November 4th 2009

Objectives SWBAT Define work SWBAT Calculate net work. SWBAT Calculate Kinetic Energy for an object

Catalyst What is the definition of work?

Agenda

Catalyst discussion WORK! Demo? Kinetic Energy Practice!

I need a volunteer

Hold the following text book at arm’s length in front of you

Is work being done to the text book?

NO!!!!!!

I need another volunteer Push the desk across the room. Is work being done to the desk?

YES!!!!

What is work?

Work is equal to the magnitude of force times the magnitude of displacement.

It tells us how much force is applied over a certain distance

W = Fd

We use “d” for displacement

How much work is done when your displacement equals zero?

Oh…that’s work?

Textbook and spring scale: Is a force being exerted on the book? Is work being done in the first situation?

If so, how much?

Is work being done in the second situation? If so, how much?

Ok…we have work

Work is only done by components of your force that are parallel to the displacement

If your Force is not parallel to your displacement, use the angle between your F and d to find work.

Wnet = Fnetd(cosΘ)

If there are multiple Forces use Net Force

d

F

Θ

Work = Newtons times meters (N * m)or Joules (J)

FYILifting an apple over your head is about 1 J of work.

3 push-ups is about 1000 J of work.

Last thing…

The sign of work is importantWork is a scalarWork is negative when the force is in the

opposite direction as displacementTheta = 180° when force is in completely

opposite direction Ex. How much work is friction doing on a box as it

slides across the table?

Problem How much work is done on a vacuum cleaner pulled 3.0m by a force of 50

N at an angle of 30.0° above the horizontal?

W= 130 J

Problem A 20 kg Suitcase is raised 3.0m above a platform by a conveyor belt. How

much work is done on the suitcase?

W= 590 J

Kinetic Energy

What is energy? The ability to do work The amount of work that can be done by a force

Kinetic Energy is the energy associated with an object in motion Kinetic energy is proportional to speed and mass Derivation!

Kinetic Energy

KE = ½mv2

Kinetic Energy = ½xmass x (speed)2

(Joules) (kg) (m/s)

If a bowling ball and a volley ball are traveling with the same speed which has more KE?

Problem A 7.00kg bowling ball moves at 3.00m/s. How much KE does the bowling

ball have? How fast must a 2.45 table-tennis ball move in order to have the same kinetic energy as the bowling ball? Is this speed reasonable for a table-tennis ball?

KE = 31.5JVt = 160m/s; not reasonable…

Practice

Pg170; 1, 2, 3, 4 Pg 171; 2, 3a, b, c, 4a, Pg174; 1, 2, 3, 4

But when they are unbalanced,

that’s when objects

accelerate

Forces are either in equilibrium or not in equilibrium

But when they are unbalanced,

that’s when objects

accelerate

Forces are either in equilibrium or not in equilibrium

But when they are unbalanced,

that’s when objects

accelerate

Forces are either in equilibrium or not in equilibrium

When an object is in equilibrium (acceleration = 0),

F = 0

If F = 0,then…

Fx = 0Fy = 0

Practice Finding the Net Force

Find the net force and tellthe direction that the velocity will change

Calculating net force in 2-Dimensions Rules:

Draw a free-body diagramSelect a coordinate system Find the x and y components of all vectors.

Gravity always acts at a 90° angle to earth.

Decide if the object is accelerating in either the x or y directions

Practice

Dafne leaves her physics book on top of a table that is inclined at a 35° angle. The table is pushing up on the book with a force of 18N, the Ffriction is 11N, and the Fgravity is 22N. Find the net external Force and determine if the book will remain at rest, or if it will slide down the table.

Example 2.

About 50 years ago the San Diego Zoo, in California, had the largest gorilla on Earth: its mass was 3.10x102kg. Suppose a gorilla with this mass hangs from two vines, each of which makes an angle of 30.0° with the vertical. Draw a free body diagram showing the various forces, and find the magnitude of the force (tension) in each vine.

Practice

Pg 133, 1, 2, 3, 4.

Force and Mass

A force is a push or pull on an objectForces will cause a change in velocity.Forces are vectors

They have magnitude and direction

Mass is a measure of the amount of matter in an object

2 Kinds of Forces

Contact Force – forces that arise from the physical contace of two objects

Field Forces – forces that exist between objects even in the absence of physical contact between the objects

Drop the ball…

Newton’s 1st and 2nd Law

An object at rest remains at rest as long as no net force acts on it

An object moving with constant velocity continues to move with the same velocity as long as no net force acts on it

1st Law – Law of Inertia 2nd Law

aFm

a=acceleration (m/s2)F=Force (N)M=mass (kg)

newtons

kilograms

This means sum

Newton’s 1st Law Demonstration

Which way will the water fall if the cup starts at rest and is pushed forward?

Which way will the water fall if the cup is in motion and is stopped?

Physics Book and pen example

Let’s think about it - T/W/P/S

What does acceleration equal if ΣF is 0?Acceleration is 0m/s2

What does 0 acceleration mean about velocity?Velocity is constant

How does this relate to Newton’s 1st Law?

Using Newton’s Laws

The net force acting on a Jaguar XK8 has a magnitude of 6800 N. If the acceleration is 3.8 m/s2, what is its mass?

Free-Body Diagrams

A Free-Body Diagram is a picture of all the forces acting on an object.

Read the free-body diagram section of the bookPages 127-128Discuss the steps for drawing a free-body diagram with

person next to you.

Car towing Example

Page 127-128

Group Practice

On whiteboards in your groups answer numbers 3 and 4 in your books

Free Body Diagram Practice

Finish the Free body diagrams practice Pg 138: 1-5