unit 3 lesson 1: work

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 matchedc. A crane lifting a car

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

unit 3 lesson 1: work

Documents