Forces · 2.1
Applied Definitions
A force is an interaction: a push or pull that can change an object’s motion or shape, or create a turning effect.
- Simple definition
- A force is a push or pull caused by an interaction, and it can change an object's motion or shape.
- Example
- When a hand pushes a trolley, the hand exerts a forward force that can accelerate the trolley.
The applied definition
You see a force through its effect.
Forces are vectors. Measure their size in newtons (N), show their direction with arrows, and name the object that experiences each force.
An interaction between objects that can also make them turn.
A force meter or newton meter measures it.
Always state magnitude and direction.
A force can…
Name the interaction
Contact, or action at a distance?
Contact forces need touching. Non-contact forces act through a field across a gap.
Applied force
A push or pull supplied by a person or another object.
- Direction
- In the direction of the push or pull.
- Example
- A hand pushes a trolley forward.
Quick check · 1/5
A hand pushes a shopping trolley.
Is the main force contact or non-contact?
Ask: must the two objects touch for this force to act?
Static friction adjusts to prevent slipping, up to a maximum. Kinetic friction acts once the surfaces slide.
Fluid resistance opposes velocity and depends on speed, shape, size, and the fluid.
Treat the whole weight as acting at one point: the centre of gravity. Suspend the shape from two points: the two plumb lines intersect at its centre of gravity.
Combine the vectors
One object. Two pulls. One result.
The resultant force is the vector sum of every force acting on the chosen object.
The 6 N resultant points right, so the object accelerates right.
Balanced does not mean “no forces.” It means the forces add to zero, so acceleration is zero. An object may be still or moving at constant velocity.
Do not mix them up
Mass stays. Weight follows gravity.
Mass measures matter and inertia in kilograms. Weight is the gravitational force on that mass in newtons.
g is the gravitational field strength. It is the gravitational force on each kilogram, g = W/m, measured in N/kg. On Earth every kilogram is pulled with about 9.8 N, so g = 9.8 N/kg — and the definition runs backwards just as well: a 60 kg astronaut weighing 96 N on the Moon puts the Moon’s g at 96 ÷ 60 = 1.6 N/kg.
9.8 N/kg and 9.8 m/s² are the same quantity. In free fall weight is the only force, so Fnet = mg, and Newton’s second law gives a = Fnet/m = mg/m = g. The mass cancels. That is why a hammer and a feather dropped where there is no air land together, and why the Moon’s 1.6 N/kg is also the 1.6 m/s² a dropped hammer accelerates at there.
60 kg × 9.8 N/kg = 588 N
Same mass · different weight
Free-body diagrams
Isolate the object before solving.
A free-body diagram keeps only the chosen object and the forces acting on it.
- 01
Choose one object. Replace it with a simple box or dot.
- 02
Find every external interaction. Include only forces acting on that object.
- 03
Draw and label arrows. Arrow direction shows force direction; length can show relative size.
- 04
Keep components separate. Resolve angled forces after the free-body diagram is complete.