Force Diagrams and Forces on Moving Objects

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1. What is a force?

A force is a push or a pull. You cannot see a force, but you can see what it does: a force can make a still object start to move, make a moving object go faster, slower or change direction, and it can change an object's shape (squash, stretch, bend or twist it).

Forces are measured in newtons (N), named after the scientist Isaac Newton. We measure a force with a forcemeter (also called a newton meter): a spring inside stretches more when the pull is bigger, and a pointer shows the force on a scale in N.

2. The forces you need to know

Notice the pattern: friction, air resistance and water resistance always act against the movement. If an object moves to the right, these forces point to the left. Air resistance and water resistance get bigger when the object moves faster, and they are smaller for streamlined shapes (smooth, pointed shapes like a fish or a racing car).

3. Drawing force diagrams

A force diagram shows every force on an object as an arrow. One arrow gives you three pieces of information at once.

How to draw a force diagram — step by step

1. Draw the object simply — a box or a circle is enough.
2. Think of every force acting on it: Is gravity pulling it down? Is a surface pushing it up? Is it moving (friction or air/water resistance)? Is something pushing or pulling it? Is it in water (upthrust)?
3. Draw each force as a straight arrow drawn with a ruler. The arrow starts on the object and points the way the force acts.
4. Make the length match the size: a force twice as big gets an arrow twice as long. Forces that are equal get arrows of the same length.
5. Label every arrow with its name and size, for example "weight 6 N".

4. Balanced and unbalanced forces

Most objects have more than one force acting on them. To find what will happen, work out the overall force (also called the resultant force).

• Forces in the same direction: add them. Two people pushing a car forwards with 200 N and 150 N give 350 N forwards.
• Forces in opposite directions: subtract the smaller from the bigger. The overall force acts in the direction of the bigger force.
• If the opposite forces are equal, the overall force is 0 N and the forces are balanced.
• If the overall force is not zero, the forces are unbalanced.

5. What forces do to objects at rest and in motion

Forces can also change the shape of an object: squeezing a sponge, stretching an elastic band, bending a ruler or kicking a football (the ball squashes for a moment).

Story of a skydive: (1) She jumps — weight is much bigger than air resistance, so she speeds up. (2) As she gets faster, air resistance grows until it equals her weight — balanced, so steady speed. (3) She opens the parachute — air resistance suddenly becomes much bigger than weight, so she slows down. (4) As she slows, air resistance falls until it equals weight again — balanced, so a new, slower steady speed. (5) She lands — the ground gives a big upward force that stops her.

6. Worked examples

Worked example 1 — An apple hangs still from a branch. Its weight is 1.5 N. Draw a force diagram for the apple. [2]

1. The apple is still, so the forces must be balanced.
2. Gravity pulls down: arrow pointing down, labelled weight 1.5 N.
3. The stalk pulls up: arrow pointing up, labelled pull of the stalk 1.5 N, the same length as the weight arrow.

Answer: two arrows starting on the apple, equal length, opposite directions, both labelled 1.5 N. Common mistake: drawing the up arrow shorter — then the apple would fall.

Worked example 2 — Two children push a heavy box to the right with forces of 40 N and 35 N. Friction on the box is 60 N. The box starts still. (a) Work out the overall force on the box. Show your working. [2] (b) Describe what happens to the box. [1]

1. Pushes in the same direction add: 40 + 35 = 75 N to the right.
2. Friction acts the opposite way (left): 75 − 60 = 15 N to the right.
3. (b) The forces are unbalanced, so the box starts to move to the right.

Common mistake: adding all three numbers (40 + 35 + 60 = 135 N). Friction is in the opposite direction, so it must be subtracted. Always give the direction with the size.

Worked example 3 — Look at the car diagram in section 4 (driving force 900 N, friction + air resistance 600 N). (a) Describe the motion of the car. [1] (b) The driver lets the driving force fall to 600 N. Describe the motion now. [1] (c) The driver then brakes, and the backward force becomes 1500 N while the driving force is 0 N. What happens? [1]

Worked example 4 — Explain why a skydiver falls more slowly after opening her parachute. [2]

Answer: The parachute has a large surface area, so air resistance increases (1 mark). Air resistance becomes bigger than her weight, so the overall force is upwards and she slows down until air resistance equals weight again and she falls at a slower steady speed (1 mark). Common mistake: saying "gravity gets smaller" — her weight does not change.

Worked example 5 — In a football match, give four different effects of forces on the ball, with an example of each. [2] (1 mark for two correct, 2 marks for four)

• Starts it moving: a player kicks a still ball from the centre spot.
• Speeds it up: a player kicks a slowly rolling ball harder.
• Slows it down / stops it: friction from the grass; the goalkeeper catches it.
• Changes its direction: a player heads the ball to the side.
• Changes its shape: the ball squashes for a moment when it is kicked.

Worked example 6 (investigation) — Laila lets a toy car roll down the same ramp onto four different surfaces and measures how far it travels on each.

1. (a) Which surface gives the most friction? Explain. [1] → Sandpaper, because the car travelled the shortest distance, so the force slowing it was biggest.
2. (b) Name the independent variable. [1] → the type of surface.
3. (c) Give two things Laila must keep the same. [2] → same car, same ramp height (same starting point), same way of releasing the car (do not push it).
4. (d) Write a conclusion. [1] → The rougher the surface, the more friction, so the shorter the distance the car travels.

Practice

All pages in this course

Start here

• Checkpoint 6 Science: The Test and How to Prepare
• Thinking and Working Scientifically: Investigations, Variables, Tables and Graphs

Biology

• The Heart and Circulatory System
• The Lungs and Respiratory System
• Human Reproductive System and Puberty
• Diseases, Hygiene and the Body's Defences
• Food Chains, Food Webs, Energy and Toxins

Chemistry

• Properties of Substances: Melting and Boiling Points, Gases, Conductors
• Reversible Changes and Dissolving
• Boiling and Evaporation
• Chemical Reactions: Reactants, Products and Evidence

Physics

• Mass, Weight and Gravity
• Force Diagrams and Forces on Moving Objects
• Floating and Sinking
• Light: Reflection and Refraction
• Electric Circuits, Symbols, Series and Parallel

Earth and space

• Rocks and Fossils
• Types of Soil
• The Rock Cycle
• The Solar System and the Phases of the Moon

Practice tests

• Checkpoint 6 Science — Practice Test 1
• Checkpoint 6 Science — Practice Test 2
• Checkpoint 6 Science — Practice Test 3