6.4 - Forces

What is a force?

A push or pull caused by an interaction.

What is the unit of force?

Newton (N).

What is a contact force?

A force that acts when objects are physically touching.

Give examples of contact forces.

Friction, air resistance, tension and normal contact force.

Give examples of non-contact forces.

Gravitational, magnetic and electrostatic forces.

What is a resultant force?

The overall force acting on an object.

What happens if the resultant force is zero?

The object remains stationary or moves at constant velocity.

What happens if there is a non-zero resultant force?

The object accelerates.

What is mass?

The amount of matter in an object.

What is the unit of mass?

Kilogram (kg).

What is weight?

The force acting on an object due to gravity.

What is the equation for weight?

W = mg

What is the unit of weight?

Newton (N).

What is gravitational field strength on Earth?

Approximately 9.8 N/kg.

Does mass change when an object moves to the Moon?

No.

Does weight change on the Moon?

Yes, because gravitational field strength is different.

What is speed?

The distance travelled per unit time.

What is the equation for speed?

speed = distance ÷ time

What is velocity?

Speed in a specific direction.

What is acceleration?

The rate of change of velocity.

What is the equation for acceleration?

a = (v − u)/t

What does u represent?

Initial velocity.

What does v represent?

Final velocity.

What does t represent?

Time.

What is the unit of acceleration?

m/s².

What does the gradient of a distance-time graph represent?

Speed.

What does a horizontal line on a distance-time graph mean?

The object is stationary.

What does a straight sloping line mean?

Constant speed.

What does a steeper gradient mean?

Greater speed.

What does a curved line mean?

The object's speed is changing.

What does the gradient of a velocity-time graph represent?

Acceleration.

What does a horizontal line on a velocity-time graph mean?

Constant velocity.

What does the area under a velocity-time graph represent?

Distance travelled.

What does a negative gradient mean?

Deceleration.

What is Newton's First Law?

An object remains at rest or moves at constant velocity unless acted on by a resultant force.

What is Newton's Second Law?

F = ma

What happens to acceleration if force increases while mass stays constant?

Acceleration increases.

What happens to acceleration if mass increases while force stays constant?

Acceleration decreases.

What is Newton's Third Law?

When two objects interact, they exert equal and opposite forces on each other.

Where do Newton's Third Law forces act?

On different objects.

What force acts on a falling object?

Weight/gravity.

What is air resistance?

A force that opposes motion through air.

What happens to air resistance as speed increases?

Air resistance increases.

What is terminal velocity?

The constant maximum velocity reached when drag equals weight.

What is the resultant force at terminal velocity?

Zero.

Is an object at terminal velocity stationary?

No. It moves at constant velocity.

How can you investigate acceleration?

Measure distance and time and use motion data to calculate speed and acceleration.

How can light gates improve accuracy?

They measure the object's motion automatically, reducing human reaction-time errors.

What is momentum?

The product of mass and velocity.

What is the equation for momentum?

p = mv

What is the unit of momentum?

kg m/s.

What happens to momentum if velocity doubles?

Momentum doubles.

What happens to momentum if mass doubles?

Momentum doubles.

What is the principle of conservation of momentum?

Total momentum before an interaction equals total momentum after the interaction, if no external resultant force acts.

What are the two parts of stopping distance?

Thinking distance + braking distance.

What is thinking distance?

The distance travelled during the driver's reaction time.

What is braking distance?

The distance travelled while the brakes slow the vehicle.

What factors increase thinking distance?

Tiredness, alcohol, drugs and distractions.

What factors increase braking distance?

High speed, poor brakes, worn tyres and poor road conditions.

Why does higher speed increase stopping distance?

The vehicle travels further during reaction time and has more kinetic energy to dissipate.

What is work done?

Energy transferred when a force causes an object to move through a distance.

What is the equation for work done?

W = Fs

What is a moment?

The turning effect of a force.

What is the equation for a moment?

Moment = force × perpendicular distance from pivot

What is the unit of a moment?

Newton metre (Nm).

What is a pivot?

The point around which an object turns.

When is an object balanced?

When the clockwise moments equal the anticlockwise moments.

Why does a falling object eventually reach terminal velocity?

As speed increases, drag increases until it equals weight, making the resultant force zero.

Why does a passenger move forward when a car suddenly stops?

Due to inertia; their body tends to continue moving at its original velocity.

Why are airbags useful in a collision?

They increase the stopping time, reducing the force on the passenger.

Why does increasing stopping time reduce force?

For a given change in momentum, a longer time produces a smaller force.

Why does doubling velocity greatly increase braking distance?

Kinetic energy depends on velocity squared, so much more energy must be dissipated.

Why are Newton's Third Law forces not balanced forces?

They act on different objects, whereas balanced forces act on the same object.

Can an object accelerate when travelling at constant speed?

Yes, if its direction changes.

Is weight measured in kg?

No. Weight is measured in newtons.

Is mass a force?

No.

At terminal velocity, is acceleration zero?

Yes.

Does zero resultant force mean the object must be stationary?

No. It can move at constant velocity.

In Newton's Third Law, are the forces on the same object?

No — they act on different objects.