Unit #4: Work and Energy

Work

Force that causes a displacement / Force applied through a distance

• Force and displacement need to be in the same direction.

• Someone trying to move an object, without the object moving is not considered as work.

• Label is Joules

The equation for work

Work(Joules) = Applied force (Newtons) x Distance (meters)

• W=Fd

• W is work

• F is for applied force

• d is for distance

Machine

Device used to change the force or increase the motion from work

• There are two categories of machines:

  • Simple Machine

  • Compound Machine

Simple Machine

A machine that does work with only one movement of the machine

Compound Machine

A machine that includes at least two simple machines

• It can include more, but the minimum is two different machines

Efficiency

Ratio of output work to input work and it usually is expressed in a percentage

• Input work: work that the person does

• Output work: work that the machine does

Mechanical Advantage

Ratio of output force to input force

• Input force: the amount of force the person uses

• Output force: the amount of force the machine uses

• Force is labeled as Newtons

Why machines cannot be 100% efficient

Real - world machines always lose some energy due to friction, heat, etc., making 100% efficiency impossible

How you can make compound machines to make life simpler?

It makes work easier by increasing mechanical advantage and efficiency, simplifying difficult jobs.

• Ex: Bicycle - gears, pedals, and wheels work together to make riding easier.

Calculating Kinetic Energy

Kinetic (J) = ½ (mass) x (velocity)²

Explain the similarities and differences between work and power

Both involve force and motion. You need a force to do work and generate power.

Work: measures the amount of energy transferred when a force causes displacement.

• Work(J) = Force(N) x Distance(M)

Power: measures the rate at which work is done (or energy is transferred)

• Power(Watts) = Energy(J) = Work(J) / Time(S)

Screw

An inclined plane that is wrapped around a cylinder

• When turned, a small force is applied over the long distance of screws thread

• The amount of work for a screw depends on the number of thread, which is actually the length of the inclined plane

Inclined Plane

A straight, slanted surface

• Ex: A ramp is a stationary inclined plane

• It is easier to push an object up a ramp than it is to lift the same object straight up to the same height

Pulley

A rope or chain wrapped around a wheel

• A load is attached to one end of the rope and a force is applied to the other end of the rope

• A double pulley makes work easier not only by changing the direction of the force, but also by multiplying the effort

• A single pulley makes work easier by changing the direction of the effort force

• When you pull down on the rope, the load moves up

Wheel and Axle

A simple machine that consists of two circular objects of different sizes

• The wheel is always larger than the axle

• Ex: door knob

• By getting more force from the effort put in when compared to the amount of distance

Lever

Made up of a bar that pivots at a fixed point called a fulcrum

• The force applied to a lever is called effort

• The object moved is the load

• They are classified into three different groups

• The groups are based upon the locations of the fulcrum, load, and the input force

  • In a first class lever, the fulcrum is in the middle, load and force on the ends

    • Ex: Seesaws

  • In a second class lever, the load (resistance) is in the middle, force and fulcrum are on the ends

    • Ex: Nutcracker, Wheelbarrow

  • In a third class lever, the force (effort) is in the middle, the load and fulcrum are on the ends

    • Ex: Baseball bat or Fishing rod

Potential Energy

Energy that is stored due to the interactions between objects

Kinetic Energy

Energy due to motion

Thermal Energy

The sum of the kinetic energy and the potential energy of all particles in the object

• It is estimated that 18% of the world believes that “heat’ and “temperature” are the same thing… however they are not!

Nuclear Energy

Energy stored in the nucleus of an atom

Fissure

Splitting apart

Fusion

Combining together

Chemical Energy

Energy that is stored due to chemical bonds

Gravitational Potential Energy

Energy that is due to the gravitational force between objects

Elastic Potential Energy

Energy stored by compressing or stretching an object

Power

The rate at which energy is converted

What is the difference between efficiency and machanical advantage?

Efficiency: The ratio of output to total work input (%)

(Output work/Input work x100)

Mechanical Advantage: The ratio of ouput force to input force

(Output force/Input force)

Temperature

Measurement of average kinetic energy

• Temperature can be describes as hot or cold

• Heat cannot!

Heat

Transfer of kinetic energy between objects / Energy transferred between objects due to a temperature difference between those objects

Specific Heat / Specific Heat Capacity

Specific Heat: The amount of heat needed to raise the temperature of one gram of a substance by one degree celcius

Specific Heat Capacity: The amount of heat needed to raise the temperature of the entire object by one degree celcius

Convection

How heat passes through fluids / the transfer of thermal energy in a fluid by the movements of warmer and colder fluid

• A fluid is anything that has loosely moving molecules that can move easily from one place to another. Liquids and gases are fluids.

Conduction

How heat transfers through direct contact with other objects / The transfer of thermal energy by collisions between the particles that make up matter

• Anytime that two objects or substances touch, the hotter objects passes heat

• Hot to cold (cannot flow from cold to hot)

• Solids conduct heat better

Radiation

How heat moves through places where there are no molecules. Radiation is actually a form of electomagnetic energy. / The transfer of energy by electromagnetic waves, such as light and microwaves.

• Radiation is heat moving in waves. It does not need molecules to pass the energy along

Thermal Insulators

A material through which thermal energy moves slowly

Heating Systems

A mechanism for maintaining temperatures at an acceptable level; by using thermal energy within a home, office, or other dwelling. Heating systems move heat from a source (like a furnace or heat pump) to inside the building, making it warmer.

Cooling Systems

The purpose of a cooling system such as a refrigerator or air conditioner is to transfer thermal energy in order to keep things cool. Cooling systems move heat from inside to outside, making the inside cooler.

First Law of Thermodynamics

Energy cannot be created or destroyed. Energy can be transferred between the system and the surroundings through the transfer of heat (q) or by the performance of mechanical work (w).

Second Law of Thermodynamics

Energy flows from regions of higher concentration to regions of lower concentration.

• This does not mean the reverse (lower to higher) isn’t true. It just does not happen naturally. Usually requires more energy and work to get this to occur.

Third Law of Thermodynamics

Absolute zero temperature can’t be reached / A crystal with no imperfections (chips, cracks, etc.) would be at perfect thermal equilibrium (no particle movement) at a temperature of absolute zero (0 kelvin)

Energy Transformation

Processes that convert energy from one type (e.g., kinetic, gravitational potential, chemical energy) into another. Any type of energy use must involve some sort of energy transformation.

The equation for Thermal Energy

Q = mc∆T

Q = (J) heat absorbed (positive) or released (negative)

M = mass(g)

C = Specific Heat J/kg°c

∆T = Final temperature - Initial temperature

Explaining how Convection, Conduction and Radiation are similar and different

Convection: fluid movement

Conduction: direct contact

Radiation: involves electromagnetic waves

Similarities: They are all methods of heat transfer moving energy from hotter to colder regions.

How energy transforms between different forms

Energy is usually converted to thermal energy. The energy is no longer useful and is transferred into the surroundings by conduction and convention.

• Ex: A laptop computer converts electrical energy into thermal energy