Energy

Energy: Main Ideas Energy is the ability to do work. The law of conservation of energy says that energy can

be neither created nor destroyed. Energy can change from one form to another, but total energy always remains unchanged.

Work and energy are closely related. When work is done on a system, the system gains potential or kinetic energy. When a system has energy, that energy can be used to do work.

Units of energy and work are the same. SI = Newton Meters or Joules English = foot-pounds.

Potential vs. Kinetic EnergyPotential Energy = Stored EnergyKinetic Energy = Energy in motion

If a force changes and object’s vertical position on earth, the object is given potential energy. Gravitational potential energy is the energy gained because of a change in an object’s vertical position.

An unbalanced force changes and object’s motion and causes an increase or decrease in it’s speed. Kinetic energy is the energy something has because of it’s speed.

In many systems, energy changes back and forth between potential and kinetic energy. If there is resistance, some kinetic energy is changed to heat energy.

Total Energy-swinging pendulum

Total Energy = Potential + Kinetic Energy

Newton’s First Law

Newton’s First Law (Law of Inertia): Every object will remain at rest, or will continue to move in a straight line with constant speed, unless the object is acted on by a non-zero net force.

Inertia: The property of an object to resist changes in it’s motion (an objects mass tells you how much inertia that it has).

Energy: Mechanical & Fluid

Potential Energy (Mechanical and Fluid) : Main Ideas If an object of weight (w) is raised a

vertical distance (h), the gravitational potential energy (Ep ) is equal to w * h.

Elastic potential energy equals the work done to stretch or compress elastic objects such as springs.

Energy (Mechanical and Fluid): Main Ideas Continued

Energy = Ability to work

Potential Energy Kinetic EnergyEp = wh (English) Ek = ½ mv2

Ep = mgh (SI) Ek = ½ Iω2

Note: (wh) = (mgh) or Nm = kg*m*m/sec2

Therefore Nm = kg *m2/sec2 = J

FluidEp = weight x height (Eng) * Weight = ρw * V ρw = 62.416 lbs/ft3

Ep = mgh (SI) * m = ρ * v ρ = density = 1000 kg/m3

W = ________lbs

W = ________N

h = ________ft

h = ________m

Student X has just finished climbing to the top of a tower to drop water balloons on unsuspecting passersby. If Student X has weight of 205 lbs (including water balloons) and the tower has a height of 40 ft how much Gravitational Potential Energy-in Joules-was added to this system??? W = _____ J

Potential Energy Problems

Elastic Potential EnergySprings have a constant (k) which indicates how flexible (stiff) the

spring is. This constant can be found using the fomula: k = f/dk = spring constantf = force required to stretch or compress the srpingd = distance the spring is displaced

After you know the spring constant (k), you can then calculate the elastic potential energy using the formula:

Elastic Examples (cont.)

Kinetic Energy ProblemsKinetic Energy is mass in motion

(Ek = ½ mv2)

Two Foot-ball players crash into each other during a game. Player A is the water boy who has a mass of 50 kg (110 lbs or 490 N) but is running at a speed of 4 m/sec. This player runs directly in to Player B who is the defensive tackle that has a mass of 100 kg (220 lbs or 980 N) and is moving at 2m/sec. Who has the greatest kinetic energy

when they crash?

If you are playing baseball, is it better to use a bat twice as big as normal, or to swing the bat two times faster than normal?

Kinetic Examples (cont.)

Given: (Ek=1/2mv2)

A wrecking ball that weighs one ton moves with a speed of 30 ft/sec just before it slams into the side of a building.

note: acceleration due to

gravity is 32 ft/s2

Find:

Kinetic energy of wrecking ball when it hits the wall

Potential Energy in a Fluid SystemWhen a water pump lifts water from a reservoir to a water tower, the work done is changed to potential energy. The formula used to calculate this fluid potential energy is the same as the formula for mechanical.

Kinetic Energy of Fluids

Newton’s First Law

Newton’s First Law (Law of Inertia): Every object will remain at rest, or will continue to move in a straight line with constant speed, unless the object is acted on by a net force.

Inertia: The property of an object to resist changes in it’s motion (an objects mass tells you how much inertia that it has).

Moment of Inertia (Spinning Mass):

A property that describes an objects resistance to change in rotational motion. The moment of inertia is determined by:

1. Location of Axis

2. Mass

3. How the mass is distributed

Moment of inertia continued

Rotational kinetic energy