november 4 th 2009 objectives swbat define work swbat calculate net work. swbat calculate kinetic...
TRANSCRIPT
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