Common force types

CIE A-Level Physics· Unit 4, Section 1· 15 min read

1. Non-Contact Force Types★★☆☆☆⏱ 3 min

📘 Definition

Non-contact force

A force that acts on an object without physical contact, arising from interactions between force fields

Example:

Gravitational force acts between the Earth and a satellite without contact

The most common non-contact force at A-Level is weight, the gravitational force on an object's mass. Other non-contact forces you will encounter are electric force (between charged objects) and magnetic force (between magnetic materials/moving charges).

Weight is always calculated with the formula $W = m g$ , where $m$ is mass and $g$ is acceleration due to gravity.

📐 Worked Example

Calculate the weight of a 12 kg uniform block on Earth's surface, where $g = 9.81 m s^{- 2}$

1
Recall the formula for weight:
2
$W = m g$
3
Substitute the given values to get the final answer:
4
$W = 12 \times 9.81 = 117.72 \approx 118 \text{ N (3 s.f.)}$

Exam tip:

Always use the value for g given in the question paper, do not assume 9.81 without checking

2. Contact Forces: Normal Reaction and Tension★★☆☆☆⏱ 4 min

📘 Definition

Contact force

A force that arises from physical contact between two objects

Example:

A table exerts a contact normal force on a book resting on it

Normal reaction is the force a surface exerts on an object in contact with it. It always acts perpendicular (normal) to the contact surface, pointing outward from the surface.

Tension is the pulling force exerted by a stretched string, rope or rod on an object attached to it. It always acts along the line of the string, pulling away from the object.

📐 Worked Example

A 5 kg mass is hung stationary from an inextensible vertical string. Calculate the tension in the string.

1
The mass is stationary, so net vertical force is zero. Upward tension balances downward weight:
2
$T - W = 0 \implies T = W$
3
Calculate weight, then substitute to find tension:
4
$W = mg = 5 \times 9.81 = 49.05 \\ T = 49.1 \text{ N (3 s.f.)}$

3. Contact Forces: Friction and Drag★★★☆☆⏱ 4 min

📘 Definition

Friction

A contact force that opposes relative motion between two solid surfaces in contact

Example:

Static friction prevents a box from sliding down a sloped ramp

Static friction acts to prevent motion, and has a maximum value given by $f_{max} = μ_{s} N$ , where $μ_{s}$ is the coefficient of static friction and $N$ is normal reaction. Kinetic friction acts when the object is moving, given by $f_{k} = μ_{k} N$ .

Drag (air resistance for objects moving through air) is a fluid friction that opposes motion of an object through a fluid. Its magnitude increases with the speed of the object, and it always acts opposite to the direction of motion relative to the fluid.

📐 Worked Example

A 3 kg block rests on a horizontal surface. The coefficient of static friction is 0.4. Calculate the maximum static friction force before the block slides.

1
For a horizontal surface with no additional vertical forces, normal reaction equals weight:
2
$N = mg = 3 \times 9.81 = 29.43 \text{ N}$
3
Use the formula for maximum static friction:
4
$f_{\text{max}} = \mu_s N = 0.4 \times 29.43 = 11.772 \approx 11.8 \text{ N (3 s.f.)}$

Exam tip:

Friction always acts parallel to the contact surface, never perpendicular to it

4. Upthrust (Buoyancy Force)★★★☆☆⏱ 4 min

📘 Definition

Upthrust

U

A buoyant contact force exerted by a fluid on an immersed or floating object, acting vertically upward

Example:

Upthrust allows boats to float on water

By Archimedes' principle, the magnitude of upthrust is equal to the weight of the fluid displaced by the object. This gives the formula $U = ρ_{fluid} V_{displaced} g$ , where $ρ$ is fluid density and $V$ is the volume of displaced fluid.

📐 Worked Example

A solid cube of volume $0.002 m^{3}$ is fully immersed in water of density $1000 kg m^{- 3}$ . Calculate the upthrust on the cube.

1
For a fully immersed object, the volume of displaced fluid equals the object's total volume. Use the upthrust formula:
2
$U = \rho V g$
3
Substitute values to calculate the result:
4
$U = 1000 \times 0.002 \times 9.81 = 19.62 \approx 19.6 \text{ N (3 s.f.)}$

5. Common Pitfalls

Wrong move:

Assuming normal reaction is always equal to an object's weight

Why:

This is only true for horizontal surfaces with no other vertical forces acting

Correct move:

Always calculate normal reaction by summing vertical forces for your specific scenario

Wrong move:

Drawing tension as pushing an object rather than pulling it

Why:

Tension is always a pulling force along a string; ideal strings cannot push

Correct move:

Draw all tension vectors pointing away from the object along the string's line

Wrong move:

Always drawing drag as acting downward

Why:

Drag direction depends on motion direction, not vertical position

Correct move:

Always point drag opposite to the object's velocity relative to the fluid

Wrong move:

Forgetting to add weight to free-body diagrams of objects in air

Why:

Weight is a non-contact force that acts on all objects near Earth's surface, regardless of contact

Correct move:

Include weight as the first force on every free-body diagram

Wrong move:

For floating objects, using the whole object volume to calculate upthrust

Why:

Only the submerged part of the object displaces fluid

Correct move:

Use only the volume of the submerged part of the object for $V_{displaced}$

6. Quick Reference Cheatsheet

Force Type	Category	Key Formula	Direction
Weight	Non-contact	$W = m g$	Vertically downward
Normal Reaction	Contact	From force equilibrium	Perpendicular outward from surface
Tension	Contact	From force equilibrium	Away from object along string
Max Static Friction	Contact	$f_{max} = μ_{s} N$	Opposes motion parallel to surface
Kinetic Friction	Contact	$f_{k} = μ_{k} N$	Opposes motion parallel to surface
Drag	Contact	Increases with speed	Opposes motion relative to fluid
Upthrust	Contact	$U = ρ V_{displaced} g$	Vertically upward

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 1
Identify direction of drag
2023 · 2
Calculate upthrust on immersed object
2021 · 1
Normal reaction on inclined plane

What's Next

Mastery of common force types is the foundational skill for all mechanics topics in CIE A-Level Physics. Once you can correctly identify, calculate and assign direction to each common force, you can build accurate free-body diagrams to solve any force problem. This topic connects directly to equilibrium, projectile motion, circular motion and density-pressure problems in this and later units. Solid understanding here will prevent many common exam errors in more complex topics.