Chapter 10 –Circular Motion

Centripetal force keeps an object in Circular motion.

10.1 Work

Axis.

9.1 Work

Work falls into two categories: 1. Work done against a force such as

gravity, friction, or to stretch a spring. 2. Work done to change the speed of

an object such as speeding up or slowing a car.

9.1 Work

Units Fd = W

(Force- Newton, Distance-meter,Nm = Joule (rymes with cool.)

One Joule of work is done when one Newton of force is exerted over the distance of one meter (- like raising an apple over your head.

9.2 Power

Power is the rate at which (or how fast) work is done.

Power equals the amount of work done divided by the time interval during which the work is done.

With more power, work is done faster. With twice the power, twice as much work can

be done in the same time, or the same amount of work can be done in half the time.

time

WorkPower

9.2 Power

Units

One Watt (W) of power is expended when one Joule of work is done in one second.

Kilowatt=1,000 Watts, Megawatt=1,000,000Watts

The space shuttle uses 33,000 MW of power. One horse-power(hp) = 765W

Wattsecond

Joule

time

WorkPower

9.3 Mechanical Energy

Energy is the property of an object or system that enables it to do work.

Energy is measured in Joules (like work.) Mechanical Energy is the energy due to

the position or movement of something. The two forms of mechanical energy are

potential energy and kinetic energy.

9.4 Potential Energy

Potential Energy (PE) is energy that is stored and held in readiness.

Some Example include: Elastic Potential Energy – stretched or

compressed. Chemical Potential Energy – Energy

stored in the molecules of matter. Gravitational Potential Energy – Energy

due to elevated position.

9.4 Potential Energy

Gravitational potential energy (PEg)= weight x height.

PEg=mgh (Remember mg = weight.) Note that the height is the distance

above some arbitrary chosen reference level. The gravitational potential energy is relative to that level and depends only on mg and h.

9.5 Kinetic Energy

Anything in motion is capable of doing work. It has kinetic energy.

Kinetic energy is the energy of motion. The kinetic energy of an object

depends on its mass and speed. Kinetic Energy = ½ mass x speed2

2speed X mass 2

1KE

9.5 Kinetic Energy

The kinetic energy of a moving object is equal to the work required to bring it to its speed from rest, or the work the object can do while being brought to rest.

net force x distance = kinetic energy. Fd = ½ mv2. Note that the speed is squared, so if the

speed is doubled, its kinetic energy is quadrupled.

A car moving at twice the speed has four time the energy.

9.6 Work-Energy Theorem

The work-energy theorem describes the relationship between work and energy.

The work-energy theorem states that whenever work is done, energy changes.

Work equals the change in energy. WorkNet = ΔKE If you push a box at constant speed (with a

force equal to friction) no net work is done. Kinetic energy is involved in heat, sound,

light, and electricity.

9.7 Conservation of Energy

The law of conservation of energy states that energy cannot be created or destroyed. It can be transformed from one form into another, but the total amount of energy never changes.

Our energy comes from the sun.

9.8 Machines

A machine is a device used to multiply forces or simply to change the direction of forces.

A machine transfers energy from one place to another or transforms it from one form to another.

A lever is a simple machine made of a bar that turns about a fixed point.

Work input = work output Since work = Fd (force x distance)input = force x distance)output

No machine can multiply work or energy.

9.8 Machines

Mechanical advantage of a machine is its ratio of output force to input force.

Without friction, MA is also the ratio of input distance to output distance.

Simple Machines:› Levers› Pulleys› Inclined Plane› Wedge› Screw› Wheel and Axle

9.8 Machines

Levers Type 1 lever – the fulcrum is between the (input) force and

the load (output force.)› Ex. Seesaw› Changes direction› Can multiply force or distance

Type 2 lever – the load is between the fulcrum and the force.› Ex. Wheelbarrow› Multiplies Force

Type 3 lever – the force is between the fulcrum and the load.› Ex. Hockey/Lax stick› Multiplies distance (Speed?)

9.8 Machines

Pulley (pulley system) – a kind of lever that can change the direction of the force or multiply the force.

Like a lever, Fulcrum, Input Force and Output Force apply.

Mechanical advantage is equal to the number of ropes that support the load.

Mechanical Advantage can also be determined by the ratio of distances.

tanceOutput Dis

anceInput DistMA

9.9 Efficiency

An Ideal Machine is 100% efficient. In any machine, some energy is transformed into atomic

or molecular kinetic energy – making the machine warmer.

Efficiency of a machine is the ratio of useful energy output to total energy input, or the percentage of the work input that is converted to work output. (Energy converted is work.)

The lower the efficiency of the machine, the greater is the amount of energy wasted as heat.

100% X input work Total

output work UsefulEffeciency

9.9 Efficiency

Inclined plane is simply a sloped surface.

MA is equal to the length divided by the height.

100% X advantage mechanical lTheoretica

advantage mechanical ActualEffeciency

height

lengthMA

9.9 Efficiency

Complex Machines. A complex machine is a combination of two or more simple machines. Scissors – What simple machines make a pair of scissors?

› Two wedges and a lever. A car jack is an example of a complex machine combining a screw

and a wheel and axle. An automobile engine is called a heat engine. It converts thermal

energy to mechanical energy (actually often chemical potential to thermal to mechanical).› Roughly 35% efficient› Energy expended through the cooling system› Exhaust gasses› Friction - Almost half heat the engine parts through friction.› Incomplete combustion

Thermal energy is the graveyard for useful energy.

9.10 Energy for Life

Every living cell in every organism is a machine.

They, like every machine, need energy to run. There is more energy stored in the molecules

of food than there is in the reaction products after the food is metabolized. This energy difference sustains life.

Cells metabolize food like engines burn fossil fuels.

In metabolism carbon joins with oxygen to form carbon dioxide.

9.10 Energy for Life

The reverse process is more difficult. Through photosynthesis, plants

combine carbon dioxide with water to produce hydrocarbon compounds like sugars. Sugar is the simplest food.

Green plants capture the sun’s energy to provide all other life on earth (almost.)

9.11 Sources of Energy

The sun is the source of practically all our energy on Earth.› Exceptions are nuclear and geothermal.

Fossil fuels are the byproducts of photosynthesis hundreds of millions of years ago.

Solar Power – Sunlight is directly transformed into electricity by photovoltaic cells

Sunlight evaporates water which falls in the mountains.

9.11 Sources of Energy

Uneven warming of the Earth produces wind.

Fuel Cells – (like a battery)› Electrolysis uses electricity to separate

hydrogen and oxygen from water H2O.

› Fuel cells combine hydrogen and oxygen to make water and give off electricity.

9.11 Sources of Energy

Nuclear and Geothermal Energy. The most concentrated form of useable energy is

stored in uranium and plutonium, which are nuclear fuels.

A byproduct of the radioactivity at the Earth’s core is geothermal energy.

“Energy sources such as nuclear, geothermal, wind, solar, and hydro power are environmentally friendly.”

“The combustion of fossil fuels , on the other hand, leads to increased atmospheric concentrations of carbon dioxide, sulfur dioxide, and other pollutants.”