The Physics Energy

Objectives Identify several forms of energy. Calculate kinetic energy for an object. Apply the work–kinetic energy theorem to solve problems. Distinguish between kinetic and potential energy. Classify different types of potential energy. Calculate the potential energy associated with an object’s position.

Kinetic Energy The energy of an object that is due to the object’s motion is called kinetic energy. Kinetic energy depends on speed and mass…but how? Bigger v= Bigger m= What type of mathematical relationship is this? Directly proportional What will the formula look like?

Formula

Kinetic Energy Units: KE= 1/2 mv 2

Sample Problem Calculate the speed of an 8.0x10 4 kg airliner with a kinetic energy of 1.1x10 9 J. (1.7x10 2 m/s)

Harder Sample Problem A 7.00 kg bowling ball moves at 3.00m/s. How fast must a 2.45g ping pong ball move in order to have the same kinetic energy? Is this speed reasonable for a ping pong ball? (160 m/s)

Work and Kinetic Energy Work-Kinetic Energy Theorem The net work done by all the forces acting on an object is equal to the change in the object’s kinetic energy. The net work done on a body equals its change in kinetic energy. W net = ∆KE net work = change in kinetic energy

Work-Kinetic Energy C:\Projects\Holt-Rinehart- Winston\HRWScience\HRWScience\Holt_Physics\Ch 05\70319.html C:\Projects\Holt-Rinehart- Winston\HRWScience\HRWScience\Holt_Physics\Ch 05\70319.html

Sample Problem A 2000kg car accelerates from rest under the actions of two forces. One is a forward force of 1140N provided by traction between the wheels and the road. The other is a 950N resistive force due to the various frictional forces. How far must the car travel to reach a speed of 2.0 m/s?

Sample Problem Work-Kinetic Energy Theorem On a frozen pond, a person kicks a 10.0 kg sled, giving it an initial speed of 2.2 m/s. How far does the sled move if the coefficient of kinetic friction between the sled and the ice is 0.10? (2.5m)

Sample Problem

POTENTIAL ENERGY Potential Energy is the energy associated with an object because of the position, shape, or condition of the object. Gravitational potential energy is the potential energy stored in the gravitational fields of interacting bodies. *gained by work done against gravity. *PE g = W=Fd Gravitational potential energy depends on height from a zero level. PE g = mgh gravitational PE = mass  free-fall acceleration  height

Elastic Potential Energy Elastic potential energy is the energy available for use when a deformed elastic object returns to its original configuration.

Elastic Potential Energy The symbol k is called the spring constant, a parameter that measures the spring’s resistance to being compressed or stretched.

Elastic Potential Energy C:\Projects\Holt-Rinehart- Winston\HRWScience\HRWScience\Holt_Physics\Ch 05\70321.html C:\Projects\Holt-Rinehart- Winston\HRWScience\HRWScience\Holt_Physics\Ch 05\70321.html

Sample Problem A 70.0 kg stuntman is attached to a bungee cord with an unstretched length of 15.0 m. He jumps off a bridge spanning a river from a height of 50.0 m. When he finally stops, the cord has a stretched length of 44.0 m. Treat the stuntman as a point mass, and disregard the weight of the bungee cord. Assuming the spring constant of the bungee cord is 71.8 N/m, what is the total potential energy relative to the water when the man stops falling?

QOTD A glass of water is dropped on the tile floor and shatters. List the types of energy present before it hits the surface and after it hits the surface. Is the total amount of Joules the same before and after the collision?

Conservation of Mechanical Energy Mechanical energy is the sum of kinetic energy and all forms of potential energy associated with an object or group of objects. ME = KE + ∑PE Mechanical energy is often conserved. ME i = ME f initial mechanical energy = final mechanical energy (in the absence of friction)

Mechanical Energy Mechanical Energy is not conserved in the presence of friction. Where does it go? It’s not lost or destroyed…it just goes somewhere else & is therefore “not conserved”. As a sanding block slides on a piece of wood, energy (in the form of heat) is dissipated into the block and surface.

Conservation of Mechanical Energy Starting from rest, a child zooms down a frictionless slide from an initial height of 3.00 m. What is her speed at the bottom of the slide? Assume she has a mass of 25.0 kg.

Sample Problem A bird flying with a speed of 18.0m/s over water when it accidentally drops a 2.00kg fish. If the altitude of the bird is 5.40m and friction is disregarded, what is the speed of the fish when it hits the water? (20.7m/s)

“The Ranger” An Olympic runner leaps over a hurdle. If the runner’s initial vertical speed is 2.2m/s, how much will the runner’s center of mass be raised during the jump? (.25m)

Divide and Conquer to become the physics master! Concept3’s2’s Work1, 2 or 4, 31,3,5,6 Work & direction of forceAll in order1,2,6,3,4 or 5 Kinetic EnergyAll in order1,2,3 Potential Energy1, 2 or 3, 41,2,3,4 Conservation of EnergyAll in order1,3,4,2,5

Q.O.T.D. Why are mountain roads designed to wrap or curve around the mountain, rather than to go straight up or down the mountain? BEWARE OF THINKING THIS IS JUST A SIMPLE ANSWER…REMEMBER IT’S PHYSICS!

Power Objectives Relate the concepts of energy, time, and power. Calculate power in two different ways. Explain the effect of machines on work and power.

Power Power is a quantity that measures the rate at which work is done or energy is transformed. P = W/∆t power = work ÷ time interval Since… P = Fv power = force  velocity

Power Units:

Power A student with a mass of 66.0 kg climbs a staircase in 44.0 s. If the distance between the base and the top of the staircase is 14.0 m, how much power will the student deliver by climbing the stairs?