WORK, POWER, AND MACHINES 9.1

WORK  A quantity that measures the effects of a force acting over a distance  Work = force x distance  W = Fd

WORK  Work is measured in: NmNm  Joules (J)

WORK EXAMPLE  A crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do?

WORK EXAMPLE  Work = Fd  Work = (5200 N)(25m)  Work = N  m = J

POWER  A quantity that measures the rate at which work is done  Power = work/time  P = W/t

POWER  Watts (W) is the SI unit for power  1 W = 1 J/s

POWER EXAMPLE  While rowing in a race, John uses 19.8 N to travel meters in 60.0 s. What is his power output in Watts?

POWER EXAMPLE  Work = Fd  Work = 19.8 N x m= 3960 J  Power = W/t  Power = 3960 J/60.0 s  Power = 66.0 W

MACHINES  Help us do work by redistributing the force that we put into them  They do not change the amount of work

MACHINES  Change the direction of an input force (ex car jack)

MACHINES  Increase an output force by changing the distance over which the force is applied (ex ramp)  Multiplying forces

MECHANICAL ADVANTAGE  A quantity that measures how much a machine multiples force or distance.

MECHANICAL ADVANTAGE Output Force Input Force Input distance Output Distance Mech. Adv = Mech. Adv. =

MECH. ADV. EXAMPLE  Calculate the mechanical advantage of a ramp that is 6.0 m long and 1.5 m high.

MECH. ADV. EXAMPLE  Input = 6.0 m  Output = 1.5 m  Mech. Adv.=6.0m/1.5m  Mech. Adv. = 4.0

ENERGY

ENERGY AND WORK  Energy is the ability to do work  whenever work is done, energy is transformed or transferred to another system.

ENERGY  Energy is measured in:  Joules (J)  Energy can only be observed when work is being done on an object

POTENTIAL ENERGY PE  the stored energy resulting from the relative positions of objects in a system

POTENTIALENERGY PE POTENTIAL ENERGY PE  PE of any stretched elastic material is called Elastic PE  ex. a rubber band, bungee cord, clock spring

GRAVITATIONAL PE  energy that could potentially do work on an object due to the forces of gravity.

GRAVITATIONAL PE  depends both on the mass of the object and the distance between them (height)

GRAVITATIONAL PE EQUATION grav. PE= mass x gravity x height PE = mgh or PE = wh

PE EXAMPLE  A 65 kg rock climber ascends a cliff. What is the climber’s gravitational PE at a point 35 m above the base of the cliff?

PE EXAMPLE  PE = mgh  PE=(65kg)(9.8m/s 2 )(35m)  PE = 2.2 x 10 4 J  PE = J

KINETIC ENERGY  the energy of a moving object due to its motion.  depends on an objects mass and speed.

KINETIC ENERGY  What influences energy more: speed or mass? ex. Car crashes  Speed does

KINETIC ENERGY EQUATION KE=1/2 x mass x speed squared KE = ½ mv 2

KE EXAMPLE  What is the kinetic energy of a 44 kg cheetah running at 31 m/s?

KE EXAMPLE  KE = ½ mv 2  KE= ½(44kg)(31m/s) 2  KE=2.1 x 10 4 J  KE = J

MECHANICAL ENERGY  the sum of the KE and the PE of large-scale objects in a system  work being done

NONMECHANICAL ENERGY  Energy that lies at the level of atoms and does not affect motion on a large scale.

ATOMS  Atoms have KE, because they are constantly in motion.  KE  particles heat up  KE  particles cool down

CHEMICAL REACTIONS  during reactions stored energy (called chemical energy)is released  So PE is converted to KE

OTHER FORMS  nuclear fusion  nuclear fission  Electricity  Light

ENERGY TRANSFORMATIONS 9.4

CONSERVATION OF ENERGY  Energy is neither created nor destroyed  Energy is transferred

ENERGY TRANSFORMATION  PE becomes KE  car going down a hill on a roller coaster

ENERGY TRANSFORMATION  KE can become PE  car going up a hill KE starts converting to PE

PHYSICS OF ROLLER COASTERS 