Regents Physics

AgendaIntroduction to ForcesIntro to Newton’s three Laws of

MotionHW: Read p. 117-125

What’s a Force?

We’ve learned that acceleration is the change in an object’s velocity..

And Velocity is the change in an objects

position.. By what causes the change in

acceleration?A Force!

What’s a Force?

Forces can be described as a push or a pull that is applied to an object by something else..

Forces are vectors – magnitude and direction

The ability to understand how forces affect us is crucial to success in many fields Ex: building of homes and bridges

Kinds of Forces

Contact force – acts on an object only by touching it Ex: book on table, friction

Long-range force – is exerted without contact Ex: magnetic force, force of gravity

Forces have agents..

Each force has a specific, identifiable, immediate cause called the agent

The agent can be animate – such as a person

or inanimate – such as a desk, floor or

magnet

What’s the agent for the pull of gravity?

Solving Force Problems - intro

First step is to draw a pictorial model, called a free body diagram, and identify the contact and long range forces Draw the vectors Example

Book on a table

F desk on book

F book on desk

Solving Force Problems - intro

Examples

Ball on a rope Skydiver

F rope on ball

F ball on rope

F air on diver

F gravity on diver

Practice Problems

Draw a free body diagram for each of the following: Book held in your hand Book pushed across the desk by your hand Book pulled across the desk by a string Book on a desk with you hand pushing

down Ball just after the string that was holding it

broke

The man…Sir Isaac Newton

300 hundred years ago an apple fell on his head…and he wondered why?

Explained the way forces influence motion

Summed it up in three famous laws

2nd Law of Motion

The force exerted on an object is equal to the objects mass times its acceleration, or F = ma

Expressed in newtons = kg x m/s2

Example: Mr. O Mass = 95 kg Acceleration = gravity = 9.80 m/s2

F = ma = (95 kg)(9.80 m/s2) = 931N

2nd law continued

F = ma can be rearranged to be

we say that the force exerted on an object is proportional to its acceleration (since the objects mass doesn’t change)

The larger the force..the greater the acceleration We have a linear relationship!

a = Fm

2nd law continued

Multiple forces can combine and act on a system

They could act in the same direction or in different directions

Because forces are vectors, the total force on an object is the vector sum of all forces on the object

This vector sum is called the net force

Finding a net force

Two horizontal forces act, 225 N and 165 N, are exerted in the same direction on a crate (assume no friction). Find the net horizontal force on the crate.

Step 1 – draw a free body diagram

F = +225 N

F = +165 N

Fnet = Facting on the crate

Fnet = 225N + 165N = +390N

worksheet

Regents Physics

AgendaNewton’s first law of motion - IntroNewton’s second law practice probsHW Chap Problems:

RC #1,3,5,6,8,9AC # 10, 15, 16Probs # 20, 22, 25, 27, 30, 32, 36, 38, 40

1st Law of Motion

An object that is at rest will remain at rest or an object that is moving will continue to move in a straight line with a constant speed, if and only if the net force acting on that object is zero. Also called the Law of Inertial Mass

What does that mean to us?

What is inertia?

1st Law of Motion

Inertia is the tendency of an object to resist change

Anything that has mass has inertia!We resist what tries to change us!

ExamplesSitting still / moving at a constant velocityObject in spaceTurning in a car

What is equilibrium?

1st Law of Motion

If an object is at rest or it is moving at a constant velocity, the net force is zeroin both cases

We are happy just chillin..

Some Types of Forces

See table 6.2 pg. 123

Common Misconceptions

When we throw a ball, the force from our hand stays on it

A force is needed to keep an object moving

Inertia, itself, is a force Air does not exert a force

Calculations with Newton’s 2nd Law

Using F = ma Mass and weight..what’s the difference? Ex: a bathroom scale

Draw a freebody diagram for this and label the forces

A system of solving..

Read the problem and visualize! Choose a coordinate system Write your known and unknowns Use f = ma to link acceleration and net

force Rearrange, plug in numbers and solve Check your answer to see if it’s

reasonable

Practice Problem

On Earth, a scale shows that you weigh 585 N a) What is your mass? B) What would the scale read on the

moon? (g = 1.60 m/s2)

Regents Physics

Agenda More on Newton’s Second Law Review Intro to Forces Worksheet Drawing Free-body Diagrams Worksheet HW: More Advanced Newton’s Second

Law Problems

Practice Problem - elevator

You still weight 585 N. Find the reading on a scale in an elevator when: a) the elevator moves up at a constant speed b) it slows at 2.0 m/s2, while moving upward c) It speeds up while moving 2 m/s2 downward d) it moves downward at a constant speed e) it slows to a stop at a constant magnitude of

acceleration

End

Working with friction...

Friction is a force that generally opposes the direction of motion

Different surfaces have different amounts of friction that affect an objects movement

There are types of friction: Kinetic Friction = uk

Static Friction = us

Ffr = Fnu

Solving Problems with Friction Find the net force on an object that has a

mass of 20.0 kg, acceleration of 2.3 m/s2 on an surface with a friction coefficient of 0.21

Fn

Fa

Ffr

Fg

Fnet = Fa - Ffr

Fnet = Fa - Fnuk

Fnet = 46N - 41.2N = 5N

Known:Fn=Fg=mg=(20.0kg)(9.8 m/s2) = 196 N Fa = ma = (20.0kg)(2.3 m/s2) = 46NFfr = Fnuk = (196N)(0.21) = 41.2 N

Worksheet

3rd Law of Motion

Every force has an equal and opposite force,

or

FA on B = -FB on A

Fyou on wall = Fwall on you