ALL OF MODULE 3 FORCES AND MOTION

ALL OF MODULE 3 FORCES AND MOTION

What does the gradient represent on motion graphs?

Displacement–time graph:
Velocity

Velocity–time graph:
Acceleration

What does the area represent on motion graphs?

Velocity–time graph:
Displacement

Force–time graph:
Impulse (change in momentum)

When can SUVAT equations be used?

Only when acceleration is constant

What makes up stopping distance?

Thinking distance + braking distance

How does speed affect stopping distance?

Thinking distance ∝ speed
Braking distance ∝ speed²

How does velocity behave in projectile motion?

Horizontal velocity: constant
Vertical velocity: changes due to g

What does a resultant force cause?

Resultant force ≠ 0 → acceleration
Resultant force = 0 → constant velocity

What are the main forces you must recognise?

Weight (mg)
Normal reaction
Friction / drag
Tension

What is terminal velocity?

Constant velocity when drag = weight

How is terminal velocity reached?

Speed increases → drag increases → acceleration decreases → eventually stops accelerating

What determines the size of a moment?

Force × perpendicular distance

What is the condition for equilibrium?

Resultant force = 0
Resultant moment = 0

What are the key formulas for density and pressure?

Density = m / V
Pressure = F / A

What affects pressure in a fluid?

p = ρgh → depends on depth, density, gravity

What are the key energy formulas?

Kinetic energy = ½mv²
GPE = mgh

Distinguish between scalar and vector quantities using examples from motion

Scalars: distance, speed
Vectors: displacement, velocity, acceleration

Key idea: vectors have magnitude AND direction

What does area under graphs represent and how is it used in exams?

Velocity–time area = displacement
Force–time area = impulse (change in momentum)

Exam use:
Area under non-linear graphs must be estimated

Define free fall and explain why acceleration may differ from g in real situations

Free fall: motion under gravity only
Real motion: air resistance acts → reduces acceleration

Describe one method to determine g and a key improvement

Method: measure time for object to fall known distance → use s = ½gt²
Improvement: use light gates to reduce reaction time error

Explain the factors affecting thinking distance and braking distance

Thinking distance: depends on speed + reaction time (fatigue, alcohol)
Braking distance: depends on speed², road conditions, brakes

State Newton’s First Law

An object will remain at rest or move with constant velocity unless acted upon by a resultant force.

State Newton’s Second Law in full form

The resultant force is equal to the rate of change of momentum.

F = Δp / Δt

State Newton’s Third Law

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

Key detail:
Forces act on different objects, same type, same magnitude.

How are forces resolved on a slope?

Weight is resolved into:

• parallel component (down slope)

• perpendicular component (into slope)

Application:
Used to find resultant force along slope.

What is drag?

A resistive force acting opposite to motion in a fluid.

Archimedes' principle

Archimedes' Principle states that the upward buoyancy force acting on any body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces.

Describe the motion of an object falling with air resistance

Acceleration decreases over time
Velocity increases then becomes constant

State the principle of moments

Sum of clockwise moments = sum of anticlockwise moments about a point.

What is a couple?

Two equal and opposite parallel forces acting on different lines.

Effect:
Produces rotation without translation.

Explain why objects float

Upthrust = weight → equilibrium
Occurs when average density < fluid

State conservation of momentum

otal momentum before = total momentum after (no external forces)

What is a brittle material?

A brittle material shows very little strain before breaking.

Application:
Snaps suddenly with no plastic deformation (e.g. glass)

What is a ductile material?

A ductile material can undergo large extension before breaking.

Application:
Can be drawn into wires (e.g. copper)

What is compression?

Forces acting towards an object causing it to shorten.

What is compressive deformation?

Change in shape due to compressive forces.

What is elastic deformation?

Deformation where the object returns to its original shape when the force is removed.

What is plastic deformation?

Permanent deformation where the object does not return to its original shape.

What is elastic potential energy?

Energy stored in an object when it is stretched or compressed.

Key detail:
Equal to work done → area under force-extension graph

What is upthrust?

Upward force exerted by a fluid on an object.

State the principle of conservation of energy

In a closed system with no external forces, total energy before an event equals total energy after.

Describe a method to determine g using a trapdoor and electromagnet

Suspend a steel ball using an electromagnet above a trapdoor.
Measure the vertical distance between the ball and trapdoor.
Switch off electromagnet → ball falls.
Timer starts automatically when current is cut and stops when trapdoor is hit.
Record time taken for fall.
Repeat and average.

Use:
s = ½gt² → rearrange for g

Describe a method to determine Young modulus

Measure wire diameter using micrometer → calculate cross-sectional area.
Clamp wire vertically and attach masses.
Measure extension using ruler or vernier scale.
Calculate stress = F/A
Calculate strain = extension/original length
Plot stress vs strain graph
Gradient = Young modulus.

Describe a method to determine the spring constant

Suspend spring from clamp stand next to ruler.
Measure original length.
Add known masses (force = mg).
Measure extension for each force.
Plot force vs extension graph.
Gradient = spring constant k.

beyond elastic limit;

Spring undergoes plastic deformation → does not return to original length.

How could terminal velocity be measured in a fluid?

Drop an object in a cylindrical fluid and measure distance travelled over time.
When equal distances are travelled in equal time intervals → velocity is constant → terminal velocity reached.

What could improve the Terminal velocity method?

• Use video/data logger instead of stopwatch

• Use large container (reduce wall effects)

• Ensure object reaches terminal velocity before measuring