Lesson 8 Objectives Define work Describe the units

Lesson 8

Objectives • Define “work” • Describe the units of measure for work • Use force and distance measurements to calculate work • Compare the work done by different forces • Compare the force exerted by a motor that is connected to three batteries in series with the force needed to lift a small sled.

Getting started • Discuss the meaning of the word “work”. • List some examples of work being done. • Read “The Meaning Of Work”. Did your exampes match the scientific meaning of work? L 8

Getting Started Con’t • Review how to calculate work and what the units of measure for work are. • Work – when a force acts on an object and it moves some distance. • Calculation: WORK = Force (N) x Distance (m) • Units of measure: » Newton-meters •

Getting Started Alice pulls a sled with a force of 12 N. She pulls the sled through a distance of 5 m. How much work does Alice do on the sled? L 8

Answer to problem Work = Force x Distance W=? F = 12 N D=5 m W = 12 N x 5 m L 8 = 60 N-m

Problem Michael lifts his book bag, which weighs 25 N, from the floor to a desktop that is 0. 80 m above the floor. How much work does Michael do on the bag? L 8

WORK Work = Force x Distance W=? F = 12 N D = 0. 80 m W = 25 N x 0. 80 m L 8 = 20. 0 N-m

Questions about work • 1. What force are you working against when you lift a backpack? • L 8. 1 b

• 2. In which of the following cases is work, as defined by scientists, being done? • A. Someone tries to move a piano, but the piano won’t budge. • L 8. 1 b

• B. A tow truck is pulling a car slowly along the street. • L 8. 1 b

• C. A student is studying for a mathematics exam. L 8 -1 b

• D. A student is pushing a grocery cart around a store. • L 8. 1 b

• E. Another student is standing in line holding a 12 -N bag of potatoes. • L 8. 1 b

• F. A student pushes against the school building. • L 8. 1 b

Vocabulary – Lesson 8 • Work: What happens when an object changes its position by moving in the direction of the force that is being applied is Work = force x distance (w = fd) Work involves a force applied across a distance. • The metric unit of work is the newton-meter (N-m). • A newton-meter is also a joule (J). • James Joule – described the relationship between work and energy.

Vocabulary – Lesson 8 • effort force: the force exerted by a person or motor when using a machine to lift a load. (In lab, this is the force is applied to lift the sled). • effort distance. the distance over which the effort force acts in a simple machine. (In lab, this is the distance the sled moves).

Vocabulary – Lesson 8 • Ampere (amp) : Electric current flows through the wire. The unit used to describe how much electric current flows through a wire is the ampere The ampere is named for Andre Ampere • Volts: Measures the electric potential of a battery • The volt is named for Alessandra Volta

Vocabulary Lesson 8 • OUTPUT WORK: The work done on a load by a machine or motor. • Load force x Load Distance = Output work • INPUT WORK: The work done by the effort force in a simple machine or motor. • Effort force x Effort distance = Input work

Notes • Work is done on an object when a force acts on an object that moves through some distance. • Work is equal to the force that pushes or pulls on the object multiplied by the distance the object moves • W = F x D Work = Force x Distance

Notes • If the object does not move any distance, no work has been done. • Work is measured in units of Newton-meters. • Force (N) x Distance (m) = Nm • Work can also be measured in Joules • One Newton-meter is equal to One Joule

Lesson 8

Question – Lesson 8 • Is the motor capable of doing enough work to lift the sled to a distance of. 10 m?

• Lesson 8 • If _________, then _____ because _________.

• Lesson 8 • • Materials 1. 2. 3.

• Lesson 8

Inquiry 8. 1 Calculating the Work Done on Different Surfaces L 8

Inquiry 8. 2 Work to Lift a Load • 1. Motor force with three batteries in series: _________ • 2. Work done by a motor when it lifts a load 10. 0 cm. (0. 10 m) • Work = _______ x _______ = ______ L 8 -2

3. Weight of six washers: _______ 4. Work to raise six washers: Work = ______ x _______ = _______ 5. Your estimate of sled’s weight: _______ 6. Sled’s actual weight: _____ L 8 -2

7. Work to lift sled 10. 0 cm. (0. 10 m): Work = _____ x ______ = _______ L 8 -2

• Lesson 8