Work and Energy

1. Work and Energy Section 1.4

2. Objectives • define force • determine direction of acting forces and transfer of energy in examples • apply conditions of work to examples • calculate work and derive its unit from fundamental units • graphically determine work done on an object

3. Force • What is it? • If I place a ball on the desk and it is at rest, will it remain at rest? Why? • Force: push or pull on an object • In newtons (N) • What happens if I apply a force to the ball? Which direction is the force? • If there were no forces acting against my force, what would be the motion of the ball?

4. Sketch time! • For the following situations, make a sketch and label the forces present and their direction: • Rolling a ball across a table • Holding a ball in the air • Bouncing a ball on the floor • What is happening to energy? How do you think it is related?

5. Work • What is work? • Work • Done when a force moves an object through a distance in the direction of the force (so it is a vector quantity) • What are the units for work? How can we derive them? • Joule (J) W = Fd

6. Practice • A tugboat is towing a tanker through a canal using a towrope. Calculate work done by the tugboat if it applies an average horizontal force of 6.50 x 103 N on the towrope while towing the tanker through a horizontal distance of 150 m. • A large crane did 2.2 x 104 J of work in lifting a demolition ball a vertical distance of 9.5 m. Calculate the average force exerted by the chain of the crane on the demolition ball.

7. Conditions for Work • What are the conditions, based on the definition? • 1) must be movement • 2) must be a force • 3) force and distance object travels must be in the same direction • All three conditions must be met for work to be done

8. Consider this… • Was work done? Which conditions are met/not met? • Pushing a chair across the floor • Person coasting on a bike • Person carrying a backpack and walking forward • Quarterback throwing a football

9. Graffiti Activity • A weightlifter lifts a 1000 N barbell a vertical distance of 2.30 m from the ground to a position above his head. He then holds the barbell above his head for a time period. • Is work being done the whole time? When does the weightlifter do work? What conditions are being met? If he isn’t doing work, what conditions are not being met? • What are the directions of the forces?

10. Follow-up • As a group, analyze each other’s responses • Were there any similarities? Differences? Why were their differences? • Come up with an answer as a group to the question • In 30 sec, tell the class what you think is happening

11. Work Input and Output • Force applied to move an object a distance • Work input • W = Fd • What happens on a force-distance graph if force is constant over a distance? What does area under the line represent? Work output= energy gained by the object

12. What if… • If there are no outside forces, like friction, how will total work input be related to total work output?

13. Energy and work • Energy = ability to do work • Describe how energy is transferred when: • A pool cue hits a ball • What loses energy? What gains energy? • Change in energy = work • What will the units of energy be?