Transcript Catalyst – October (Prime # between 11 and 17

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.

W
It tells us how much force is applied over a certain distance
= 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?

 Is

If so, how much?
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)
d
Θ
F
Work = Newtons times meters (N * m)
or Joules (J)

If there are multiple Forces use Net
Force
FYI
Lifting 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 important
 Work
is a scalar
 Work is negative when the force is in the
opposite direction as displacement
 Theta = 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 = 31.5J
Vt = 160m/s; not reasonable…
Practice
Pg170; 1, 2, 3, 4
 Pg 171; 2, 3a, b, c, 4a,
 Pg174; 1, 2, 3, 4

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
But when they
are unbalanced,
that’s when
objects
accelerate
When an object is
in equilibrium
(acceleration = 0),
SF = 0
If SF = 0,
then…
SFx = 0
SFy = 0
Practice Finding the Net Force
Find the net
force and tell
the direction
that the velocity
will change
Calculating net force in 2Dimensions

Rules:
 Draw
a free-body diagram
 Select 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 object
 Forces
 Forces


will cause a change in velocity.
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
1st Law – Law of Inertia
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

2nd Law
a=acceleration (m/s2)
F=Force (N)
M=mass (kg)
This means sum
newtons
kilograms


 F  ma
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
book
 Pages
127-128
 Discuss 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
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