IB MYP Sciences – Forces Study Guide

This study guide teaches forces for IB MYP Sciences. It is aligned to the International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014, which lists forces among the topics explored in the MYP sciences on-screen examinations. The page suits MYP years 4 and 5. MYP sciences has no SL/HL split, so everything here applies to every student; check with your teacher which topics your own on-screen examination or school course includes.

One point first. MYP has no prescribed content list: schools design their own units. This page covers a topic the IB’s brief names, using the physics that is standard at this level. For the course as a whole, see the IB MYP Sciences course hub and the printable checklist. When you have worked through this guide, use the forces revision notes and then test yourself with the forces practice questions.

What this unit covers

Area	What you must be able to do	Level
Types of force	Name contact and non-contact forces; draw free-body diagrams; tell mass from weight	All students
Resultant force	Add forces acting along one line; decide if forces are balanced	All students
Newton’s laws	State and apply the three laws; use F = ma	All students
Describing motion	Use speed, velocity and acceleration; read distance-time and velocity-time graphs	All students
Pressure	Use p = F/A; explain pressure in liquids	All students
Moments	Use moment = F × d; apply the principle of moments	All students

The brief gives movement, energy and models as examples of related concepts in MYP sciences. Forces is where movement becomes measurable, and free-body diagrams and motion graphs are models: simplified pictures that let you predict what happens.

Types of force

A force is a push or a pull. It has size (magnitude) and direction, so it is a vector. Forces are measured in newtons (N) with a newton meter or force sensor.

Contact forces act only when objects touch:

• friction – opposes sliding between surfaces
• air resistance or drag – friction from a gas or liquid, which grows with speed
• tension – the pull in a stretched string, rope or cable
• normal (reaction) force – the push of a surface at right angles to it
• upthrust – the upward push of a fluid on an object in it

Non-contact forces act across a gap: gravitational (weight), magnetic and electrostatic.

Mass and weight

Mass is the amount of matter in an object, in kilograms. It does not change from place to place. Weight is the gravitational force on the object, in newtons:

W = m × g

where g is the gravitational field strength. Near Earth’s surface g is about 9.8 N/kg. Use whatever value a question gives you.

Worked example 1. An astronaut has a mass of 60 kg. The Moon’s g is about 1.6 N/kg.

On Earth: W = 60 × 9.8 = 588 N
On the Moon: W = 60 × 1.6 = 96 N
Mass in both places: 60 kg

Free-body diagrams

A free-body diagram shows one object as a dot or box, with an arrow for each force acting on it. Arrow length shows size; each arrow is labelled. Do not draw forces the object exerts on other things.

Resultant force

The resultant (or net) force is the single force that has the same effect as all the forces together. For forces along one line, take one direction as positive, then add.

Worked example 2. A boat’s engine gives a forward force of 850 N. Water resistance is 320 N backwards.

Take forward as positive.
Resultant = 850 − 320 = 530 N forward

If the resultant is zero, the forces are balanced. If not, they are unbalanced and the motion changes.

Newton’s three laws

First law. An object stays at rest, or keeps moving at constant velocity, unless a resultant force acts on it. Balanced forces do not mean “not moving”; they mean “not changing its motion”.

Second law. A resultant force makes an object accelerate in the direction of the force:

F = m × a     (resultant force in N, mass in kg, acceleration in m/s²)

Third law. When object A exerts a force on object B, B exerts a force on A that is equal in size and opposite in direction. The two forces act on different objects and are the same type of force. That is why they never cancel.

Worked example 3. A car of mass 1200 kg has a driving force of 3000 N. Friction and air resistance total 600 N.

Resultant = 3000 − 600 = 2400 N
a = F / m = 2400 / 1200 = 2.0 m/s²

A common error is to put 3000 N into F = ma. The law uses the resultant force.

Terminal velocity

A falling skydiver starts with weight much bigger than air resistance, so she accelerates. As speed rises, air resistance rises. When it equals her weight, the resultant is zero and she falls at a constant terminal velocity. Opening a parachute increases air resistance sharply, she decelerates, and a new, lower terminal velocity is reached.

Speed, velocity and acceleration

Speed = distance ÷ time (m/s). Velocity is speed in a stated direction, so it is a vector. A car going round a roundabout at a steady 10 m/s has constant speed but changing velocity.

Acceleration is the rate of change of velocity:

a = (v − u) / t     (u = initial velocity, v = final velocity)

A negative answer means the object is slowing down (deceleration).

Worked example 4. A runner covers 400 m in 50 s. Later a cyclist speeds up from 4 m/s to 22 m/s in 6 s.

Runner: speed = 400 / 50 = 8.0 m/s
Cyclist: a = (22 − 4) / 6 = 18 / 6 = 3.0 m/s²

Graphs of motion

Distance-time graphs

• The gradient is the speed.
• A straight sloping line means constant speed.
• A horizontal line means the object is stationary.
• A curve that gets steeper means the object is speeding up.

Velocity-time graphs

• The gradient is the acceleration.
• A horizontal line means constant velocity (zero acceleration).
• The area under the graph is the distance travelled.

Worked example 5. A tram starts from rest and reaches 12 m/s in 4 s. It travels at 12 m/s for 6 s, then slows steadily to rest in 3 s. Find the acceleration in each phase, the total distance and the average speed.

Phase 1: a = 12 / 4 = 3.0 m/s²
Phase 2: a = 0
Phase 3: a = (0 − 12) / 3 = −4.0 m/s²  (deceleration 4.0 m/s²)

Distance = area under graph
Phase 1 (triangle):  ½ × 4 × 12 = 24 m
Phase 2 (rectangle): 6 × 12     = 72 m
Phase 3 (triangle):  ½ × 3 × 12 = 18 m
Total = 114 m

Average speed = total distance / total time = 114 / 13 = 8.77 m/s (3 s.f.)

Note that the average speed is not the average of 0 and 12 m/s.

Pressure

Pressure is force per unit area, acting at right angles to a surface:

p = F / A     (pressure in pascals, Pa; 1 Pa = 1 N/m²)

The same force on a smaller area gives a larger pressure. This explains why knives are sharpened and why snowshoes stop you sinking.

Worked example 6. A walker weighs 600 N. Her boots touch the snow over 0.030 m². Snowshoes spread her weight over 0.24 m².

Boots:     p = 600 / 0.030 = 20 000 Pa
Snowshoes: p = 600 / 0.24  = 2 500 Pa
The snowshoes cut the pressure by a factor of 8.

Pressure in liquids

Pressure in a liquid increases with depth and acts in all directions. The extra pressure at depth h is:

p = h × ρ × g     (ρ = density of the liquid in kg/m³)

At 5.0 m below the surface of fresh water (ρ = 1000 kg/m³), the water adds 5.0 × 1000 × 9.8 = 49 000 Pa. This is why dam walls are built thicker at the bottom. The difference in pressure between the top and bottom of a submerged object is what produces upthrust.

Moments

A moment is the turning effect of a force about a pivot:

moment = F × d     (N m; d = perpendicular distance from the pivot to the line of the force)

A longer spanner gives a bigger moment for the same effort. A 40 N push at 0.25 m from a nut gives a moment of 40 × 0.25 = 10 N m.

Principle of moments. For an object in equilibrium, the total clockwise moment about any pivot equals the total anticlockwise moment. For full equilibrium the resultant force must also be zero.

Worked example 7. On a seesaw pivoted at its centre, a child weighing 300 N sits 2.0 m to the left of the pivot. How far to the right must a 400 N child sit to balance it? Ignore the weight of the plank.

Anticlockwise moment = clockwise moment
300 × 2.0 = 400 × d
600 = 400d
d = 1.5 m to the right of the pivot

The heavier child sits closer to the pivot.

Investigating forces

The brief’s investigation-skills task carries 50 marks and covers criteria B and C, so expect forces content inside an investigation. Typical forces investigations include:

• how resultant force affects a trolley’s acceleration (mass kept constant)
• how the area of a parachute affects its terminal velocity
• how the surface type affects the friction on a sliding block
• how the load on a beam affects the distance needed to balance it

For each, you should be able to name the independent, dependent and control variables, choose equipment (light gates, data logger, video analysis, newton meter), repeat readings, and process data into a graph whose gradient means something physical. The investigation skills exam preparation page covers the general method.

Common errors

• Using the driving force instead of the resultant force in F = ma.
• Giving weight in kilograms, or mass in newtons.
• Saying an object with balanced forces “must be stationary”. It can move at constant velocity.
• Calling a book’s weight and the table’s normal force a third-law pair. They act on the same object and are different types of force. The pair to the book’s weight is the book’s gravitational pull on the Earth.
• Reading the height of a velocity-time graph as the distance. Distance is the area.
• Using an area in cm² with a force in N and calling the answer pascals. Convert to m² first (1 cm² = 0.0001 m²).
• Measuring the distance for a moment along a slanted lever instead of at right angles to the force.
• Writing “the speed is 12” with no unit, or giving velocity without a direction when direction matters.

Where this fits in the course

Forces sits beside other MYP content topics. For how the examinations and criteria are set up, read the MYP Sciences syllabus guide, the course models guide and the criteria in practice guide. Your next steps on this topic are the revision notes and the practice set.

Official syllabus

International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014. The brief lists forces among the topics explored in the MYP sciences on-screen examinations, and gives energy, movement, transformation and models as examples of related concepts.