The amount of energy transferred when an external force causes an object to move over a certain distance .

When a block is pushed along a rough surface, work is done against .......... , transferring energy to heat and sound.

When a block is pushed along a rough surface, work is done against friction , transferring energy to heat and sound.

True or False? Work is done whenever a force is applied, even if the object does not move.

False. Work is only done when the force causes the object to move over a distance. If there is no displacement, no work is done .

If a force acts in the direction an object is moving, does the object gain or lose energy?

The object gains energy. If the force acts in the opposite direction to the motion, the object loses energy.

Define kinetic energy .

The energy an object has due to its motion (or velocity).

When an object falls, its kinetic energy increases as it is transferred from its .......... energy.

When an object falls, its kinetic energy increases as it is transferred from its gravitational potential energy.

When finding the 'loss of kinetic energy', should the answer be given as a negative value?

No. Energy is a scalar quantity with no direction, so the value is not written with a negative sign.

What happens to an object's kinetic energy if its speed does not change?

It stays constant . An object maintains its kinetic energy unless its speed changes.

Define gravitational potential energy .

The energy stored in a mass due to its position in a gravitational field .

The equation for gravitational potential energy can be derived from the .......... in lifting an object against gravity.

The equation for gravitational potential energy can be derived from the work done in lifting an object against gravity.

What happens to an object's gravitational potential energy when it is lifted, and when it falls?

When lifted , it gains gravitational potential energy (converted from other forms). When it falls , it loses gravitational potential energy (converted to other forms).

For a falling object, how does the gravitational potential energy at the start compare with the kinetic energy at the end?

Since energy is conserved , the gravitational potential energy at the start equals the kinetic energy at the end (ignoring resistive forces).

Define the principle of conservation of energy .

Energy cannot be created or destroyed , only transferred from one form to another — the total energy of a closed system stays constant.

In conservation of energy calculations, what is usually ignored, and why must it be mentioned when comparing to real life?

Heat losses are usually ignored during the calculation. In reality there are always some energy losses , so these should be mentioned when comparing ideal calculated values with real situations.

Conservation of energy is often applied to exchanges between .......... energy and gravitational potential energy.

Conservation of energy is often applied to exchanges between kinetic energy and gravitational potential energy.

Give two common examples where conservation of energy is applied to a kinetic–gravitational energy exchange.

Any from: a swinging pendulum an object in free fall sports such as skiing or skydiving , where gravity causes motion with few drag forces

True or False? In real situations, all of an object's gravitational potential energy is transferred to kinetic energy as it falls.

False. In reality there are always some energy losses (for example to heat through air resistance), so not all the gravitational potential energy becomes kinetic energy.

What two quantities can a gravitational–kinetic energy exchange be used to find?

The final velocity from the distance an object moved, or the height of a drop from the final velocity.

The rate at which energy is transferred, or the rate of doing work (work done per unit time).

True or False? Power measures the total amount of energy transferred.

False. Power is the rate of energy transfer (energy transferred per unit time ), not the total energy.

What do the prefixes kW, MW and GW represent?

kW = × 10 3 W MW = × 10 6 W GW = × 10 9 W

The ratio of the useful power or energy transfer output from a system to its total power or energy transfer input.

Because efficiency is a ratio, it has .......... .

Because efficiency is a ratio, it has no units .

In a lightbulb, which energy output is useful and which is wasted?

The light energy is useful; the heat energy produced is wasted.

True or False? Whether a form of energy is useful or wasted is the same for every system.

False. It depends on the system . For example, heat is wasted in a lightbulb but is the useful output of a heater.

What is the range of efficiency values as a ratio and as a percentage?

As a ratio , between 0 and 1 . As a percentage , between 0 and 100% .

A LevelPhysicsEdexcelFlashcards2. MechanicsWork, Energy & Power

Work, Energy & Power (Edexcel A Level Physics): Flashcards

Exam code: 9PH0

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Define work.

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Cards in this collection (36)

Define work.
The amount of energy transferred when an external force causes an object to move over a certain distance.
State the equation for the work done when a force is parallel to the displacement.
$Δ W = F Δ s$
where F is the average force in the direction of motion and Δs is the displacement.
How is the work done found when a force acts at an angle θ to the displacement?
Use the component of the force parallel to the displacement:
$W = F s \cos θ$
Use cos when θ is measured to the horizontal, and sin when θ is measured to the vertical.
When a block is pushed along a rough surface, work is done against .........., transferring energy to heat and sound.
When a block is pushed along a rough surface, work is done against friction, transferring energy to heat and sound.
True or False?
Work is done whenever a force is applied, even if the object does not move.
False.
Work is only done when the force causes the object to move over a distance. If there is no displacement, no work is done.
If a force acts in the direction an object is moving, does the object gain or lose energy?
The object gains energy. If the force acts in the opposite direction to the motion, the object loses energy.
Define kinetic energy.
The energy an object has due to its motion (or velocity).
State the equation for kinetic energy.
$E_{k} = \frac{1}{2} m v^{2}$
where m is the mass and v is the speed.
When an object falls, its kinetic energy increases as it is transferred from its .......... energy.
When an object falls, its kinetic energy increases as it is transferred from its gravitational potential energy.
True or False?
Doubling the speed of an object doubles its kinetic energy.
False.
Since $E_{k} = \frac{1}{2} m v^{2}$ , kinetic energy depends on the speed squared, so doubling the speed makes the kinetic energy four times larger.
When finding the 'loss of kinetic energy', should the answer be given as a negative value?
No. Energy is a scalar quantity with no direction, so the value is not written with a negative sign.
What happens to an object's kinetic energy if its speed does not change?
It stays constant. An object maintains its kinetic energy unless its speed changes.
Define gravitational potential energy.
The energy stored in a mass due to its position in a gravitational field.
State the equation for the change in gravitational potential energy.
$Δ E_{p} = m g Δ h$
where m is the mass, g is the gravitational field strength and Δh is the change in height.
The equation for gravitational potential energy can be derived from the .......... in lifting an object against gravity.
The equation for gravitational potential energy can be derived from the work done in lifting an object against gravity.
What happens to an object's gravitational potential energy when it is lifted, and when it falls?
When lifted, it gains gravitational potential energy (converted from other forms). When it falls, it loses gravitational potential energy (converted to other forms).
True or False?
The equation $Δ E_{p} = m g Δ h$ can be used to find the energy of a satellite far from Earth.
False.
The equation only applies in a uniform gravitational field, such as near the Earth's surface. It does not apply where the field varies with distance.
For a falling object, how does the gravitational potential energy at the start compare with the kinetic energy at the end?
Since energy is conserved, the gravitational potential energy at the start equals the kinetic energy at the end (ignoring resistive forces).
Define the principle of conservation of energy.
Energy cannot be created or destroyed, only transferred from one form to another — the total energy of a closed system stays constant.
In conservation of energy calculations, what is usually ignored, and why must it be mentioned when comparing to real life?
Heat losses are usually ignored during the calculation. In reality there are always some energy losses, so these should be mentioned when comparing ideal calculated values with real situations.
Conservation of energy is often applied to exchanges between .......... energy and gravitational potential energy.
Conservation of energy is often applied to exchanges between kinetic energy and gravitational potential energy.
Give two common examples where conservation of energy is applied to a kinetic–gravitational energy exchange.
Any from:
- a swinging pendulum
- an object in free fall
- sports such as skiing or skydiving, where gravity causes motion with few drag forces
True or False?
In real situations, all of an object's gravitational potential energy is transferred to kinetic energy as it falls.
False.
In reality there are always some energy losses (for example to heat through air resistance), so not all the gravitational potential energy becomes kinetic energy.
What two quantities can a gravitational–kinetic energy exchange be used to find?
The final velocity from the distance an object moved, or the height of a drop from the final velocity.
Define power.
The rate at which energy is transferred, or the rate of doing work (work done per unit time).
What is the unit of power?
The watt (W).
One watt is equal to one .......... per second, so $1 W = 1 {J s}^{- 1}$ .
One watt is equal to one joule per second, so $1 W = 1 {J s}^{- 1}$ .
Write an equation for power in terms of energy transferred and time.
$P = \frac{E}{t}$
Power is the energy transferred (or work done) divided by the time taken.
True or False?
Power measures the total amount of energy transferred.
False.
Power is the rate of energy transfer (energy transferred per unit time), not the total energy.
What do the prefixes kW, MW and GW represent?
- kW = × 10³ W
- MW = × 10⁶ W
- GW = × 10⁹ W
Define efficiency.
The ratio of the useful power or energy transfer output from a system to its total power or energy transfer input.
Write the equation for the efficiency of a system.
$efficiency = \frac{useful energy (or power) output}{total energy (or power) input}$
It can be given as a ratio or a percentage.
Because efficiency is a ratio, it has ...........
Because efficiency is a ratio, it has no units.
In a lightbulb, which energy output is useful and which is wasted?
The light energy is useful; the heat energy produced is wasted.
True or False?
Whether a form of energy is useful or wasted is the same for every system.
False.
It depends on the system. For example, heat is wasted in a lightbulb but is the useful output of a heater.
What is the range of efficiency values as a ratio and as a percentage?
As a ratio, between 0 and 1. As a percentage, between 0 and 100%.

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