SP8+SP9: Energy: Forces doing Work & Forces and their Effects

What is the law of conservation of energy?

Energy can be neither created or destroyed, but transferred from one form to another

What are the energy stores?

Thermal, kinetic, gravitational potential, elastic potential, chemical, magnetic, electrostatic, nuclear

What is a system?

An object or group of objects

What happens when a system changes?

Energy is transferred - it can be transferred into/away from the system, between different objects in the system or between different types of energy stores.

How does heating a pan of water transfer energy?

Energy is transferred by heating into the system (pan of water) to the thermal energy stores of the pan and the water, which increases their temperature. However, if the system is the fuel e.g. the stove and the pan, energy is transferred from the chemical energy store of the gas to the thermal energy stores of the pan and the water.

How can energy be transferred by electrical equipment?

In the instance of e.g. an electric tooth brush, the toothbrush is the system. It transfers energy electrically from the chemical energy store of the batter to the kinetic energy store of its bristles. Some of this energy is transferred out of the system to the surroundings by sound and by heating.

What is work done?

A transfer of energy, as a result of a force acting on an object. If the work is done on an object, it gains energy; if the work is done by an object, it loses energy. The total amount of energy remains the same (it is conserved).

How can energy be transferred by forces doing work?

If e.g. a box is lifted up off a floor, the box is the system. As the box is lifted, work is done against gravity. This causes energy to be transferred to the box's kinetic and gravitational potential energy stores.

What is the equation for Work Done?

Work Done (J) \= Force (N) x Distance (m)

Why does a force doing work cause a rise in temperature?

Energy is dissipated to the thermal energy stores of the moving object and its surroundings. This means that the process is wasteful and the efficiency of the process is reduced.

What is the equation for efficiency?

Efficiency (%) \= useful energy output/total energy input

What is being done when you push an object alone a rough surface?

Work is being done against frictional forces. Energy is being transferred to the kinetic energy stores of the object because it starts moving, but some is also being transferred to thermal energy stores due to friction. This causes the overall temperature of the object to increase.

What is power?

The rate at which work is done - the rate of energy transferred.

What is the equation for power?

Power (W) \= Work Done (J)/Time (S)

What is one Watt equivalent to?

1 joule per second (J/s)

What does it mean if an object has a large power?

It means the object does more work per second e.g. if an electric heater has a power of 600W, this means it transfers 600J of energy every second. A 1200W heater would transfer twice as much energy per second and so would heat a room quicker than the 600W heater.

What is a force?

A push or pull on an object that is caused by it interacting with something

Is force a scalar or vector quantity?

Vector quantity, as it has both magnitude and direction

What is a contact force?

When two objects have to be touching for a force to act

Give four examples of contact forces

Friction, air resistance, tension and normal contact force

What is a non-contact force?

When two objects do not need to be touching for a force to act

Give three examples of non-contact forces

Gravitational forces, electrostatic forces, magnetic forces

What law of gravity applies when two objects are touching each other?

Newton's Third Law - whenever two objects interact, both objects feel an equal, but opposite force. This pair of forces is called an interaction pair.

What is the Resultant force (Rf)?

The single force that replaces multiple forces acting at a single point - the net force of an object

What causes an object to rotate?

A force or system of forces

What is the moment?

The turning effect of a force

How do you calculate the size of the moment?

Moment of a force (Nm) \= Force (N) x Distance perpendicular to the direction of the force (M)

How do you get the maximum moment?

You push perpendicular (right angles) to the turning surface e.g. a spanner.

What is the principle of moments?

For an object in rotational equilibrium the sum of the clockwise moments about any point is equal to the sum of anticlockwise moments about that point.

How is a turning effect of a spanner made?

The force on the spanner cause a turning effect on the nut, which acts as a pivot. A larger force or a longer distance i.e. a longer spanner would mean a larger turning effect (moment)

How do levers make it easier to do work and make use of moments?

They transfer the turning effect of a force by pushing on end of a lever down and the rotation around the pivot causes the other end to rise. They increase the perpendicular distance of the force from the pivot, so decrease the force needed to produce the same moment.

How are levers used to decrease the force required to lift something?

They use a pivot point and the concept of moments to reduce the force. The further you are away from the pivot, the less force you need to lift the object up.

How do gears make use of moments?

Gears are circular cogs with 'teeth' around the edge. Their teeth interlock so that turning one causes another to turn in the opposite direction. They are used to transmit the rotational effect of a force from one place to another. A force applied to a small gear creates a small moment. This gear applies the same force to the next gear. If the next gear is larger, this force is applied further from its pivot, so its moment is larger.

How does the size of a gear affect its speed?

The larger the gear, the slower it spins.

How does lubricating levers/gears improve efficiency?

There is less resistance; less energy is wasted/dissipated as heat, overcoming frictional forces.