Aim: How can we apply work- energy to motion problems? Do Now: In your own words, what does energy mean to you? In your own words, what does energy mean to you? List and describe different forms of energy. List and describe different forms of energy.

How can we define energy? Energy is the ability to produce change in an object or its environment. Energy is the ability to produce change in an object or its environment. Examples of forms of energy: solar, thermal, mechanical (PE and KE), chemical Examples of forms of energy: solar, thermal, mechanical (PE and KE), chemical Physics definition of Energy: the ability to do work Physics definition of Energy: the ability to do work

What does work mean to you?  Lifting a weight?  Holding a weight?  Pushing a cart?  Pushing a wall?

What does work mean to scientists? Definition: Work is done when a force transfers energy to an object and moves it. Definition: Work is done when a force transfers energy to an object and moves it.  Lifting a weight?  Holding a weight stationary?  Pushing a cart?  Pushing a wall?

Symbol: W Symbol: W Equation: W = Fd (force times displacement) Equation: W = Fd (force times displacement) Unit: Joule (J) Unit: Joule (J) One Joule equals one Newtonmeter One Joule equals one Newtonmeter Scalar quantity Scalar quantity Horizontal Work involves applying a force over a distance Horizontal Work involves applying a force over a distance Vertical Work involves lifting your weight (F g =mg) a certain height. Vertical Work involves lifting your weight (F g =mg) a certain height.

Rule for calculating work W = Fd=ΔE T W = Fd=ΔE T ΔE T Change in energy ΔE T Change in energy The force must be parallel (horizontal component) to the displacement. The force must be parallel (horizontal component) to the displacement.

List two examples of zero work. Applying a force and the object is not moving. Applying a force and the object is not moving. Applying a force that is perpendicular to the displacement of the object. Applying a force that is perpendicular to the displacement of the object. F d

Example (horizontal motion) 1. How much work is done in pushing a crate 3 meters with a force of 10 Newtons? 1. How much work is done in pushing a crate 3 meters with a force of 10 Newtons?

Example: A student is pulling a cart with a 25 newton force at a 60° angle for 20 meters. Calculate the work done. A student is pulling a cart with a 25 newton force at a 60° angle for 20 meters. Calculate the work done.

Examples: vertical motion against gravity How much work does it take to lift a 10 kg object 4 meters? How much work does it take to lift a 10 kg object 4 meters? It takes 250 Joules to lift an object 10 meters. Calculate the force required. It takes 250 Joules to lift an object 10 meters. Calculate the force required.

A constant horizontal force of newtons applied to a box causes it to move at a constant speed of 5.0 m/s. Determine how much work is done against friction on the box in 10 seconds. A constant horizontal force of newtons applied to a box causes it to move at a constant speed of 5.0 m/s. Determine how much work is done against friction on the box in 10 seconds.

Summary How can we describe work? How can we describe work? Give one example of applying a force to an object but no work is done. Give one example of applying a force to an object but no work is done. Identify two units for work. Identify two units for work.

Aim: How can we calculate power? 1) It takes 500 Joules to lift a 20-kilogram object. Calculate the height of the object. 1) It takes 500 Joules to lift a 20-kilogram object. Calculate the height of the object. 2) A constant force of 10.-newtons is used to push a 3.0 kilogram block 4 meters across a counter top. How much work is done on the block. 2) A constant force of 10.-newtons is used to push a 3.0 kilogram block 4 meters across a counter top. How much work is done on the block. 3) Identify the two units used for work. 3) Identify the two units used for work.

How can we calculate work from a graph? Displacement (m) Force (N) Force vs Displacement On a force vs displ. Graph Area under line equals work.

Calculate work Displacement (m) Force (N) Force vs Displacement

How can we describe power? Definition: amount of work done per unit time, or rate at which work is done. Definition: amount of work done per unit time, or rate at which work is done. Unit: 1 Watt (W) = 1 Joule/second Unit: 1 Watt (W) = 1 Joule/second Scalar quantity Scalar quantity Note: Don’t confuse W(ork) with W(att)! Note: Don’t confuse W(ork) with W(att)!

Example 1 You lift a 10 kg box a vertical distance of 2 meters in 3 second. How much power did you generate? You lift a 10 kg box a vertical distance of 2 meters in 3 second. How much power did you generate? You lift the same box 2 meters in 1 second. Calculate the power. You lift the same box 2 meters in 1 second. Calculate the power.

Example 2 Your friend lifts a 50 kg object the same vertical distance of 2 meters, but in only 2min. Calculate the rate at which work is done. Your friend lifts a 50 kg object the same vertical distance of 2 meters, but in only 2min. Calculate the rate at which work is done.

Example 3 An electric motor develops 65 kW of power as it lifts a loaded elevator 17.5 meters in 0.5 minutes. How much force does the elevator exert? An electric motor develops 65 kW of power as it lifts a loaded elevator 17.5 meters in 0.5 minutes. How much force does the elevator exert?

Example 4 A 50 Newton force exerted on an object is powered by a 250-Watt motor. Calculate the velocity. A 50 Newton force exerted on an object is powered by a 250-Watt motor. Calculate the velocity.

Summary 1) How can we calculate power? 1) How can we calculate power? 2) As the time increases the power _____________. 2) As the time increases the power _____________. 3) Describe the units for power. 3) Describe the units for power.