Interactions Over Small and Large Distances: Forces and Energy Changes — AQA Combined Science: Synergy
This topic covers forces as vectors, work, weight, and energy stored in raised and stretched objects.
Forces as vectors
- Scalars have magnitude only (e.g. mass, energy, speed).
- Vectors have magnitude and direction (e.g. force, weight, velocity), drawn as arrows.
Forces are pushes or pulls from an interaction. They may be contact (friction, tension, normal force) or non-contact (gravity, magnetic, electrostatic).
Resolving forces (Higher Tier)
Several forces can be replaced by a single resultant force. A single force can also be split (resolved) into two perpendicular components. Free body diagrams and scale drawings help find the resultant. If the resultant force is zero, the object is in equilibrium.
Work
When a force moves an object, work is done and energy is transferred: $$W = F \times d$$ (work in joules = force × distance moved in the direction of the force). Work done against friction transfers energy to thermal stores.
Mass and weight
- Mass (kg) is the amount of matter; constant everywhere.
- Weight (N) is the force of gravity: $$W = m \times g$$ (g ≈ 9.8 N/kg on Earth). Weight is measured with a calibrated spring balance (newtonmeter) and changes with gravitational field strength.
Gravitational potential energy
Raising an object stores gravitational potential energy: $$E_p = m , g , h$$ When an object falls, this is transferred to kinetic energy ($E_k = \tfrac12 mv^2$).
Elastic deformation
Stretching, bending or compressing needs more than one force. For an elastic object up to the limit of proportionality, Hooke's law applies: $$F = k \times e$$ (force = spring constant × extension). Extension is directly proportional to force (straight-line graph until the limit).
Required practical: investigating the relationship between force and extension of a spring.
Energy stored in a stretched spring
$$E_e = \tfrac{1}{2} k e^2$$ This elastic potential energy is stored in a stretched (or compressed) spring within the limit of proportionality.
Exam tips
- Classify quantities as scalar or vector.
- Learn W = Fd, W = mg, E_p = mgh and F = ke, and practise rearranging.
- Distinguish mass (constant) from weight (depends on gravity).
- For a force–extension graph, the straight section obeys Hooke's law up to the limit of proportionality.