Hand in labs. Get into groups of 2-3 Discuss the following question: Which requires more work? Holding a heavy chair at arm length for a few minutes Carrying a bucket of water along a horizontal path while walking at constant velocity

Work and Energy

Recognize the difference between the scientific and ordinary definitions of work Define work, relating it to force and displacement Identify where work is being performed Calculate the net work done when many forces are applied to an object Identify several forms of energy Calculate kinetic energy for an object

Work is the force exerted on an object times the displacement of the object in that direction. Measured in Joules! (J) The force and displacement must be in the same direction Perpendicular force = no work W=Fd

Holding a heavy chair at arm length for a few minutes Carrying a bucket of water along a horizontal path while walking at constant velocity Actually, they both require the same amount of work. NONE!

The force in this situation is not in the same direction as displacement. However, a component of the force is!

A 20.0 kg suitcase is raised 3.0 m above a platform by a conveyor belt. How much work is done on the suitcase? 5.9 x 10 2 J

In 1453, during the siege of Constantinople, the Turks used a cannon capable of launching a stone cannonball with a mass of 5.40  10 2 kg. Suppose a soldier dropped a cannonball with this mass while trying to load it into the cannon. The cannonball rolled down a hill that made an angle of 30.0  with the horizontal. If 5.30  10 4 J of work was done by gravity on the cannonball as it rolled down a hill, how far did it roll? 20.0 m

If force acts in the opposite direction of displacement, negative work is done

Kinetic Energy is energy of an object due to its motion Units? Also Joules! A 6.0 kg cat runs 10.0 m/s. What is its kinetic energy? 3.0 x 10 2 J

Suppose a strong man pushes a 4.50 x 10 3 kg car with a force of 1840 N over 15 m. How fast is the car now moving? About 3.5 m/s W= ΔKE

Section Review, p. 171 P. 194, 19, 21

Take out the labs that you worked on Thursday (10/31) We’re going to have a discussion on this lab, which is due Wednesday as a Formal Lab Report

Potential Energy is stored energy. Potential Energy comes in many different forms. Gravitational Elastic Electric Magnetic Chemical

Potential Energy stored in the gravitational fields of interacting bodies Height must be defined from a certain reference point

Energy available for use when a deformed elastic object returns to its original configuration x= distance compressed or stretched k= spring constant

When a 2.00 kg mass is attached to a vertical spring, the spring is stretched 10.0 cm such that the mass is 50.0 cm above the table What is the gravitational potential energy associated with this mass relative to the table? What is the spring’s elastic potential energy if the spring constant is N/m? What is the total potential energy of the system? 9.81 J 2.00 J J

Why do things slow down? Friction causes Kinetic energy to become thermal energy (heat) We sometimes say this energy is ”lost” Total energy is still conserved!

A small 10.0 g ball is held to a slingshot that is stretched 6.0 cm. The spring constant is 2.0 x 10 2 N/m a.What is the elastic potential energy of the slingshot before it is released? b.What is the kinetic energy of the ball just after the slingshot is released? c.What is the ball’s speed at that instant? d.How high does the ball rise if it is shot directly upward? A= 0.36 J, B= 0.36 J, C= 8.5 m/s, D= 3.7 m

5.3 Lesson Check. Labs due Wednesday. Test Friday. ENJOY!

Power is the rate at which work is done. Change in work/time Force x speed Measured in watts. Measured in horsepower in American

Two horses pull a cart. Each exerts a force of N at a speed of 2.0 m/s for 10.0 min a)Calculate the power delivered by the horses. b)How much work is done by the two horses? a. 1.0 x 10 3 W, b. 6.0 x 10 5 W