Weight, normal force and free-body diagrams
G2.1 G2.2
By the end of this lesson you should be able to
- Solve problems involving force due to gravity (weight) and mass using F_g = mg. G2.1
- Describe the concept of normal force. G2.2
Key ideas
- Mass (kg) is the amount of matter and doesn't change with location. Weight (N) is the gravitational force on it.
- The normal force is the push of a surface on an object, always perpendicular to the surface.
- A free-body diagram shows the object as a dot with every force acting on it as an arrow from that dot, labelled and roughly to scale.
- On a flat surface with no other vertical forces, the normal force equals the weight. That's not true on slopes or when pulling at an angle.
Key formulas (QCAA formula book notation)
\[
F_g = m g
\]
\[
F_{net} = m a
\]
Worked example
A 12 kg box rests on the floor. Find its weight and the normal force.
- \(F_g = 12 \times 9.8 = 118\) N downwards
- Balanced vertically, so \(F_N = 118\) N upwards
Common mistakes
- Giving weight in kg.
- Drawing the normal force vertically on a slope.
- Including forces the object exerts on other things.
Do
- Read: Textbook (Oxford Physics for Queensland Units 3 & 4) section 3.1-3.2
- Do: CYL 3.1 Q1, 5, 8; CYL 3.2 Q4
Try it
Watch
- Introduction to Free Body Diagrams · Flipping Physics
Further reading
- OpenStax Physics 5.4 Inclined planes (OpenStax, CC BY 4.0)
- Full topic notes: Gravity & Motion