Chapter 7

1 / 10

# Chapter 7 - PowerPoint PPT Presentation

Chapter 7. Work and Kinetic Energy. Outline. Work Done by a Constant Force Case 1: Work done when the force is in the direction of the displacement Case 2: Work done when the force is at an angle to the displacement Negative Work Finding Total Work Various Examples.

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

## PowerPoint Slideshow about 'Chapter 7' - tyler-stuart

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

### Chapter 7

Work and Kinetic Energy

Dr. Jie Zou PHY 1151G Department of Physics

Outline
• Work Done by a Constant Force
• Case 1: Work done when the force is in the direction of the displacement
• Case 2: Work done when the force is at an angle to the displacement
• Negative Work
• Finding Total Work
• Various Examples

Dr. Jie Zou PHY 1151G Department of Physics

Work Done by a Force in the Direction of the Displacement
• Definition of work when force is in the direction of displacement: W = Fd.
• SI units for work: newton-meter (N m) = joule (J)
• The work W is zero if the distance d is zero, regardless of how great the force might be.

Dr. Jie Zou PHY 1151G Department of Physics

An Example
• An intern pushes a 72-kg patient on a 15-kg gurney, producing an acceleration of 0.60 m/s2.
• How much work does the intern do by pushing the patient and gurney through a distance of 2.5 m? Assume the gurney moves without friction.

Dr. Jie Zou PHY 1151G Department of Physics

Work Done by a Force at an Angle to the Displacement
• Definition of work when the angle between the force and displacement is :
• W = (F cos)d = Fd cos.
• When  = 0, force is in the same direction as the displacement and W = Fd cos 0 = Fd.
• When  = 90, where force and displacement are at right angles to each other, W = Fd cos 90= 0.

Dr. Jie Zou PHY 1151G Department of Physics

An Example
• A 75.0-kg person slides a distance of 5.00 m on a straight water slide, dropping through a vertical height of 2.50 m.
• How much work does gravity do on the person?

Dr. Jie Zou PHY 1151G Department of Physics

Negative Work
• Whenever we calculate work we must be careful about its sign, and not just assume it to be positive.

Dr. Jie Zou PHY 1151G Department of Physics

Finding Total Work
• Method 1: If force F1 does work W1, force F2 does work W2, and so on, the total work is:
• Wtotal = W1 + W2 +… =  Wi.
• Method 2: The total work can also be calculated by first performing a vector sum of all the forces acting on an object to find the resultant (total or net) force F and then using the basic definition of work:
• Wtotal = (F)d cos.
• Here  is the angle between the total force F and the displacement d.

Dr. Jie Zou PHY 1151G Department of Physics

An Example
• A car of mass m coasts down a hill inclined at an angle  below the horizontal. The car is acted on by three forces: (i) the normal force exerted by the road, (ii) a force due to air resistance, and (iii) the force of gravity. Find the total work done on the car as it travels a distance d along the road.

Dr. Jie Zou PHY 1151G Department of Physics

Homework
• See online homework assignment at www.masteringphysics.com

Dr. Jie Zou PHY 1151G Department of Physics