What Is a Force? Contact and Non-contact Forces – Force and Equilibrium of Objects (Senior 1 Physics) | Ganzaa

Force and Equilibrium of Objects

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Senior 1 · Physics

Force and Equilibrium of Objects

5 lessons

Lesson 1 of 5

What Is a Force? Contact and Non-contact Forces

22 min

Three puzzles

A ripe mango breaks off a tree in Rusizi and falls to the ground, though nothing seems to touch it. A magnet pulls a steel pin across a gap. A football kicked across the grass slows down and stops by itself. In each case something starts, stops or changes the motion of an object. That "something" is a force.

Definition

A force is a push or a pull that one object exerts on another. A force has a size and a direction, so it is a vector, and it is drawn as an arrow.

The SI unit of force is the newton (N). One newton is the force that gives a mass of 1 kg an acceleration of $1\,\mathrm{m\,s^{-2}}$ : $1\,\mathrm{N} = 1\,\mathrm{kg\,m\,s^{-2}}$ . The weight of a 100 g apple is about 1 N.

Experiment: measuring forces with a spring balance

A spring balance (or newton meter) measures force: the bigger the pull on its hook, the more its spring stretches and the further the pointer moves along a scale marked in newtons.

Figure 1.1: Measuring the weight of slotted masses with a spring balance.

Hang a spring balance from a clamp and check that it reads zero with nothing on it. If not, adjust its zero screw.
Hang a mass hanger with one 100 g mass on the hook. Read the scale at eye level.
Add masses 100 g at a time, up to 500 g, and record each reading.

Mass (g)	100	200	300	400	500
Spring balance reading (N)	1.0	2.0	2.9	3.9	4.9
$W = m g$ with $g = 9.8\,\mathrm{N\,kg^{-1}}$ (N)	0.98	1.96	2.94	3.92	4.90

Each extra 100 g adds about 1 N. The readings agree with $W = m g$ to within the precision of the scale (0.1 N).

Important

Safety. Never hang more than the largest reading on the scale, or the spring is stretched for good. Keep your feet clear of the hanging masses, and clamp the stand firmly.

Definition

A contact force acts only when two objects touch: a push, friction, tension, the normal reaction of a surface, air resistance, upthrust.

A non-contact force (or field force) acts across a distance, without touching: gravitational force (weight), magnetic force and electrostatic force.

Contact and non-contact forces

Figure 1.2: Contact forces act where objects touch; non-contact forces act across a gap.

Force	Type	What it does	Example
Friction	contact	opposes sliding between two surfaces	tyres gripping a murram road; a moto's brakes
Tension	contact	the pull in a stretched rope, string or cable	the rope holding a bucket in a well
Normal reaction	contact	the push of a surface on an object resting on it, at right angles to the surface	the floor pushing up on your feet
Air resistance (drag)	contact	opposes motion through air	a cyclist crouching low to go faster
Upthrust	contact	the upward push of a liquid or gas	a fishing boat floating on Lake Kivu
Gravitational force (weight)	non-contact	pulls every mass towards the Earth	a mango falling from a tree
Magnetic force	non-contact	pull or push between magnets, or a magnet and iron	a magnet picking up iron nails
Electrostatic force	non-contact	pull or push between electric charges	a rubbed plastic comb picking up bits of paper

Definition

Weight ( $W$ ) is the gravitational force of the Earth on an object:

W = m g

where $m$ is the mass in kg and $g$ is the gravitational field strength, $9.8\,\mathrm{N\,kg^{-1}}$ on Earth (often rounded to $10\,\mathrm{N\,kg^{-1}}$ for quick estimates) and $1.6\,\mathrm{N\,kg^{-1}}$ on the Moon.

Worked example 1: the weight of a bag of rice

Find the weight of a 25 kg bag of rice on Earth.

W = mg = 25\,\mathrm{kg} \times 9.8\,\mathrm{N\,kg^{-1}} = 245\,\mathrm{N}

On the Moon the same bag would weigh $25 \times 1.6 = 40\,\mathrm{N}$ , but its mass would still be 25 kg.

Worked example 2: finding a mass from a spring balance

A bunch of bananas hung from a spring balance gives a reading of 7.35 N. What is its mass?

Rearrange $W = m g$ :

m = \frac{W}{g} = \frac{7.35\,\mathrm{N}}{9.8\,\mathrm{N\,kg^{-1}}} = 0.75\,\mathrm{kg}

Warning

Common mistakes. (1) Giving a force in kilograms: forces, including weight, are in newtons. (2) Forgetting that a force needs a direction: "20 N" is incomplete; "20 N downwards" is complete. (3) Thinking that non-contact forces need air: gravity and magnetism act through empty space too.

Tip

Ask "are they touching?" If the force disappears when the objects are pulled a little apart, it is a contact force. Gravity, magnetism and electrostatic forces still act across a gap.

Forces on Rwanda's hills

On steep murram roads, friction is the force that matters most. It lets tyres grip when a moto climbs, and it is what slows a loaded icuguti or a braking moto on the way down. When rain makes the surface wet and muddy, friction becomes much smaller: tyres slip, and braking takes longer. That is why drivers and riders must slow down in the rainy season.

Contact and non-contact forces (Freesciencelessons). Watch for the two lists of forces and the examples of each, then compare them with the table above.

✦

Practice

Say whether each force is a contact or a non-contact force: (a) friction on the soles of your shoes; (b) the Earth's pull on a satellite; (c) a magnet turning a compass needle; (d) the pull in a skipping rope; (e) the upthrust on a canoe; (f) a rubbed balloon pulling your hair.
Find the weight of a 60 kg pupil on Earth ( $g = 9.8\,\mathrm{N\,kg^{-1}}$ ).
A spring balance reads 14.7 N when a bag hangs from it. Find the mass of the bag.
Find the weight of a 12 kg bag of maize flour (a) on Earth, (b) on the Moon ( $g = 1.6\,\mathrm{N\,kg^{-1}}$ ).
Name the force that (a) slows down a book sliding across a table; (b) holds up a bucket hanging from a rope; (c) holds up a boat on a lake.
Use the results table to predict the reading for a mass of 250 g.
Explain why weight is a vector but mass is a scalar.

Show answersHide answers

(a) contact; (b) non-contact; (c) non-contact; (d) contact; (e) contact; (f) non-contact.
$W = 60 \times 9.8 = 588\,\mathrm{N}$
$m = \frac{14.7}{9.8} = 1.5\,\mathrm{kg}$
(a) $12 \times 9.8 = 117.6\,\mathrm{N} \approx 118\,\mathrm{N}$ ; (b) $12 \times 1.6 = 19.2\,\mathrm{N}$
(a) friction; (b) tension (in the rope); (c) upthrust.
About 2.5 N (half-way between 2.0 N and 2.9 N; $0.25 \times 9.8 = 2.45\,\mathrm{N}$ ).
Weight is a force: it has a size and a direction (downwards, towards the centre of the Earth). Mass is an amount of matter: it has a size only.

Key points

A force is a push or a pull; it is a vector measured in newtons (N) with a spring balance.
Contact forces need touching (friction, tension, normal reaction, air resistance, upthrust); non-contact forces act across a gap (gravity, magnetism, electrostatic force).
Weight is the pull of gravity: $W = m g$ , with $g = 9.8\,\mathrm{N\,kg^{-1}}$ on Earth.

Sources and credits

4.1: Development of Force Concept, from College Physics (1st edition) by Paul Peter Urone and Roger Hinrichs, OpenStax, hosted by LibreTexts, licensed under CC BY 4.0. Adapted by Ganzaa.
Photo: ICUGUTI-A WOODEN BIKE.jpg by Mutiganda, Wikimedia Commons, licensed under CC BY-SA 4.0.
Video: GCSE Physics Revision "Contact and Non-contact Forces", Freesciencelessons, embedded from YouTube.

The worked examples, diagrams, Rwandan applications and practice exercises are written by Ganzaa.

1Three puzzles

2Experiment: measuring forces with a spring balance

3Contact and non-contact forces

4Forces on Rwanda's hills

Three puzzles

Experiment: measuring forces with a spring balance

Contact and non-contact forces

Forces on Rwanda's hills