State the principle of the conservation of energy .

The principle of the conservation of energy states that energy cannot be created or destroyed, it can only be transferred from one form to another.

Define the term system as it is used in physics.

The term system in physics is defined as an object or group of objects .

What is the purpose of defining a system in physics?

The purpose of defining the system in physics is to narrow the parameters to focus only on what is relevant to the situation observed.

State three types of energy.

Examples of different types of energy are: kinetic gravitational potential elastic potential chemical nuclear internal thermal

State the three energy types that are considered mechanical energy types.

The three energy types that are considered as mechanical energy types are: kinetic gravitational potential elastic potential

True or False? The total amount of energy in a closed system can change over time.

False. The total amount of energy in a closed system remains constant ; it cannot change over time according to the principle of conservation of energy.

Define the term dissipated energy .

The term dissipated energy means wasted energy transferred to the surroundings that cannot easily be used for another purpose. Energy is usually dissipated as heat, light and sound.

State the three types of energy involved in the vertical oscillation of a mass on a spring.

The three types of energy involved in the vertical oscillation of a mass on a spring are: gravitational potential energy kinetic energy elastic potential energy

For a horizontal mass-spring system, the gravitational potential energy does not need to be considered. Why is this?

For a horizontal mass-spring system, the gravitational potential energy does not need to be considered because the objects are moving horizontally, so the gravitational potential energy does not change.

What are Sankey diagrams used for?

Sankey diagrams are used to represent energy transfers .

True or False? Work is done when a force is applied to an object, regardless of whether the object moves or not.

False. Work is done when a force is applied to an object and results in the object's movement over a distance in the direction of the force.

True or False? Work done is equivalent to energy transferred.

True. The work done by a resultant force on a system is equivalent to the energy transferred.

State the units of work done.

The units of work done are newton metres (Nm) or joules (J). 1 Nm = 1 J

Define mechanical work done .

Mechanical work done is the transfer of energy that occurs when an external force causes an object to move over a distance .

Define kinetic energy .

Kinetic energy is the energy an object has due to its translational motion .

What are the units for kinetic energy?

The units for kinetic energy are joules (J). The units for all types of energy are joules.

Define gravitational potential energy .

Gravitational potential energy is the energy that an object has due to its position in a gravitational field.

True or False? The gravitational potential energy of an object is directly proportional to its change in height in the gravitational field.

True. The gravitational potential energy of an object is directly proportional to its change in height in the gravitational field.

True or False? The gravitational field strength on Earth is 8.9 N kg –1 .

False. The gravitational field strength on Earth is 9.8 N kg –1 .

True or False? As an object with mass is raised in a uniform gravitational field, its gravitational potential energy decreases.

False. As an object with mass is raised in a uniform gravitational field, its gravitational potential energy increases .

Define elastic potential energy .

Elastic potential energy is defined as the energy stored within a material when it is stretched or compressed .

Why is it dangerous when a wire under stress suddenly breaks?

It is dangerous when a wire under stress suddenly breaks because the elastic potential energy stored in the strained wire is converted into kinetic energy. The greater the extension of a wire, the greater the speed it will have when it breaks.

True or False? The change in the total mechanical energy of a system is the work done on the system by any non-conservative force.

True. The change in the total mechanical energy of a system is the work done on the system by any non-conservative force.

Define a non-conservative force .

A non-conservative force is one that dissipates energy away from the system.

Give one example of a system with mechanical energy.

Examples of systems with mechanical energy are: mass-spring system simple pendulum any system in simple harmonic motion

True or False? In the absence of non-conservative forces, mechanical energy is always conserved.

True. In the absence of non-conservative forces, mechanical energy is always conserved.

Power is the rate at which energy is transferred.

True or False? Power is equal to work done.

False. Work done is equal to energy transferred. Therefore, power is equal to the rate of work done, or work done per unit time.

The watt is the unit of power. 1 watt = 1 joule per second.

Efficiency is defined as the ratio of the useful power (or energy transfer) output from a system to its total power (or energy transfer) input.

True or False? An energy transfer with a higher efficiency has a lower amount of wasted energy.

True. An energy transfer with a higher efficiency has a lower amount of wasted energy.

What are the units of efficiency?

Efficiency has no units because it is a ratio . Efficiency is expressed as a decimal between zero and one, or as a percentage.

A fuel is anything that can be burned to produce heat , which can be used for an engine to work.

Define energy density .

Energy density is the amount of energy that a fuel can provide per unit volume of fuel.

What are the units of energy density?

The units of energy density are joules per cubic metre (J m –3 ).

What is one litre (1 L) in cubic metres (m 3 )?

One litre is 0.001 cubic metres. 1 L = 0.001 m –3 .

What factors are taken into consideration when choosing a fuel for a specific purpose?

The factors taken into consideration when choosing a fuel for a specific purpose are: energy density safety of use pollutants released in combustion

IBPhysicsDPSLFlashcardsSpace, Time & MotionWork, Energy & Power

Work, Energy & Power (DP IB Physics)

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State the principle of the conservation of energy.

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State the principle of the conservation of energy.
The principle of the conservation of energy states that energy cannot be created or destroyed, it can only be transferred from one form to another.
Define the term system as it is used in physics.
The term system in physics is defined as an object or group of objects.
What is the purpose of defining a system in physics?
The purpose of defining the system in physics is to narrow the parameters to focus only on what is relevant to the situation observed.
State three types of energy.
Examples of different types of energy are:
- kinetic
- gravitational potential
- elastic potential
- chemical
- nuclear
- internal
- thermal
State the three energy types that are considered mechanical energy types.
The three energy types that are considered as mechanical energy types are:
- kinetic
- gravitational potential
- elastic potential
True or False?
The total amount of energy in a closed system can change over time.
False.
The total amount of energy in a closed system remains constant; it cannot change over time according to the principle of conservation of energy.
Define the term dissipated energy.
The term dissipated energy means wasted energy transferred to the surroundings that cannot easily be used for another purpose. Energy is usually dissipated as heat, light and sound.
State the three types of energy involved in the vertical oscillation of a mass on a spring.
The three types of energy involved in the vertical oscillation of a mass on a spring are:
- gravitational potential energy
- kinetic energy
- elastic potential energy
For a horizontal mass-spring system, the gravitational potential energy does not need to be considered. Why is this?
For a horizontal mass-spring system, the gravitational potential energy does not need to be considered because the objects are moving horizontally, so the gravitational potential energy does not change.
What are Sankey diagrams used for?
Sankey diagrams are used to represent energy transfers.
What does the straight horizontal (green) arrow represent on a Sankey diagram?
The straight horizontal (green) arrow on a Sankey diagram represents the useful energy output.
What do the curved downwards pointing (blue) arrows represent on a Sankey diagram?
The curved downward pointing (blue) arrows on a Sankey diagram represent wasted energy.
How can the total energy input be determined from a Sankey diagram?
The total energy input can be determined from a Sankey diagram by the flat edge of the arrows or by the useful plus the wasted energy values.
State the efficiency of this energy transfer.
The efficiency of this energy transfer is 24% or 0.24.
- $efficiency = \frac{useful energy out}{total energy in}$
- $efficiency = \frac{120}{500} \times 100 % = 24 %$
True or False?
Work is done when a force is applied to an object, regardless of whether the object moves or not.
False.
Work is done when a force is applied to an object and results in the object's movement over a distance in the direction of the force.
True or False?
Work done is equivalent to energy transferred.
True.
The work done by a resultant force on a system is equivalent to the energy transferred.
State the units of work done.
The units of work done are newton metres (Nm) or joules (J).
1 Nm = 1 J
Define mechanical work done.
Mechanical work done is the transfer of energy that occurs when an external force causes an object to move over a distance.
State the equation for mechanical work done.
The equation for mechanical work done is $W = F s$
Where:
- $W$ = work done, measured in newtons metres (Nm) or joules (J)
- $F$ = force applied, measured in newtons (N)
- $s$ = displacement, measure in metres (m)
True or False?
The work done equation, $W = F s$ , can only be used if the force applied is constant.
True.
The work done equation, $W = F s$ , can only be used if the force applied is constant.
State the equation for mechanical work done when the force is applied at an angle to the object's displacement.
The equation for mechanical work done is $W = F s \cos θ$
Where:
- $W$ = work done, measured in newtons metres (Nm) or joules (J)
- $F$ = force applied, measured in newtons (N)
- $s$ = displacement, measure in metres (m)
- $θ$ = angle between force applied and displacement, measured in degrees (°)
Define kinetic energy.
Kinetic energy is the energy an object has due to its translational motion.
What are the units for kinetic energy?
The units for kinetic energy are joules (J).
The units for all types of energy are joules.
State the equation for kinetic energy.
The equation for kinetic energy is $E_{k} = \frac{1}{2} m v^{2}$
Where:
- $E_{k}$ = kinetic energy, measured in joules (J)
- $m$ = mass, measured in kilograms (kg)
- $v$ = velocity, measured in metres per second (m s^–1)
State the equation for kinetic energy when velocity is the subject.
The equation for kinetic energy when velocity is the subject is $v = \sqrt{\frac{2 E_{k}}{m}}$
Where:
- $v$ = velocity, measured in metres per second (m s^–1)
- $E_{k}$ = kinetic energy, measured in joules (J)
- $m$ = mass, measured in kilograms (kg)
State the equation for kinetic energy in terms of momentum.
The equation for kinetic energy in terms of momentum is $E_{k} = \frac{p^{2}}{2 m}$
Where:
- $E_{k}$ = kinetic energy, measured in joules (J)
- $p$ = momentum, measured in kilogram metres per second (kg m s^–1)
- $m$ = mass, measured in kilograms (kg)
How can the equation for kinetic energy in terms of momentum be derived from the kinetic energy equation?
The equation for kinetic energy in terms of momentum be derived from the kinetic energy equation as follows:
- $E_{k} = \frac{1}{2} m v^{2} = \frac{m v^{2}}{2} = \frac{m^{2} v^{2}}{2 m} = \frac{{(m v)}^{2}}{2 m} = \frac{p^{2}}{2 m}$
Define gravitational potential energy.
Gravitational potential energy is the energy that an object has due to its position in a gravitational field.
State the equation for gravitational potential energy in a uniform gravitational field.
The equation for gravitational potential energy in a uniform gravitational field is $∆ E_{p} = m g ∆ h$
Where:
- $∆ E_{p}$ = change in gravitational potential energy, measured in joules (J)
- $m$ = mass, measured in kilograms (kg)
- $g$ = gravitational field strength, measured in newtons per kilogram (N kg^–1)
- $∆ h$ = change in height, measured in metres (m)
True or False?
The gravitational potential energy of an object is directly proportional to its change in height in the gravitational field.
True.
The gravitational potential energy of an object is directly proportional to its change in height in the gravitational field.
State the equation for gravitational potential energy in a uniform gravitational field when mass is the subject.
The equation for gravitational potential energy in a uniform gravitational field is $m = \frac{E_{p}}{g ∆ h}$
Where:
- $m$ = mass, measured in kilograms (kg)
- $∆ E_{p}$ = change in gravitational potential energy, measured in joules (J)
- $g$ = gravitational field strength, measured in newtons per kilogram (N kg^–1)
- $∆ h$ = change in height, measured in metres (m)
True or False?
The gravitational field strength on Earth is 8.9 N kg^–1.
False.
The gravitational field strength on Earth is 9.8 N kg^–1.
True or False?
As an object with mass is raised in a uniform gravitational field, its gravitational potential energy decreases.
False.
As an object with mass is raised in a uniform gravitational field, its gravitational potential energy increases.
Define elastic potential energy.
Elastic potential energy is defined as the energy stored within a material when it is stretched or compressed.
State the equation for the elastic potential energy of an object that obeys Hooke's law.
The equation for the elastic potential energy of an object that obeys Hooke's law is $E_{H} = \frac{1}{2} k ∆ x^{2}$
Where:
- $E_{H}$ = elastic potential energy, measured in joules (J)
- $k$ = spring constant, measured in newtons per metre (N m^–1)
- $∆ x$ = extension of material, measured in metres (m)
State the equation linking the elastic potential energy of an object and the restoring force.
The equation linking the elastic potential energy of an object and the restoring force is $E_{H} = \frac{1}{2} F ∆ x$
Where:
- $E_{H}$ = elastic potential energy, measured in joules (J)
- $F$ = restoring force, measured in newtons (N)
- $∆ x$ = extension of material, measured in metres (m)
State the expression showing a perfect transfer of elastic potential energy into kinetic energy.
The expression showing a perfect transfer of elastic potential energy into kinetic energy is $\frac{1}{2} k ∆ x^{2} = \frac{1}{2} m v^{2}$
Where:
- $k$ = restoring force, measured in newtons (N)
- $m$ = mass, measured in kilograms (kg)
- $v$ = speed of wire, measured in metres per second (m s^-1)
- $∆ x$ = extension of material, measured in metres (m)
Why is it dangerous when a wire under stress suddenly breaks?
It is dangerous when a wire under stress suddenly breaks because the elastic potential energy stored in the strained wire is converted into kinetic energy. The greater the extension of a wire, the greater the speed it will have when it breaks.
State the mathematical expression for mechanical energy.
The mathematical expression for mechanical energy is $mechanical energy = E_{k} + ∆ E_{p} + E_{H}$
Where:
- $E_{k}$ = kinetic energy, measured in joules (J)
- $∆ E_{p}$ = change in gravitational potential energy, measured in joules (J)
- $E_{H}$ = elastic potential energy, measured in joules (J)
True or False?
The change in the total mechanical energy of a system is the work done on the system by any non-conservative force.
True.
The change in the total mechanical energy of a system is the work done on the system by any non-conservative force.
Define a non-conservative force.
A non-conservative force is one that dissipates energy away from the system.
Give one example of a system with mechanical energy.
Examples of systems with mechanical energy are:
- mass-spring system
- simple pendulum
- any system in simple harmonic motion
True or False?
In the absence of non-conservative forces, mechanical energy is always conserved.
True.
In the absence of non-conservative forces, mechanical energy is always conserved.
Define power.
Power is the rate at which energy is transferred.
True or False?
Power is equal to work done.
False.
Work done is equal to energy transferred. Therefore, power is equal to the rate of work done, or work done per unit time.
State the equation for power.
The equation for power is: $P = \frac{∆ W}{∆ t}$
Where:
- $P$ = power, measured in watts (W)
- $∆ W$ = change in work done, measured in joules (J)
- $∆ t$ = change in time, measured in seconds (s)
State the equation for power in terms of force and velocity.
The equation for power is: $P = F v$
Where:
- $P$ = power, measured in watts (W)
- $F$ = constant force applied, measured in newtons (N)
- $v$ = constant velocity, measured in metres per second (m s^–1)
Define 1 watt (W).
The watt is the unit of power. 1 watt = 1 joule per second.
Define efficiency.
Efficiency is defined as the ratio of the useful power (or energy transfer) output from a system to its total power (or energy transfer) input.
State the symbol for efficiency.
The symbol for efficiency is the Greek letter eta, $η$ .
True or False?
An energy transfer with a higher efficiency has a lower amount of wasted energy.
True.
An energy transfer with a higher efficiency has a lower amount of wasted energy.
What are the units of efficiency?
Efficiency has no units because it is a ratio. Efficiency is expressed as a decimal between zero and one, or as a percentage.
State the efficiency equation in terms of power.
The efficiency equation in terms of power is $η = \frac{useful power out}{total power in}$
Where:
- $η$ = efficiency (no units as it is a ratio)
State the efficiency equation in terms of work done.
The efficiency equation in terms of work done is $η = \frac{useful work out}{total work in}$
Where:
- $η$ = efficiency (no units as it is a ratio)
State the efficiency equation in terms of energy transferred.
The efficiency equation in terms of energy is $η = \frac{useful energy out}{total energy in}$
Where:
- $η$ = efficiency (no units as it is a ratio)
Motor A has an efficiency of 0.8 and Motor B has an efficiency of 0.7. The total power input to each motor is 1000 J.
What is the amount of wasted energy for each motor?
Motor A has 200 J of wasted energy, and Motor B has 300 J of wasted energy.
- $wasted energy = (1 - η) \times total energy in$
- Motor A: $0.2 \times 1000 = 200 J$
- Motor B: $0.3 \times 1000 = 300 J$
What is a fuel?
A fuel is anything that can be burned to produce heat, which can be used for an engine to work.
Define energy density.
Energy density is the amount of energy that a fuel can provide per unit volume of fuel.
What are the units of energy density?
The units of energy density are joules per cubic metre (J m^–3).
What is one litre (1 L) in cubic metres (m³)?
One litre is 0.001 cubic metres. 1 L = 0.001 m^–3.
What factors are taken into consideration when choosing a fuel for a specific purpose?
The factors taken into consideration when choosing a fuel for a specific purpose are:
- energy density
- safety of use
- pollutants released in combustion

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