subunit (a) Units

What is the SI base unit for mass?

The kilogram (kg).

What is mass a measure of?

The amount of "stuff" or matter contained within an object.

Is mass the same as weight?

No. Mass is not the same as weight. Mass stays the same everywhere in the universe.

What is the common exam mistake when using mass in equations?

Using "grams" (g) in a formula that requires kilograms. All IGCSE physics equations (like F = ma) require mass to be in kilograms.

If a question gives you 500 g, what must you do before using it in an equation?

Convert it to 0.5 kg (divide by 1000).

How do you convert grams to kilograms?

Divide by 1000.

How do you convert kilograms to grams?

Multiply by 1000.

What is the SI base unit for length or distance?

The metre (m).

What is the common exam mistake when using distance in equations?

Forgetting to convert cm or km into metres before using speed, velocity, or acceleration equations.

How do you convert kilometres to metres?

Multiply by 1000 (1 km = 1000 m).

How do you convert metres to centimetres?

Multiply by 100 (1 m = 100 cm).

How do you convert centimetres to metres?

Divide by 100.

How do you convert metres to millimetres?

Multiply by 1000 (1 m = 1000 mm).

How do you convert millimetres to metres?

Divide by 1000.

What is the SI base unit for time?

The second (s).

What is the common exam mistake when using time in equations?

Using minutes in a speed or acceleration equation without converting to seconds first.

How do you convert minutes to seconds?

Multiply by 60 (1 minute = 60 seconds).

How do you convert hours to seconds?

Multiply by 3600 (1 hour = 3600 seconds).

If a question says "a car travels for 1.5 hours," what must you do before using it in a formula?

Convert to seconds: 1.5 × 3600 = 5400 seconds.

What is the SI derived unit for force?

The newton (N).

Is the newton a base unit or a derived unit?

It is a derived unit (built from kg, m, and s).

What is the definition of one newton?

The force required to accelerate a 1 kg mass at a rate of 1 m/s².

What are the base units of a newton?

kg m/s² (derived from F = m × a).

What is the common exam mistake regarding weight and mass?

Thinking weight (in N) and mass (in kg) are the same.

What equation do you use to convert between mass and weight?

W = mg (where g = 10 N/kg on Earth).

What is the SI unit for speed and velocity?

Metre per second (m/s).

What is speed?

A scalar quantity (magnitude only).

What is velocity?

A vector quantity (magnitude AND direction).

What is the common exam mistake when writing the unit for speed?

Writing it as "mps" or "ms" (which stands for milliseconds). The correct format is m/s or m s⁻¹.

How do you convert km/h to m/s?

Divide by 3.6 (because 1 m/s = 3.6 km/h).

What are the base units of speed?

m/s (derived from Speed = Distance / Time).

What is the SI unit for acceleration?

Metre per second squared (m/s²).

What does acceleration tell you?

How much the velocity (in m/s) changes every single second.

What is acceleration the rate of change of?

Velocity.

What are the base units of acceleration?

m/s² (derived from Acceleration = Change in Velocity / Time Taken).

What does an acceleration of 2 m/s² mean in reality?

The object's velocity increases by 2 m/s every second.

What is the common exam mistake regarding deceleration?

Forgetting that deceleration (negative acceleration) still has the unit m/s². You show the negative sign in the number (e.g., -2 m/s²), but the unit stays m/s².

What is the unit for gravitational field strength (g)?

Newton per kilogram (N/kg).

What does gravitational field strength tell you?

The amount of gravitational force (in newtons) acting on every 1 kilogram of mass.

What is the value of g on Earth?

10 N/kg (unless stated otherwise).

What is the equation linking weight, mass, and gravitational field strength?

W = mg (Weight = Mass × Gravitational Field Strength).

What is the crucial link between N/kg and m/s²?

1 N/kg is identical to 1 m/s² (because from F = ma, 1 N = 1 kg m/s², so dividing by 1 kg gives 1 m/s²).

What is the common exam mistake regarding g in the W = mg formula?

Using g = 10 m/s² and forgetting it can be written as N/kg. Either unit is correct, but when calculating weight, the unit of g is effectively N/kg to leave you with newtons.

What is the SI unit for the moment of a force (torque)?

The newton metre (Nm).

What is a moment?

The turning effect of a force around a pivot (fulcrum).

Is moment a scalar or vector quantity?

A vector quantity (it has a clockwise or anticlockwise direction).

What is the equation for moment?

Moment = Force × Perpendicular distance from the pivot to the line of action of the force.

What are the units of moment?

Nm (or N m), derived from N × m.

What is the crucial rule for the "perpendicular" distance in moment calculations?

The distance must be the perpendicular (90°) distance from the pivot to the line of action of the force.

What do you do if the force is applied at an angle to the distance?

You must either find the perpendicular distance to the line of action, OR find the perpendicular component of the force (F × sin θ) and multiply it by the full distance.

What is a common exam mistake when calculating moment?

Using the distance from the pivot to the point where the force is applied, even if the force is not at 90° to that line.

What must you do if a distance is given in cm before calculating moment?

Divide by 100 to convert to metres (e.g., 50 cm = 0.50 m).

What is the SI unit for momentum?

The kilogram metre per second (kg m/s).

What is momentum equivalent to in terms of N and s?

N s (Newton-second).

Is momentum a scalar or vector quantity?

A vector quantity (it has a direction).

What is the definition of momentum?

The "quantity of motion" an object has.

What is the equation for momentum?

Momentum = Mass × Velocity, or p = mv.

What are the units of momentum?

kg m/s (or kg m s⁻¹).

What is the crucial link between kg m/s and N s?

They are equivalent. From F = ma and a = Δv/t, you get F × t = m × Δv, so N × s = kg × m/s.

What is a common exam mistake when calculating momentum?

Using speed instead of velocity. Momentum is a vector, so you must use velocity (which has direction).

How do you calculate change in momentum if an object changes direction?

Δp = m(v

What is a common exam mistake regarding the unit of momentum?

Forgetting the unit entirely, or writing "kg m/s²" (which is a newton) or "kg/m/s". It is specifically kg m/s.

What is a common exam mistake in conservation of momentum questions?

Forgetting that momentum is a vector. If two objects collide and move in opposite directions, you must subtract one momentum from the other (assign a negative sign to the leftward velocity).

What is the unit for mass?

kg (kilogram).

What is the unit for distance/length?

m (metre).

What is the unit for time?

s (second).

What is the unit for force?

N (newton).

What is the unit for speed and velocity?

m/s (metre per second).

What is the unit for acceleration?

m/s² (metre per second squared).

What is the unit for gravitational field strength?

N/kg (newton per kilogram).

What is the unit for moment (EXTENDED)?

Nm (newton metre).

What is the unit for momentum (EXTENDED)?

kg m/s (kilogram metre per second).