Work done is the amount of energy transferred when an external force causes an object to move over a distance.

Power is the rate of doing work, or the rate of energy transfer.

Define variable force .

A variable force is a force that does not remain constant, changing as an object moves — this is more representative of a force in real life.

How can the work done by a variable force be found from a force-displacement graph?

The work done is equal to the area under the force-displacement graph .

How should the area under a force-displacement graph be found if it is not a simple standard shape?

The graph is split into sections (such as triangles, rectangles or trapeziums) and the total area is the sum of the areas of each section.

A small force acting over a long displacement can do the same work as a large force acting over a .......... displacement.

A small force acting over a long displacement can do the same work as a large force acting over a small displacement.

If a varying force acting on an object increases, what happens to its acceleration?

The acceleration increases (and if the force decreases, the acceleration decreases).

Efficiency is the ratio of the useful power output from a system to its total power input.

Efficiency is usually expressed as a .......... , found by multiplying the ratio of useful output to total input by 100.

Efficiency is usually expressed as a percentage , found by multiplying the ratio of useful output to total input by 100.

In a lightbulb converting electrical energy to light, which energy output is useful and which is wasted?

Light energy is useful Heat (thermal) energy produced is wasted

True or False? Efficiency has units, since it is often quoted as a percentage.

False. Efficiency has no units — it is a ratio of two quantities with the same units (power or energy), so the units cancel.

Does a high-efficiency system waste more or less energy than a low-efficiency system?

A high-efficiency system wastes less energy, since most of the input energy is usefully transferred; a low-efficiency system wastes more .

Define the principle of conservation of energy .

Energy cannot be created or destroyed ; it can only be transferred from one form to another, so the total energy in a closed system remains constant.

Why is dissipated energy usually described as "wasted" energy?

Dissipated energy is wasted because it is transferred to the surroundings (usually as thermal, light or sound energy) and cannot easily be used for a specific purpose.

In a television, electrical energy is transferred to light energy, sound energy and .......... energy, of which the last is wasted.

In a television, electrical energy is transferred to light energy, sound energy and thermal energy, of which the last is wasted.

State the energy transfer that occurs as an object falls freely in a vacuum.

Gravitational potential energy is transferred to kinetic energy .

Give the main energy transfer occurring in a battery.

Chemical energy is transferred to electrical energy .

When an object travels up a rough inclined surface, how is its loss in kinetic energy related to its gain in gravitational potential energy?

Loss in kinetic energy = Gain in gravitational potential energy + Work done against friction

True or False? The thermal energy produced by a system is always classed as wasted energy.

False. Whether energy counts as useful or wasted depends on the system — for example, thermal energy is the useful output of a heater, but wasted in a television.

Define kinetic energy .

Kinetic energy is the energy an object has due to its motion (or velocity); the faster an object moves, the greater its kinetic energy.

Define gravitational potential energy .

Gravitational potential energy is energy stored in a mass due to its position in a gravitational field; it increases if the mass is lifted and decreases if it falls.

What position is normally taken as zero gravitational potential energy?

Ground level (the Earth's surface) is usually taken as zero, although any position may be chosen as zero when calculating a change in gravitational potential energy.

What type of relationship exists between gravitational potential energy and height on a graph of one against the other?

A linear relationship — the graph is a straight line.

A LevelPhysicsAQAFlashcardsMechanics & MaterialsWork, Energy & Power

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

Exam code: 7408

1/35

0Still learning

Know0

FrontWork & Power
Define work done.
FrontWork & Power
What is the equation for the work done by a constant force acting parallel to an object's displacement?
BackWork & Power
$W = F s$
W = work done (J)
F = average force applied (N)
s = displacement (m)

Was this flashcard helpful?

Cards in this collection (35)

Define work done.
Work done is the amount of energy transferred when an external force causes an object to move over a distance.
What is the equation for the work done by a constant force acting parallel to an object's displacement?
$W = F s$
- W = work done (J)
- F = average force applied (N)
- s = displacement (m)
When a force acts at an angle θ to an object's displacement, the work done is given by ..........
When a force acts at an angle θ to an object's displacement, the work done is given by $W = F s \cos θ$
Define power.
Power is the rate of doing work, or the rate of energy transfer.
Which equation gives power when an object moves at constant velocity under a constant force parallel to that velocity?
$P = F v$
- F = force (N)
- v = velocity (m s^-1)
True or False?
The equation $P = F v$ can be used with the full force, even when that force is not directed along the object's velocity.
False.
If the force is not parallel to the velocity, only the component of the force in the direction of the velocity must be used.
What is the relationship between a newton metre and a joule?
$1 N m = 1 J$
Define variable force.
A variable force is a force that does not remain constant, changing as an object moves — this is more representative of a force in real life.
How can the work done by a variable force be found from a force-displacement graph?
The work done is equal to the area under the force-displacement graph.
How should the area under a force-displacement graph be found if it is not a simple standard shape?
The graph is split into sections (such as triangles, rectangles or trapeziums) and the total area is the sum of the areas of each section.
A small force acting over a long displacement can do the same work as a large force acting over a .......... displacement.
A small force acting over a long displacement can do the same work as a large force acting over a small displacement.
Which two equations can only be used when a force is constant, not when it varies?
$W = F s$ and $P = F v$
For a varying force, the work done must instead be found from the area under a force-displacement graph.
True or False?
The equation $W = F s$ can be used to calculate the work done by any force, whether constant or varying.
False.
$W = F s$ only applies to a constant force; for a varying force, the work done must be calculated from the area under a force-displacement graph.
If a varying force acting on an object increases, what happens to its acceleration?
The acceleration increases (and if the force decreases, the acceleration decreases).
Define efficiency.
Efficiency is the ratio of the useful power output from a system to its total power input.
Efficiency is usually expressed as a .........., found by multiplying the ratio of useful output to total input by 100.
Efficiency is usually expressed as a percentage, found by multiplying the ratio of useful output to total input by 100.
What is the equation for efficiency in terms of power?
$Efficiency = \frac{Power output}{Power input} \times 100$
What is the equation for efficiency in terms of energy?
$Efficiency = \frac{Useful energy output}{Total energy input} \times 100$
In a lightbulb converting electrical energy to light, which energy output is useful and which is wasted?
- Light energy is useful
- Heat (thermal) energy produced is wasted
True or False?
Efficiency has units, since it is often quoted as a percentage.
False.
Efficiency has no units — it is a ratio of two quantities with the same units (power or energy), so the units cancel.
Does a high-efficiency system waste more or less energy than a low-efficiency system?
A high-efficiency system wastes less energy, since most of the input energy is usefully transferred; a low-efficiency system wastes more.
Define the principle of conservation of energy.
Energy cannot be created or destroyed; it can only be transferred from one form to another, so the total energy in a closed system remains constant.
Why is dissipated energy usually described as "wasted" energy?
Dissipated energy is wasted because it is transferred to the surroundings (usually as thermal, light or sound energy) and cannot easily be used for a specific purpose.
In a television, electrical energy is transferred to light energy, sound energy and .......... energy, of which the last is wasted.
In a television, electrical energy is transferred to light energy, sound energy and thermal energy, of which the last is wasted.
State the energy transfer that occurs as an object falls freely in a vacuum.
Gravitational potential energy is transferred to kinetic energy.
Give the main energy transfer occurring in a battery.
Chemical energy is transferred to electrical energy.
When an object travels up a rough inclined surface, how is its loss in kinetic energy related to its gain in gravitational potential energy?
Loss in kinetic energy = Gain in gravitational potential energy + Work done against friction
True or False?
The thermal energy produced by a system is always classed as wasted energy.
False.
Whether energy counts as useful or wasted depends on the system — for example, thermal energy is the useful output of a heater, but wasted in a television.
Define kinetic energy.
Kinetic energy is the energy an object has due to its motion (or velocity); the faster an object moves, the greater its kinetic energy.
What is the equation for kinetic energy, and what does each symbol represent?
$E_{k} = \frac{1}{2} m v^{2}$
- E_k = kinetic energy (J)
- m = mass (kg)
- v = velocity (m s^-1)
Define gravitational potential energy.
Gravitational potential energy is energy stored in a mass due to its position in a gravitational field; it increases if the mass is lifted and decreases if it falls.
The equation $Δ E_{p} = m g Δ h$ is only valid for energy changes in a .......... gravitational field.
The equation $Δ E_{p} = m g Δ h$ is only valid for energy changes in a uniform gravitational field.
What position is normally taken as zero gravitational potential energy?
Ground level (the Earth's surface) is usually taken as zero, although any position may be chosen as zero when calculating a change in gravitational potential energy.
What type of relationship exists between gravitational potential energy and height on a graph of one against the other?
A linear relationship — the graph is a straight line.
True or False?
In the kinetic energy equation, both the mass and the velocity are squared.
False.
Only the velocity is squared in $E_{k} = \frac{1}{2} m v^{2}$ ; the mass and the ½ are not squared.

Sign up to unlock flashcards

By signing up you agree to our Terms and Privacy Policy

Previous:Linear Momentum & ConservationNext:Bulk Properties of Solids