Forces
Map Your Gaps
Card 1 of 12
Swipe right if you know it, left if you don't
✔ Know
✖ Don't know
Scalar and Vector Quantities
Physical quantities are either scalar (magnitude only) or vector (magnitude and direction).
Forces are vectors — direction matters when combining them.
Scalar and Vector Quantities — Key Knowledge
- Scalar quantities have magnitude only — e.g. speed, distance, temperature, mass
- Vector quantities have magnitude and direction — e.g. velocity, displacement, force, acceleration
Contact and Non-Contact Forces
Forces are pushes or pulls that arise from the interaction of two objects. They are either contact or non-contact.
Identifying force type is the first step in any free body diagram question.
Contact and Non-Contact Forces — Key Knowledge
- Contact forces objects are physically touching — friction, air resistance, tension, normal contact force
- Non-contact forces objects are separated — gravitational, electrostatic, magnetic
Weight and Mass
Mass is the amount of matter in an object, measured in kilograms. Weight is the force of gravity acting on a mass, measured in newtons.
W = mg
weight = mass x gravitational field strength
Mass stays the same everywhere — weight changes depending on the strength of the gravitational field.
Weight and Mass — Key Knowledge
- Mass amount of matter, measured in kg, does not change with location
- Weight gravitational force on an object, measured in N, varies with gravitational field strength
- Gravitational field strength g = 10 N/kg on Earth, lower on the Moon
Resultant Forces
A resultant force is a single force that has the same effect as all the individual forces acting on an object combined.
If the resultant force is zero, the object is either stationary or moving at constant velocity.
Resultant Forces — Key Knowledge
- Resultant force the overall net force on an object — found by adding forces in the same direction and subtracting forces in opposite directions
- Free body diagrams show the size and direction of all forces acting on a single object
Work Done and Energy Transfer
When a force causes an object to move, energy is transferred — the amount of energy transferred is the work done.
W = Fs
work done = force x distance moved in the direction of the force
If a force acts but nothing moves, no work is done.
Work Done and Energy Transfer — Key Knowledge
- Work done energy transferred by a force, measured in joules, J
- Friction work done against friction causes a temperature increase in the objects involved
Forces and Elasticity
A force can stretch, compress or bend an object. The extension is proportional to the applied force up to the limit of proportionality.
F = ke
force = spring constant x extension
The linear region of a force-extension graph represents Hooke's law — the curve begins at the limit of proportionality.
Forces and Elasticity — Key Knowledge
- Hooke's law extension is directly proportional to force, up to the limit of proportionality
- Spring constant k, a measure of stiffness, units N/m
- Limit of proportionality beyond this point, extension is no longer proportional to force
- Elastic deformation object returns to original shape when force removed
- Inelastic deformation object does not return to original shape
Moments, Levers and Gears
A moment is the turning effect of a force about a pivot point.
M = Fd
moment = force x perpendicular distance from the pivot
Exam questions often involve a balanced beam — set clockwise moments equal to anticlockwise moments and solve.
Moments, Levers and Gears — Key Knowledge
- Moment turning effect of a force, measured in Nm
- Principle of moments for a balanced object, total clockwise moments = total anticlockwise moments
- Levers increase the distance from the pivot to reduce the force needed
- Gears transmit turning forces — a larger driven gear turns slower with more force
Pressure and Pressure in Fluids
Pressure is the force per unit area. In liquids, pressure increases with depth due to the weight of liquid above.
p = F/A
pressure = force / area, p = hρg
A smaller area means a greater pressure for the same force — this is why sharp objects cut more easily.
Pressure and Pressure in Fluids — Key Knowledge
- Pressure force per unit area, measured in pascals, Pa
- Pressure in a liquid column increases with depth and density of the liquid
- Atmospheric pressure caused by air molecules colliding with surfaces, decreases with altitude as there are fewer molecules above
- Upthrust the upward force a fluid exerts on a submerged or partially submerged object
Speed, Distance and Velocity
Speed is a scalar quantity (how fast something is going). Velocity is a vector quantity (speed in a stated direction).
s = vt
distance = speed x time
A flat line on a distance-time graph means the object is stationary.
Speed, Distance and Velocity — Key Knowledge
- Speed distance travelled per unit time, measured in m/s
- Velocity speed in a given direction — a vector quantity
- Distance-time graphs gradient = speed; a steeper line means faster; a curved line means changing speed
- Typical speeds walking ~1.5 m/s, running ~3 m/s, cycling ~6 m/s, car ~13-30 m/s, sound in air ~330 m/s
Acceleration
Acceleration is the rate of change of velocity — how quickly an object speeds up, slows down or changes direction.
a =
v - u / t
A flat line on a velocity-time graph means constant velocity (zero acceleration), not zero velocity.
Acceleration — Key Knowledge
- Acceleration change in velocity per unit time, measured in m/s², a vector quantity
- Deceleration negative acceleration — the object is slowing down
- Velocity-time graphs gradient = acceleration; area under the graph = distance travelled
Newton's Laws of Motion
Three laws describing how forces affect the motion of objects.
F = ma
resultant force = mass x acceleration
Third Law pairs always act on two different objects, are the same type of force, and are equal in size but opposite in direction.
Newton's Laws of Motion — Key Knowledge
- Newton's First Law an object remains at rest or at constant velocity unless acted on by a resultant force — this is inertia
- Newton's Second Law the acceleration of an object is directly proportional to the resultant force and inversely proportional to mass
- Newton's Third Law when two objects interact, they exert equal and opposite forces on each other — the forces act on different objects
Stopping Distances and Terminal Velocity
Stopping distance is the total distance a vehicle travels from when the driver sees a hazard to when the vehicle stops. Terminal velocity is the constant speed reached when drag equals the driving force.
Greater speed means greater braking distance — the braking force also causes the brakes to heat up.
Stopping Distances and Terminal Velocity — Key Knowledge
- Stopping distance thinking distance + braking distance
- Thinking distance distance travelled during the driver's reaction time — affected by tiredness, drugs, alcohol, distractions
- Braking distance distance travelled while brakes are applied — affected by speed, road conditions, tyre condition, brake condition, vehicle mass
- Terminal velocity reached when air resistance equals weight, so resultant force is zero and acceleration stops