Chapter 5 – WORK and ENERGY

TODAY’s OBJECTIVES Recognize the difference between the “scientific” and “ordinary” work Define work by relating it to force and displacement

5.1 – 5.3 Work and Energy Vocabulary Work Work done on an object Joule Kinetic energy Potential energy Gravitational Potential Energy Elastic Potential Energy Spring constant Mechanical Energy

5.1 WORK

What is work? In science In everyday language The product of the magnitude of the displacement and the component of a force parallel to that displacement To do something: To learn To kick a ball To think To hold a chair

Work is done – when a force causes a displacement Work done on a car W = F ∙ d Constant horizontal force = F Displacement = Δx = d NO WORK is done on the object unless the objects moves (i.e. has a displacement)!!!

Constant horizontal force = F ∙ cos𝜃 Displacement = Δx = d Work is done – when a force acts on an object AND the object must move in the direction of that force Work done on a crate W = F ∙ d Constant horizontal force = F ∙ cos𝜃 Displacement = Δx = d NO WORK is done on the object if the force is non – parallel to the direction of the displacement

IS WORK DONE ON AN OBJECT? Example 1 (on a worksheet) IS WORK DONE ON AN OBJECT? A teacher holds a chair at arm’s length for several minutes. A person carries a bucket of water along a horizontal path while walking at constant velocity

IS WORK DONE ON AN OBJECT? Example 1 (on a worksheet) IS WORK DONE ON AN OBJECT? No, chair does not move = no displacement in the direction of force applied No, the upward force that holds the buckets is perpendicular to the displacement of the bucket

1.1 x 102 J 1.72 ×104 J, or 17.2 kJ W = F × d = 300.0 x 0 = 0 J

If WORK is done against GRAVITY Reminder... If WORK is done against GRAVITY W = F ∙ d W = mg ∙ h

POWER the rate at which work is done or energy is transformed Rate of Energy Transfer the rate at which work is done or energy is transformed

WHY? POWER UNITS = J/s = Watt = W the rate at which work is done or energy is transformed Power = work time interval Power = force x speed WHY? UNITS = J/s = Watt = W

73 W 5.4 ×105 J/h Useable work: 2.7 ×1011J Heat: 8.1×1011 J

HORSE POWER (hp) Another unit used 1 hp = 746 watts

Convert to horsepower.

If WORK is done against GRAVITY Reminder... If WORK is done against GRAVITY W = F ∙ d W = mg ∙ h

HOMEWORK Page: 139 - 141 Problems: all

WORKSHEET EXAMPLE Many mountain roads are built so that they zigzag up the mountain rather than go straight up toward the peak. Discuss the advantages of such a design from the viewpoint of energy conservation and power

WORKSHEET EXAMPLE A light bulb is described as “having 60 watts.” What’s wrong with this statement?

WORKSHEET EXAMPLE A 193.0 kg curtain needs to be raised 7.5 m, at constant speed, in as close to 5.0 s as possible. The power ratings for three motors are listed as 1.0 kW, 3.5 kW, and 5.5 kW. Which motor is best for the job?

P = ? watts Worksheet EXAMPLE m = 193.0 kg Δt = 5.0 s d = 7.5 m GIVEN? A 193.0 kg curtain needs to be raised 7.5 m, at constant speed, in as close to 5.0 s as possible. The power ratings for three motors are listed as 1.0 kW, 3.5 kW, and 5.5 kW. Which motor is best for the job? GIVEN? UNKNOWN? P = ? watts m = 193.0 kg Δt = 5.0 s d = 7.5 m

WORKSHEET EXAMPLE Two horses pull a cart. Each exerts a force of 250.0 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?

Answer all the Concluding Questions Investigation 5 – 1A Read the lab procedure Answer all the Concluding Questions

Investigation 5 – 1A and 5 – 1B Two horses pull a cart. Each exerts a force of 250.0 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?

ANSWERS TO WORKSHEET EXAMPLES

IS WORK DONE ON AN OBJECT? Example 1 (on a worksheet) IS WORK DONE ON AN OBJECT? No, chair does not move = no displacement in the direction of force applied No, the upward force that holds the buckets is perpendicular to the displacement of the bucket

WORKSHEET EXAMPLE Many mountain roads are built so that they zigzag up the mountain rather than go straight up toward the peak. Discuss the advantages of such a design from the viewpoint of energy conservation and power Assuming mechanical energy is conserved, the same amount of energy is needed to reach the top in both cases. Because the same amount of work must be done, the path with a longer distance takes more time and hence requires less power.

WORKSHEET EXAMPLE Light bulbs don’t have the A light bulb is described as having 60 watts. What’s wrong with this statement? Light bulbs don’t have the energy stored within them; energy is transferred to them in the form of electricity at a rate of 60 J/s.

WORKSHEET EXAMPLE Two horses pull a cart. Each exerts a force of 250.0 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?