Forces and energy

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Forces and energy- Energy transfer and systems

Energy can never be created or destroyed

Its only transferred between forms and objects

Different energy stores

Kinetic- anything moving

Thermal- any object- the hotter the object the more energy it has got in its store

Chemical- anything that can release energy by chemical reaction

Gravitational potential energy- anything that uses gravity

Elastic potential- anything that stretches

Electrostatic- Two charges that attract or repel

Magnetic- 2 magnets that attract or repel

Nuclear- Anything that has a nuclear store

Energy can be transferred between stores ;

Mechanically- by a force doing work

Electrically- by a moving charge doing work

Heating- light or sound

Radiation – energy transferred as a wave eg light, infrared, sound - the Sun emits light radiation and infrared radiation

A system is a single object or a group of objects that you're intrested in

Energy can be transferred into or away from a system. It can be transferred between objects r between different energy stores .

When system changes some energy gets dissipated

A closed system is one that the net change in energy is 0

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Forces and energy- Energy transfer and systems

Energy can be transferred by heating

Bringing water to a boil on a gas hob

Energy from the chemical store in the gas increases the internal (thermal) energy of the water in the pan.

Energy can be transferred electrically.

An electric motor lifting a weight

The chemical energy store decreases as energy is transferred via the electrical pathway and the gravitational potential energy store increases.

Energy can be transferred by forces doing work (mechanically)

A toy car rolls to a stop

Kinetic energy store decreases as energy is transferred into the thermal energy store of the surroundings by the work done by the force of friction.

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Forces and energy- Power

Power is the rate at which energy is transferred.

Rate at which work is done.

P= E/T

P= Power (Watts/W) E= Work done (Joules/J) / Energy transferred T= Time (Seconds/S)

Power is equal to 1 joule

Which means that 1 Joule of energy is transferred per second- And 1 joule of work is done per second

Kilo- Thousand

Mega= Million

Question;

Two electric motors are used to lift a 2 N weight through a vertical height of 10 m.

Motor one does this in 5 seconds.

Motor two does this in 10 seconds.

For both motors, the energy transferred - the work done - is 20 J.

E= F x D 2 x 10= 20J

For motor one:

P=E/t = 20/ 5= 4W

For motor two:

P=E/t= 20/10 =2W

Since twice as much energy is transferred by motor one each second, it is possible to say that motor one is twice as powerful as motor two.

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Forces and energy- Work done

When a force causes a body to move, work is being done on the object by the force. Work is the measure of energy transfer when a force ‘F’ moves an object through a distance ‘d’. So when work is done, energy has been transferred from one energy store to another, and so:

energy transferred = work done

Energy transferred and work done are both measured in joules (J).

The amount of work done when a force acts on a body depends on two things:

the size of the force acting on the object

the distance through which the force causes the body to move in the direction of the force

work done = force × distance

The equation used to calculate the work done is:

E = F x D

work done (E) is measured in joules (J)

force (F) is measured in newtons (N)

distance (d) is in the same direction as the force and is measured in metres (m)

Question

12,000 J of energy is supplied to move a small truck a distance of 80 m. What is the size of the force applied?

E = F x D

F= E/D

12000/80

=150N

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Forces and energy- Forces

A force is a Vector it has both size and direction

Its a pull or a push on an object causing it to move and interact with something

Objects can be contacting or non contacting forces

Contact forces are forces that act between two objects that are physically touching each other. Examples of contact forces include:

Tension

An object that is being stretched experiences a tension force. For example, a cable holding a ceiling lamp.

Friction

Two objects sliding past each other experience friction forces. For example, a box sliding down a slope.

Air resistance

An object moving through the air experiences air resistance. For example, a skydiver falling through the air.

When a contact force acts between two objects, both objects experience the same size force, but in opposite directions. This is Newtons 3rd Law.

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Forces and energy- Forces

Non-contact forces are forces that act between two objects that are not physically touching each other. Examples of non-contact forces include:

Magnetic force

A magnetic force is experienced by any magnetic material in a magnetic field.

Opposite magnetic poles (N–S or S–N) attract each other. Like magnetic poles (N–N or S–S) repel each other:

Electrostatic force

An electrostatic force is experienced by any charged particle in an electric field.

Opposite charges (+ and –) attract

Like charges (– and –, or + and +) repel

Gravitational force

A gravitational force is experienced by any mass in a gravitational field.

Masses are attracted towards each other by gravitational force:

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Forces and energy- Forces

A free body diagram models the forces acting on an object. The object or ‘body’ is usually shown as a box or a dot. The forces are shown as thin arrows pointing away from the centre of the box or dot.

Free body diagrams do not need to be drawn to scale but it can sometimes be useful if they are. It is important to label each arrow to show the magnitude of the force it represents. The type of force involved may also be shown.

If you have a number of forces acting at a single point you can replace them with a single force this single force is called resultant force.

Objects at equilibrium have a resultant force of 0 objects in equilibrium are either stationary or moving at steady speed Newtons 1st law.

Formula for Gravitational potential energy

gravitational potential energy = mass × height × gravitational field strength (g) This is when: gravitational potential energy is measured in joules (J)

Formula for kinetic energy

KE = ½ × m × v2. where: KE is the kinetic energy in joules, J. m is the mass in kilograms, kg. v is the speed in metres per second, m/s.

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Questions from spec

Q. Describe the changes involved in the way energy is stored when systems change

A. Energy is a quantity that is conserved - it cannot be created or destroyed. Energy can be stored and transferred .It could be stored as Kinetic energy, Gravitational potential energy, Thermal energy, Chemical energy, Elastic potential energy, Electrostatic energy, Magnetic energy, Nuclear energy.

Q. What happens in a closed system?

A. Where there are energy transfers in a closed system there is no net change to the total energy in that system

Q. Identify the different ways that the energy of a system can be changed

A. Heating/ Mechanical/ Work done by forces

Q. Describe how to measure the work done by a force

A. work done = force × distance This is when: work done is measured in joules (J) force is measured in newtons (N) distance moved along the line of action of the force is measured in metres (m)

Q. What is energy transferred equal to?

A. Energy transferred (joule, J) is equal to work done (joule, J)

Q. Describe the formula for Gravitational potential energy?

A. gravitational potential energy = mass × height × gravitational field strength (g) This is when: gravitational potential energy is measured in joules (J)

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Questions from spec

Q. Describe the formula for kinetic energy?

A. KE = ½ × m × v2. where: KE is the kinetic energy in joules, J. m is the mass in kilograms, kg. v is the speed in metres per second, m/s.

Q. Explain, using examples, how in all system changes energy is dissipated so that it is stored in less useful ways

A. for a tumble dryer, the electrical work is transferred into useful internal (thermal) energy which helps to dry clothes - energy is dissipated by sound waves

Q. How do mechanical processes become less useful?

A. When they cause a rise in temperature so dissipating energy in heating the surroundings

Q. Define power?

A. It is the rate at which energy is transferred

Q. What is an example of power?

A. The power involved in moving a ground vehicle is the product of the traction force on the wheels and the velocity of the vehicle. The output power of a motor is the product of the torque that the motor generates and the angular velocity of its output shaft.

Q. Provide the formula for power

A. P= E/T

P= Power (Watts/W) E= Work done (Joules/J) / Energy transferred T= Time (Seconds/S)

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Questions from spec

Q. What is one Watt equal to?

A. One watt is equal to one joule per second, J/s

Q. What is the formula for efficiency?

A. efficiency= useful energy transferred by the device/ total energy supplied to the device)