Energy

• Part 1: Overview of Energy and Mechanics

• Working in your group, you should answer the question:

• What is ENERGY?

Warm Up - What is energy?

• Energy is defined as the ABILITY TO DO WORK

• So, what is “WORK” ?

What is ENERGY?

• Work- application of a force causing an object to move a certain distance

• So, what is FORCE?

What is WORK?

• Force- result of an interaction that is capable of changing the state of motion of an object

• “net” force is end result (sum) of all forces

• If working together, added

• If opposing, greater force “wins”

• Indicated by force arrows

What is FORCE?

Let’s Play!

• Roll the ball gently on your desk.• What happens?• Now, roll the ball gently on the towel.• What happens? • Are they different?• WHY?

What forces are acting on the ball?

• The air and the floor are applying force to the ball in the direction opposite of movement. If no force is applied to the ball it will roll to a stop.

• The different resistance of the floors (table vs. towel) result in different forces applied by the floor.

What would you need to keep the ball rolling?

In action…

Here net force is zero

Acceleration is directly proportional to force, if you pedal twice as hard you double acceleration

F1=tire frictionF2=air resistanceFa=force applied

• First Law of Motion• Every object retains its state of rest or its state of

uniform straight line motion unless acted upon by an unbalanced force

• Second Law of Motion• The acceleration of an object is directly proportional to

the net force acting on it and inversely proportional to the mass of the object

• Third Law of Motion• Whenever two objects interact, the force exerted on

one object is equal in strength and opposite in direction to the force exerted on the other object

Newton’s Laws of Motion

• Based on Newton’s Second law of motion: F=ma

Force (N) = mass (kg) X acceleration (m/s2)

Unit for mass is kg, unit for acceleration is m/s2

The combination of kg*m/s2 is called a Newton

What are the units of force?

• Work- application of a force causing an object to move a certain distance

• Work = Force (N) x distance (m) W=Fd

So back to WORK…

What is WORK? If you move a book to a higher shelf, it is work

If you just hold the book, it is not work (your arm may get tired from opposing a force to equal the weight of the book!)

• Work = Force x distance• W=(Newton)(meter)• W=Nm• Newton-meters are also called joules

• Therefore, since a newton is a kg·m/s2 and this is multiplied by a meter, the units for a joule are kg·m2/s2

In the English system, force is measures in pounds and distance in feet, so work is measured in ft·lbs

Work Calculations

Work Calculations

• Power is the WORK per unit time• P=W/t = joules/second• J/s is called a Watt

• “Horsepower” is 550 ft·lb/s• (To convert a power rating in the English system,

must divide by 550 ft·lb/s) –• 500 ft·lb/s ÷ 550 ft·lb/s = .9 hp

Power

Power