards at 5.0 m/s?

a) Just when it leaves the hand?

b) At the apex of its flight?

c) Just before it is grabbed by the hand?

Advanced 2nd law problems

#1 Have to use the acceleration equation first then use 2nd law

#2 Have to determine Fnet first, then use 2nd law

Example

• Nicole slows down in her BMW from 15 m/s to 3 m/s in 42 seconds. What is the mass of her car if the net force on it is 7312 N?

Example

• A small car has a mass of 957 kg rests on a level icy patch of road. If a person can apply 500 N of horizontal force, what will be the car’s acceleration?

USING 2ND AND 3RD LAWS TO SOLVE QUESTIONS

Connections

• How does first law explain the change of direction of the ball when hit by the bat?

• Concerning bat and ball, what are the action and reaction forces?

Which undergoes a greater change of velocity, ball or bat?

What is true about the action and reaction forces between

ball and bat?

Draw free body diagrams of the ball and bat

Are both action and reaction forces effecting the ball?

Are the masses of the ball and bat the same?

If the same force was applied to both ball and bat would it have the same effect?

Third law problems

• Interaction of forces

• Two boxes are placed one on top of the other on a table

• What can you say about the applied forces

3rd law problems

• Boxes and string problems

Third law problems

• Stacked Box problem

• What is the force on the top box?

• What is the force on the table?

• What are the forces on the middle box?

Third law

• Accelerating boxes with ropes

• What is true about the acceleration of both boxes?

• How do the forces on the ropes compare to each other?

Rope problems

• Look at the whole , look at the parts

• Acceleration of all parts is constant

Pulley problems

• Pulley is a simple machine that, at minimum, changes the direction of the force

• 2 blocks starting from rest. What is the magnitude of the acceleration of either?

Pulley problems

• Pulley is a simple machine that, at minimum, changes the direction of the force

• The tension on the rope must be the same throughout

• Ropes do not stretch or elongate in our questions

• 2 blocks starting from rest. What is the magnitude of the acceleration of either?

AUTO ACCIDENTS AND NEWTON’S LAWS

Auto Accidents

Have you been in an auto accident?

In 2004, there were over 42,000 deaths involving automobiles and between 3-6 million injuries in the

United States

There may be as many as 750,000 million deaths and 24-36 million

injuries resulting from auto-accidents worldwide per year

What are the ways people get hurt if they inside the autos during the accident?

• Be thrown through windshield

• Crushed

• Injured by steering wheel, or dashboard

• Injured by loose material in car

• Injured by objects outside of cars

Frontal accidents

• Which way does the person travel after impact?

• How about stuff on the seat?

• How about stuff in the person’s hands?

Can you explain, using Newton’s Lawswhy the face is against the air bag instead of the

headrest?

Rear accidents

• Which way does a person’s head go upon impact from behind?

• What would happen to the head without a headrest?

• Should the airbag deploy in a rear-end collision?

How does speed relate to auto accidents?

At low speeds

At somewhat higher speeds

Collisions and the laws

• Be able to use the laws of motion to explain what happens in different types of collisions

• Front crash• Rear crash• At least 2 reasons for head rests• Does the bus and the car exert the same

amount of force on each other?• Why are people less injured on the bus?

Collisions and the laws

• Seat-belt configurations

• What is wrong with just lap-belts?

• Why are seat-belts flat and wide?

• What are uses for EKG’s, MRI’s and Xrays?

• Newton’s second law

• Triangle

• Metric units of force

Wrap up of forces

• Any questions about the material over the last 2 weeks

• Questions on seat belt paper

• Wall questions

• Yellow booklet (due Wednesday) pages: – 2,3,14-19 (only pre-lab questions on page 19), 21,22,45

To review

• Chapters 10,11 (mostly 11)

• What is a force? Types of forces?• What are Newton’s 3 Laws of motion?• Examples of the 3 laws• Free-body diagrams• Calculating with the 2nd law• Gravity, Weight, Black holes

Newton’s 3 laws

• !st

• 2nd

• 3rd

• When does an object change its motion (accelerate)?

Newton’s 3 laws

• 1st: Things will continue move or not move as they have unless a net external force is applied to them

• 2nd: Fnet = ma

• 3rd: For every action force, there is an equal and opposite reaction force