work and energysection 1 unit 3 lesson 1: work goals: recognize the difference between the...

Work and Energy Section 1

Unit 3Lesson 1: Work

Goals:•Recognize the difference between the scientific and ordinary definitions of work.•Define work by relating it to force and displacement.•Identify where work is being performed in a variety of situations.•Calculate the net work done when many forces are applied to an object.


Bellringer

• List five examples of things you have done in the last year that you would consider work.

• Based on these examples, how do you define work?


Work

• Work is done on an object when a force causes a displacement of the object.

• Work is done only when components of a force are parallel to a displacement.


Work

• In physics, work is the magnitude of the force (F) times the magnitude of the displacement (d) in the same direction as the force.

• W = Fd• What are the SI units for work?

– Force units (N) distance units (m)– N•m are also called joules (J).

• How much work is 1 joule?– Lift an apple weighing about 1 N from the floor to the

desk, a distance of about 1 m.


Work • Pushing this car is work because F and d are in the same direction.

• Why aren’t the following tasks considered work?– A student holds a heavy chair at

arm’s length for several minutes.– A student carries a bucket of

water along a horizontal path while walking at a constant velocity.


Practice Problem

• 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?


Practice Problem

• 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 102 J


Work• How would you calculate

the work in this case?– What is the component of F

in the direction of d?• F cos

– If the angle is 90°, what is the component of F in the direction of d?

• F cos 90° = 0

– If the angle is 0°, what is the component of F in the direction of d?

• F cos 0° = F


Work


Practice Problem

• How much work is done on a vacuum cleaner pulled 3.0 m by a force of 50.0 N at an angle of 30.0O above the horizontal?


Practice Problem

• How much work is done on a vacuum cleaner pulled 3.0 m by a force of 50.0 N at an angle of 30.0O above the horizontal?

• 130 J


Practice Problem

• A tugboat pulls a ship with a constant net horizontal force of 5.00 x 103 N and causes the ship to move through a harbor. How much work is done on the ship if it moves a distance of 3.00 km?


Practice Problem

• A tugboat pulls a ship with a constant net horizontal force of 5.00 x 103 N and causes the ship to move through a harbor. How much work is done on the ship if it moves a distance of 3.00 km?

• 1.50 x 107 J


Practice Problem

• A weight lifter lifts a set of weights a vertical distance of 2.00 m. If a constant net force of 350 N is exerted on the weights, what is the net work done on the weights?


Practice Problem

• A weight lifter lifts a set of weights a vertical distance of 2.00 m. If a constant net force of 350 N is exerted on the weights, what is the net work done on the weights?

• 7.0 x 102 J


Practice Problem

• A shopper in a supermarket pushes a cart with a force of 35 N directed at an angle of 25O downward from the horizontal. Find the work done by the shopper on the cart as the shopper moves along a 50.0 m length of aisle.


Practice Problem

• A shopper in a supermarket pushes a cart with a force of 35 N directed at an angle of 25O downward from the horizontal. Find the work done by the shopper on the cart as the shopper moves along a 50.0 m length of aisle.

• 1.6 x 103 J


Practice Problem

• If 2.0 J of work is done in raising a 180 g apple, how far is it lifted?


Practice Problem

• If 2.0 J of work is done in raising a 180 g apple, how far is it lifted?

• 1.1 m


Practice Problem

• A bumble bee has a mass of about 0.3 grams. Suppose the bee does 0.04 J of work against gravity, so that it ascends straight up with a net acceleration of 1.0 m/s2. How far up does it move?


Practice Problem

• A bumble bee has a mass of about 0.3 grams. Suppose the bee does 0.04 J of work against gravity, so that it ascends straight up with a net acceleration of 1.0 m/s2. How far up does it move?

• Answer


Work is a Scalar• Work can be

positive or negative but does not have a direction.

• What is the angle between F and d in each case?


HomeworkDue Next Class


Ticket Out

• Determine whether work is being done in each of the following examplesa. A train engine pulling a loaded boxcar initially at rest.

b. A tug of war that is evenly matched

c. A crane lifting a car

• Choose one of the examples above and explain your answer.

work and energysection 1 unit 3 lesson 1: work goals: recognize the difference between the...

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