Energy & Forces

What unit is normally used on a food label to state how much energy a food contains?

• Akilowatts (kW)
• Bdegrees Celsius (°C)
• Ckilojoules (kJ)✓
• Dnewtons (N)

Answer: Food labels state energy content in kilojoules, often alongside kilocalories. Kilowatts measure power, which is the rate of using energy, not the total energy stored in food. Newtons measure force and degrees Celsius measure temperature, so neither describes an amount of energy.

Which unit is used to describe the power rating of a domestic appliance, such as a kettle or toaster?

• Aamperes or volts
• Bjoules or kilojoules
• Cpascals or newtons
• Dwatts or kilowatts✓

Answer: Domestic appliances such as kettles and toasters are rated in watts or kilowatts, showing how quickly they transfer energy. Joules and kilojoules measure the total energy transferred, not the rate of transfer. Amperes and volts describe current and potential difference, while pascals and newtons relate to pressure and force.

A hairdryer has a power rating of 2 kW and is used for 30 minutes (0.5 hours). How much energy does it transfer?

• A1 kilowatt hour✓
• B2 kilowatt hours
• C4 kilowatt hours
• D0.5 kilowatt hours

Answer: Energy transferred in kilowatt hours equals power in kilowatts multiplied by time in hours. Here 2 kW multiplied by 0.5 hours gives 1 kilowatt hour. Using the power rating alone, the time alone, or multiplying by the wrong figures gives each of the other values.

A household uses 5 kWh of electricity in a day, and the supplier charges 20p per kWh. What is the cost of that day's electricity?

• A£2.00
• B£5.20
• C£1.00✓
• D£0.25

Answer: The cost of electricity is found by multiplying the energy used in kilowatt hours by the price charged per unit. Here 5 kWh at 20p per kWh gives 100p, which is £1.00. The other amounts come from dividing or multiplying the wrong pair of numbers together.

A domestic energy bill is calculated mainly from which two things?

• Athe power rating of the appliances and the room temperature
• Bthe voltage supplied and the number of appliances owned
• Cthe amount of energy used and the price per unit of energy✓
• Dthe time of day and the colour of the electricity meter

Answer: A domestic energy bill is worked out from how much energy was used and the price charged for each unit of energy. The power rating of individual appliances and the room temperature do not by themselves determine the total bill. Voltage, the number of appliances owned, the time of day and the meter's colour also play no direct part in the calculation.

Coal, oil and natural gas are burned in power stations to release energy. What term describes all three of these?

• Aconductors
• Binsulators
• Ccatalysts
• Dfuels✓

Answer: Coal, oil and natural gas are fuels, meaning they release stored chemical energy when they are burned. Insulators are materials that reduce heat transfer rather than release energy themselves. Conductors allow heat or electric charge to flow easily, and catalysts speed up chemical reactions without being used up, so neither term fits.

A crowbar lets a person lift a heavy rock using a smaller force than lifting it directly. What is the trade-off for using this smaller force?

• Athe force needed is larger, but the crowbar end moves a noticeably shorter distance each time
• Bthe force needed is smaller, and the crowbar end also moves a much shorter distance overall
• Cthe force needed stays exactly the same, but far less overall movement is ever needed at all
• Dthe force needed is smaller, but the crowbar end must move further✓

Answer: A crowbar is a simple machine that lets a smaller force lift a heavy load, but the end applying the force must move further than the load itself moves. Because force multiplied by distance moved stays unchanged, a smaller force always needs a bigger movement, never a smaller one. The force required does not stay the same either, since the whole point of the crowbar is to reduce it.

Heat travels along a metal saucepan handle mainly by which process?

• Aradiation, because energy travels outward as electromagnetic waves through empty space
• Bevaporation, because liquid particles are escaping rapidly from an exposed liquid surface
• Cinsulation, because the handle is specially designed to block heat from escaping outward
• Dconduction, because vibrating particles transfer energy to neighbouring particles✓

Answer: Heat travels through the solid metal handle by conduction, because vibrating particles pass energy on to their neighbouring particles. Radiation transfers energy as waves and does not rely on particles bumping into one another. Insulation describes reducing heat transfer rather than causing it, and evaporation involves liquid particles escaping a surface, which does not apply to a solid handle.

The Sun warms the Earth even though space is a vacuum containing no particles. Which process transfers this energy?

• Aconvection
• Bconduction
• Cradiation✓
• Dfriction

Answer: Energy from the Sun reaches Earth by radiation, which can travel through the vacuum of space because it does not need particles to carry it. Conduction needs touching particles, and convection needs a fluid that can circulate, so neither can work across empty space. Friction only occurs between surfaces in contact, which is not relevant to energy crossing space.

Loft insulation in a house roof reduces heat loss mainly by:

• Atrapping air and reducing the rate of heat transfer by conduction✓
• Bincreasing the rate of heat transfer by conduction
• Creflecting all light back into the house
• Dconverting heat energy into electrical energy

Answer: Loft insulation traps air, which reduces the rate at which heat is transferred out of the house by conduction. It does not work by reflecting light, since heat loss through a roof is not mainly a light effect. Insulation should reduce rather than increase heat loss, and it does not convert heat into electricity.

Which of the following is not usually described as an energy transfer process?

• Adropping an object from a height
• Bcompleting an electrical circuit
• Cchanging an object's speed or direction
• Dleaving an object at rest on a shelf✓

Answer: Changing an object's motion, dropping it, or completing an electrical circuit are all processes in which energy is transferred from one store to another. Leaving an object at rest on a shelf involves no change, so no energy transfer is taking place. The other three options each describe a recognised way that energy moves between stores.

In an ideal simple machine, if a smaller input force is used to lift a load, what must increase to keep force multiplied by distance unchanged?

• Athe distance moved by the input force✓
• Bthe total energy transferred by the machine
• Cthe weight of the object being lifted
• Dthe output force needed to lift the load

Answer: In an ideal simple machine, force multiplied by the distance moved stays the same throughout. If the input force is made smaller to ease the effort, the distance moved by that force must increase to make up for it. The energy transferred and the output force needed to lift a given load stay about the same, and the weight of the object is not affected by how the machine is used.

A ball rolls down a hill and speeds up as it goes. What happens to the total amount of energy in the system?

• Ait increases, because the ball is speeding up and gaining energy
• Bit stays the same overall, but it is transferred between different energy stores✓
• Cit stays entirely in one energy store the whole time
• Dit decreases as some energy disappears completely

Answer: When a ball rolls downhill and speeds up, the total energy in the system stays the same overall, but it is transferred between different energy stores as the ball loses height and gains speed. The total does not increase just because the ball is moving faster, since energy cannot be created from nothing. It is not lost completely either, and it cannot stay in a single store while both the ball's height and speed are changing.

Which pair correctly matches an energy store with an example?

• Aelastic distortion — a stretched spring✓
• Bposition in a field — a moving car
• Cmovement — a stationary rock on a hill
• Dchemical composition — a hot cup of tea

Answer: A stretched spring holds energy in an elastic distortion store, since work has been done to change its shape. A moving car's energy is mainly stored by its movement, not by its position in a field. A stationary rock has no store linked to movement, and a hot cup of tea holds energy through a temperature change rather than through its chemical composition.

A falling apple speeds up as it falls. Which explanation best fits how this change is described in physics?

• Aenergy is somehow created inside the apple out of nothing as it continues falling downward
• Bthe apple's total mass gradually increases as it gets closer and closer to the ground
• Cenergy simply flows into the apple from below, coming directly from the ground itself
• Dgravity acts on the apple, doing work that increases its store of kinetic energy✓

Answer: As the apple falls, gravity acts on it and does work, transferring energy into its store of kinetic energy, which is why it speeds up. Energy does not simply flow into the apple from the ground, since the ground is not the source of the change. Energy is not created inside the apple, and its mass stays constant as it falls.

Just before a firework explodes, its energy is stored mainly as chemical energy. Just after it explodes, this energy has mainly transferred into which stores?

• Amovement, light and sound✓
• Belastic distortion only
• Cchemical composition, unchanged
• Dposition in a field only

Answer: Just after a firework explodes, the chemical energy that was stored inside it has mainly transferred into the movement of fragments, light and sound given off by the explosion. It does not end up only as elastic distortion or only as position in a field, since the explosion clearly changes the firework's motion and surroundings. The chemical composition also changes as the explosive reacts, so it cannot remain, unchanged, afterwards.

What is the correct word equation for average speed?

• Aaverage speed = distance ÷ time✓
• Baverage speed = distance + time
• Caverage speed = time ÷ distance
• Daverage speed = distance × time

Answer: Average speed is calculated by dividing the distance travelled by the time taken, written as distance ÷ time. Swapping the two quantities round, as in time ÷ distance, gives a completely different measurement. Multiplying or adding distance and time instead of dividing does not produce a speed at all.

A runner covers 400 m in 50 s. What is their average speed?

• A40 m/s
• B450 m/s
• C0.8 m/s
• D8 m/s✓

Answer: Average speed equals distance divided by time, so 400 m divided by 50 s gives 8 m/s. Misplacing a decimal point or dividing the wrong way round produces incorrect values such as 0.8 m/s or 40 m/s. Adding the two numbers together to get 450 is not a valid way to calculate speed.

On a distance-time graph, what does a steeper line indicate?

• Aa change in direction only
• Ba faster speed✓
• Ca slower speed
• Da constant, unchanging position

Answer: On a distance-time graph, the gradient of the line represents speed, so a steeper line shows a faster speed. A shallower line would show a slower speed instead. A constant, unchanging position appears as a flat line, and a change of direction alone does not by itself make a line steeper.

On a distance-time graph, a flat horizontal line shows that an object is:

• Astationary, not changing distance over time✓
• Bmoving at a constant, fast speed
• Cmoving backwards at a steady speed
• Daccelerating steadily as time passes

Answer: A flat horizontal line on a distance-time graph shows that the distance from the starting point is not changing, meaning the object is stationary. A constant, fast speed would instead appear as a straight sloped line. Accelerating steadily would produce a curved line, and moving backwards at a steady speed would produce a downward-sloping straight line, not a flat one.

Two trains travel in the same direction on parallel tracks at the same speed. To a passenger on one train, the other train appears to be:

• Amoving faster than both trains combined
• Bmoving very quickly forward
• Cstationary relative to their own train✓
• Dmoving very quickly backward

Answer: Because both trains move in the same direction at the same speed, the distance between them stays constant, so the other train appears stationary relative to their own train. It would not appear to move quickly forward or backward, since neither train is gaining on the other. Its apparent speed also cannot be greater than both trains combined, since relative motion compares the difference between their speeds, not the sum.

A force is best described as:

• Aa special unit that is used only to measure the speed of a moving object
• Ba push or a pull resulting from the interaction between two objects✓
• Ca measure of how heavy an object is
• Da type of energy that is stored permanently inside a single, isolated object

Answer: A force is a push or a pull that results from the interaction between two objects, such as a hand pushing a door. It is not a type of energy stored inside one object on its own. It is also not a direct measure of weight, and it is not a unit for measuring speed.

A book rests on a table without moving. What can be said about the forces acting on it?

• Athe forces are balanced, causing constant acceleration
• Bthere are no forces acting on the book at all
• Cthe forces are unbalanced, causing a resultant upward force
• Dthe forces are balanced, so there is no resultant force✓

Answer: A book resting still on a table has balanced forces acting on it, so there is no resultant force and it does not accelerate. If the forces were unbalanced, there would be a resultant force and the book would start to move. Forces such as gravity and the table's support are still acting on the book, so it is wrong to say no forces act at all, and balanced forces cannot cause constant acceleration.

Pushing on a door far from its hinges opens it more easily than pushing close to the hinges. This turning effect of a force is called a:

• Areaction
• Bextension
• Cresultant
• Dmoment✓

Answer: The turning effect of a force, such as pushing a door away from its hinges, is called a moment. A resultant is the overall combined effect of several forces, not specifically a turning effect. An extension describes how much a spring stretches, and a reaction is the force a surface exerts back on an object, so neither term fits here.

A spring stretches by 4 cm when a force of 2 N is applied to it. Assuming the spring obeys Hooke's Law, how far would it stretch under a force of 4 N?

• A16 cm
• B12 cm
• C8 cm✓
• D4 cm

Answer: Hooke's Law states that extension is proportional to the applied force, provided the spring is not stretched beyond its limit. Doubling the force from 2 N to 4 N therefore doubles the extension from 4 cm to 8 cm. An extension of 4 cm would mean the force had not changed at all, while 12 cm or 16 cm would mean the extension had grown by more than the force did.

A cyclist freewheeling on a flat road gradually slows down and stops, even though no brakes are applied. This is mainly because of:

• Athe road surface pushing the cyclist forward
• Ba change in the cyclist's mass
• Cgravity pulling the cyclist backwards
• Dfriction and air resistance opposing the motion✓

Answer: A freewheeling cyclist slows down mainly because friction between moving parts and air resistance oppose the motion, gradually removing energy from the bicycle's movement. Gravity acting straight down would not by itself slow forward motion on a flat road. The cyclist's mass does not change while cycling, and the road surface does not push the cyclist forward.

Which of these is an example of a non-contact force?

• Athe magnetic attraction between a magnet and a fridge door✓
• Bthe tension that builds up in a long rope while it is being pulled tightly
• Cthe push needed to open a heavy door
• Dthe friction that acts between the sole of a shoe and the rough pavement surface

Answer: The magnetic attraction between a magnet and a fridge door is a non-contact force, since it acts without the two objects touching. Friction between a shoe and the pavement only happens when the two surfaces are in contact. The push needed to open a door and the tension in a pulled rope are also contact forces, since they need direct touching to act.

As a hiker climbs higher up a mountain, what generally happens to atmospheric pressure?

• Ait increases
• Bit decreases✓
• Cit fluctuates randomly with no pattern
• Dit stays exactly the same

Answer: Atmospheric pressure decreases with height, because there is less air above pressing down as altitude increases. It does not increase with height, and it does not stay exactly the same at different altitudes. Although weather can cause small local changes, pressure does not simply fluctuate with no pattern as a hiker climbs.

A diver notices that water pressure increases the deeper they swim. This is mainly because:

• Athere is more water, and therefore more weight, pressing down from above✓
• Bthe diver's body becomes denser at greater depths
• Cthe water becomes colder, which increases its pressure
• Dsunlight cannot reach deep water, which raises the pressure

Answer: Water pressure increases with depth because there is more water, and therefore more weight, pressing down from above at greater depths. Water temperature does not directly determine pressure in this way. A diver's body does not become denser simply by swimming deeper, and sunlight reaching the water has no effect on its pressure.

A box exerts a force of 60 N on the ground through its base, which has an area of 2 m². What pressure does the box exert on the ground?

• A15 Pa
• B30 Pa✓
• C120 Pa
• D62 Pa

Answer: Pressure equals force divided by the area over which it acts, so 60 N divided by 2 m² gives 30 Pa. Multiplying instead of dividing, or adding the two numbers together, gives incorrect values such as 120 Pa or 62 Pa. Halving the correct answer incorrectly, as in 15 Pa, also does not match the calculation.