# Energy, Work and Simple Machines

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Energy, Work and Simple Machines
Chapter 6: Energy, Work and Simple Machines

6A Work and Power

6A Objectives Describe the relationship between work and energy.
Display an ability to calculate work done by a force. Identify the force that does work. Differentiate between work and power and correctly calculate power used.

Concept Development Map
What is it? Applications Definitions The property of an object that allows it to produce change in itself or its environment. Latin energia = en (in) + ergon (work) = active Chemical Energy Thermal Energy Internal or Inherent Power Energy Nuclear Energy Energetic, energize Motion Energy (momentum) Examples Potential Energy I ran out of energy. I need to re-energize. I don’t feel very energetic.

Concept Development Map
What is it? Applications Definitions The process of changing energy of a system by means of forces. Engines Middle European werk, wirk, work = to do Springs; Pulleys Exertion of Strength Work The matter on which someone labors on. Human Efforts Examples She’s a real piece of work! Torque This is a work of art! I need to get to work.

WORK MOSHER’S

Work Defined Work (W): The product of the force on an object and the distance through which the object is moved. or in Symbols: Let’s compare this new W with I (impulse)…

Work = Force X Distance Where is the Force and where is the distance in this picture?

Work Defined

Work Clarified Be careful!
Only the force in the same direction as the motion counts towards the work.

Work Clarified Case 1. A man pushes against a car stuck in a snow bank while his date sits nervously behind the steering wheel trying not to make the tires spin. However, the car does not move. How much work did he do on the car? Answer: The man did no work on the car since d=0. He may have burned calories, converting chemical energy into heat, but still, the car did not move.

Work Clarified Case 2. Sally carries a 0.5 kg textbook under her arm along a horizontal path. How much work was done on the text book? Answer: None, since both gravity and the force Sally exerted against gravity are perpendicular to the distance the book moved. (cosø = 0 so W = 0).

Work Clarified Case 3. An asteroid traveling at constant velocity out of reach of gravitational fields [etc.]... How much work is done on the satellite? Answer: None, since F = 0, W = 0.

Power Power - the rate of doing work. Work per unit time. 1 Watt = 1 Joule/sec. James Watts Compare: Units: Units:

6A Conclusions Work - a force applied over a certain distance. Force times distance has units of 1 Nm = 1 Joule. Power - energy of an object due to its motion. Units of 1 kgm2/s3 = 1 Watt=1 J/s.

6B Mechanical Energy: Potential and Kinetic

Mechanical Energy Defined
Mechanical Energy (ME): Energy due to the position or the movement of something; potential energy or kinetic energy or a combination of both. But just what is potential energy and kinetic energy?

What do these have to do with Potential?

Potential Energy Defined
Potential Energy (PE): “Height” energy, position energy. It is usually related to the relative position of two things, such as a stone and the earth (gravitational PE), or an electron and a nucleus (Electric PE). h – relative to reference level; hground = 0.

What does this have to do with potential
Energy? Work?

Kinetic Energy Defined
Kinetic Energy (KE): Motion energy. Equal to half the mass multiplied by the speed (scalar!) squared.

Kinetic Energy Defined

(when does HEIGHT energy become
potential become kinetic? (when does HEIGHT energy become MOTION energy)

When does potential become kinetic?

When does potential become kinetic?

Bill Nye: Energy (0:00 to 6:00)

6B Conclusions Potential Energy – energy due to position = height energy = mgh Kinetic Energy - energy of an object due to its motion. Units of 1 kgm2/s2 = 1 Joule. Energy Transfer – Potential energy can be turned into kinetic energy and visa versa. Roller coasters and pendulums are examples of this.

6C Energy Theorems and Conservation

6C Objectives Solve problems using the work-energy theorem.
Solve problems using the law of conservation of energy.

Work versus Impulse Starting with the Impulse-Momentum Theorem:
Multiplying both sides by d/t:

Work versus Impulse (cont’d)
This simplifies to: Substitution of vavg = d/t = (v2+v1)/2:

Work - Energy Theorem This simplifies to:
This is the Work-Energy Theorem:

Work - Impulse Comparison
Let’s compare the two theorems: Impulse-Momentum Theorem: Work-Energy Theorem:

Kinetic Energy vs. Momentum
Another conclusion: Kinetic Energy is the derivative of Momentum

Work - Impulse Comparison
Let’s compare the two graphs: Impulse (Force versus Time) Work (Force versus Distance) Force Time

Conservation of Mechanical Energy
Mechanical Energy is the sum of kinetic energy and gravitational energy. It cannot change in an ideal system. The decrease in potential energy is equal to the increase in kinetic energy. The decrease in kinetic energy is equal to the increase in potential energy.

Conservation in a Pendulum
Simple Harmonic Motion conserves energy on each swing. The decrease in potential energy is equal to the increase in kinetic energy. The decrease in kinetic energy is equal to the increase in potential energy.

Momentum and Kinetic Energy Conservation

Conservation Of Energy
Hew35: Bowling Ball Conservation Of Energy

Conservation Of Energy
Hew36: Math Example Conservation Of Energy

Bill Nye: Energy (6:00 to 12:56)

6D Machines

6D Objectives Demonstrate Knowledge of why simple machines are useful.
Communicate an understanding of mechanical advantage in ideal and real machines. Analyze compound machines and describe them in terms of simple machines. Calculate efficiencies for simple and compound machines.

6D Vocabulary Machine - A device that changes the magnitude or the direction of the force needed to do work, making the task easier to accomplish. Simple Machine - A lever, pulley, gear, wheel and axle, inclined plane, wedge, or screw. Compound Machine - A device that consists of two or more simple machines linked so that the resistance force of one machine becomes the effort force of the second machine.

Japanese Rube Goldberg Machines

Mechanical Advantage Mechanical Advantage - The ratio of the resistance (r) force to the effort (e) force. Ideal Mechanical Advantage - The ratio of the resistance distance to the effort distance. Torque Balance - The resistance torque equals the effort torque.

Efficiency Percent Efficiency - The ratio of the output work to the input work times 100%.

Energy Transfer in a Coupled Pendulum

Demo: Place a baseball on top of a basketball. Drop both at the same time on the floor and see what happens. What do you think will happen? Why?

What does all this have to do with baseball or sports in general? When you bounce a baseball off a basketball, you are transferring energy from the deformation of the basketball to the baseball. When you bounce a baseball off a bat, you are transferring energy from the bat to the baseball. How well a ball bounces off the basketball has to do with timing. When the basketball hits the floor, it squashes the bottom a bit. When it springs back to its original shape, it pushes off the floor -- it bounces. The baseball indents into the basketball on the top. When the basketball returns to its round shape all the energy is transferred to the baseball. The effect is similar to a man on a trampoline.

Conservation of Mechanical Energy
The Amazing Oscillating Spring Thing

Efficiency Ideal Machines: Real Machines:

Bill Nye: Energy (12:56 to END)