November 4 th 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. W net = F net d(cosΘ) If there are multiple Forces use Net Force d F Θ Work = Newtons times meters (N * m) or Joules (J) 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 = ½mv 2 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 V t = 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…  F x = 0  F y = 0

Practice Finding the Net Force Find the net force and tell the direction that the velocity will change

Calculating net force in 2- Dimensions 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 F friction is 11N, and the F gravity 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 3.10x10 2 kg. 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 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 1 st and 2 nd 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 1 st Law – Law of Inertia2 nd Law a=acceleration (m/s 2 ) F=Force (N) M=mass (kg) newtons kilograms This means sum

Newton’s 1 st 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/s 2 What does 0 acceleration mean about velocity?  Velocity is constant How does this relate to Newton’s 1 st 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/s 2, 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  Discuss the steps for drawing a free-body diagram with person next to you.

Car towing Example Page

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