subunit (c) Forces, movement, shape and momentum

What is a force?

A push or a pull that acts on an object due to its interaction with another object. Forces are measured in newtons (N).

What are the three main effects of a force?

A force can change speed, change direction, or change shape.

Can a force change an object's speed?

Yes. A force can make an object speed up (accelerate) or slow down (decelerate).

Can a force change an object's direction?

Yes. A force can make an object change the direction in which it is moving.

Can a force change an object's shape?

Yes. A force can stretch, compress, or bend an object.

Can an object experience multiple effects of a force at the same time?

Yes. For example, kicking a football changes its speed and its direction.

Is a force needed to keep an object moving?

No. A force is only needed to change the motion (speed up, slow down, or change direction). If an object is already moving at a constant speed in a straight line, no force is needed to keep it moving (ignoring friction).

What are the two main categories of forces?

Contact forces and non-contact forces.

What is a contact force?

A force that acts when two objects are physically touching.

What is a non-contact force?

A force that acts between objects even when they are not touching.

What is gravitational force (weight)?

A non-contact force of attraction between any two objects with mass. On Earth, it pulls objects towards the centre of the Earth.

What is electrostatic force?

A non-contact force of attraction or repulsion between two charged objects.

What is magnetic force?

A non-contact force of attraction or repulsion between magnets or magnetic materials.

What is friction?

A contact force that opposes the relative motion of two surfaces in contact.

What is air resistance (drag)?

A contact force (a type of friction) that acts on objects moving through air.

What is tension?

A contact force – the pulling force exerted by a stretched rope, string, or cable.

What is thrust (or driving force)?

A contact force – the forward force produced by an engine or motor.

What is upthrust (buoyancy)?

A contact force – an upward force exerted on an object immersed in a fluid (liquid or gas).

What is normal reaction force (contact force)?

A contact force that acts perpendicular (at 90°) to a surface, pushing back against an object placed on it.

What is mass?

The amount of matter in an object. It stays the same everywhere. Measured in kilograms (kg).

What is weight?

The gravitational force acting on an object. Measured in newtons (N).

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

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

What is the value of g on Earth?

10 N/kg.

What is a common exam mistake regarding weight and mass?

Saying that weight is measured in kilograms. Weight is measured in newtons (N). Mass is measured in kilograms (kg).

What is a scalar quantity?

A quantity that has magnitude (size) only. It does not have a direction.

What is a vector quantity?

A quantity that has both magnitude (size) and direction.

Is speed a scalar or vector quantity?

Scalar (magnitude only).

Is velocity a scalar or vector quantity?

Vector (magnitude AND direction).

Is distance a scalar or vector quantity?

Scalar (magnitude only).

Is displacement a scalar or vector quantity?

Vector (magnitude AND direction).

Is mass a scalar or vector quantity?

Scalar (magnitude only).

Is weight a scalar or vector quantity?

Vector (magnitude AND direction – acts downwards).

Is time a scalar or vector quantity?

Scalar (magnitude only).

Is force a scalar or vector quantity?

Vector (magnitude AND direction).

Is acceleration a scalar or vector quantity?

Vector (magnitude AND direction).

Is energy a scalar or vector quantity?

Scalar (magnitude only).

Is momentum a scalar or vector quantity?

Vector (magnitude AND direction).

Is moment a scalar or vector quantity?

Vector (magnitude AND direction – clockwise or anticlockwise).

How can you remember the difference between scalar and vector?

Scalar = Size only. Vector = Velocity (which has direction) – think of a "V" for vector and velocity.

What is a common exam mistake regarding speed and velocity?

Saying that speed is a vector. Speed is a scalar (it only has magnitude). Velocity is a vector (it has magnitude AND direction).

Why is force a vector quantity?

Because it has both magnitude (size) and direction.

Why does the direction of a force matter?

Because when multiple forces act on the same object, you cannot simply add their magnitudes together. You must consider their directions.

How do you combine forces in the same direction?

Add their magnitudes. The resultant force acts in the same direction.

How do you combine forces in opposite directions?

Subtract their magnitudes. The resultant force acts in the direction of the larger force.

What happens when forces in opposite directions are equal in magnitude?

They cancel each other out. The resultant force is zero (0 N). The object is in equilibrium.

What is the resultant force?

The single force that has the same effect on an object as all the individual forces acting together.

What is a common exam mistake when calculating resultant force?

Forgetting to state the direction of the resultant force. Since force is a vector, you must state both the magnitude AND the direction.

What is a common exam mistake when forces are in opposite directions?

Adding forces when they are in opposite directions. You must subtract them.

Do balanced forces mean the object is stationary?

No. Balanced forces mean the object is either stationary OR moving at a constant speed in a straight line (Newton's First Law).

What is a common exam mistake regarding units for resultant force?

Forgetting the units (N) for the resultant force.

What is the resultant force when forces of 5 N right and 3 N right act on an object?

8 N to the right (add them).

What is the resultant force when forces of 5 N right and 3 N left act on an object?

2 N to the right (subtract: 5

What is the resultant force when forces of 5 N right and 5 N left act on an object?

0 N (balanced forces)

What is friction?

A force that opposes the relative motion of two surfaces in contact. It always acts in the opposite direction to the motion (or intended motion).

Is friction a contact or non-contact force?

A contact force – it only acts when two surfaces are touching.

Does friction oppose or assist motion?

Friction opposes motion, meaning it tries to slow things down or prevent them from starting to move.

Can friction be useful?

Yes. Examples include car tyres gripping the road, brakes slowing a vehicle, and walking without slipping.

Can friction be unwanted?

Yes. Examples include wear and tear on moving parts, energy wasted as heat, and reduced efficiency of machines.

How can friction be reduced?

Lubrication (oil/grease), streamlining (aerodynamic shapes), using ball bearings, and making surfaces smoother.

How can friction be increased?

Using rough surfaces (tyre treads), increasing the force pushing surfaces together, and using materials with high friction (rubber soles).

What is a common exam mistake regarding friction?

Saying friction always slows things down. Friction opposes motion, but it can also cause motion (e.g., a car accelerates because friction from the road pushes the tyres forward).

What is an unbalanced force (resultant force)?

When the forces acting on an object do not cancel each other out, there is a resultant (unbalanced) force. This causes the object to accelerate.

What is Newton's Second Law?

The acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass.

What is the equation for Newton's Second Law?

F = m × a (Force = Mass × Acceleration).

What are the units for force, mass, and acceleration?

Force (N), Mass (kg), Acceleration (m/s²).

How do you find force?

F = m × a.

How do you find mass?

m = F / a.

How do you find acceleration?

a = F / m.

What is a common exam mistake when using F = ma?

Using F = ma when the forces are balanced (resultant force = 0). If the forces are balanced, there is no acceleration, so F = 0.

What is a common exam mistake regarding mass in F = ma?

Forgetting to convert mass into kilograms. Mass must be in kg, not grams.

What is a common exam mistake confusing mass and weight?

Mass is in kg; weight is in N (W = mg).

What is a common exam mistake regarding units for acceleration?

Acceleration must be in m/s².

What happens if the resultant force is zero?

The acceleration is zero. The object will either remain stationary or continue moving at a constant velocity.

What happens if the resultant force is constant?

The acceleration is constant (uniform acceleration).

What is the relationship between force and acceleration?

If the resultant force increases, the acceleration increases (directly proportional).

What is the relationship between mass and acceleration?

If the mass increases, the acceleration decreases (inversely proportional) for the same force.

What is mass?

The amount of matter in an object. It is a scalar quantity measured in kilograms (kg). Mass stays the same everywhere.

What is weight?

The gravitational force acting on an object. It is a vector quantity (acts downwards) measured in newtons (N).

What is gravitational field strength?

The force per unit mass acting on an object in a gravitational field. On Earth, g = 10 N/kg.

What is the equation for weight?

W = m × g (Weight = Mass × Gravitational Field Strength).

What is the value of g on Earth?

10 N/kg (or 10 m/s²).

Does mass change with location?

No – mass stays the same everywhere.

Does weight change with location?

Yes – weight depends on g (changes on different planets).

What instrument measures mass?

Balance / Scale.

What instrument measures weight?

Spring balance / Newton meter.

What is a common exam mistake regarding weight and mass?

Saying weight is measured in kilograms. Weight is measured in newtons (N). Mass is measured in kilograms (kg).

What is a common exam mistake regarding the direction of weight?

Forgetting that weight acts downwards (towards the centre of the Earth) – it is a vector.

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

Using g = 10 m/s² and forgetting that the unit for g in this equation is N/kg. Both are correct, but N/kg makes it clearer that you are calculating force.

What is stopping distance?

The total distance a vehicle travels from the moment the driver first sees a hazard until the vehicle comes to a complete stop.

What is thinking distance?

The distance a vehicle travels during the driver's reaction time (from seeing the hazard to pressing the brake pedal).

What is braking distance?

The distance a vehicle travels from the moment the brakes are applied until the vehicle comes to a complete stop.

What is the equation for stopping distance?

Stopping Distance = Thinking Distance + Braking Distance.

What factors affect thinking distance?

Driver's reaction time, speed, tiredness, alcohol/drugs, distractions, age.

What factors affect braking distance?

Speed (speed²!), mass, condition of brakes, condition of tyres, road conditions, grip/friction.

Does speed affect thinking distance or braking distance?

Speed affects BOTH thinking distance AND braking distance. However, it has a much larger effect on braking distance because braking distance is proportional to speed².

What happens to braking distance if speed doubles?

Braking distance quadruples (increases by a factor of 4).

Does thinking distance depend on the driver or the vehicle?

Thinking distance depends on the driver (reaction time, tiredness, distractions, alcohol/drugs).

Does braking distance depend on the driver or the vehicle?

Braking distance depends on the vehicle and the road (speed, mass, brakes, tyres, road conditions).

What is a common exam mistake regarding stopping distance?

Thinking that stopping distance is just the braking distance. You must remember that stopping distance = thinking distance + braking distance.

What is a common exam mistake regarding units for stopping distance?

Forgetting to state the units (metres, m) when calculating stopping distance.