Chapter revision
Revision summary
Key ideas, equations and common mistakes. Open any topic below for the full explanation.
- Weight
- W = mg. W in N; m in kg; gravitational field strength g in N/kg. An unchanged object's mass stays the same in a different field, while its weight can change.
- Density
- ρ = m / V; m = ρV; V = m / ρ. Use g with cm3, or kg with m3. Density is mass per volume, not weight per volume.
- Average pressure at a surface
- p = F / A; F = pA; A = F / p. F is perpendicular to the area. Use N and m2 for a result in Pa; 1 Pa = 1 N/m2.
Identify the quantity and interaction
- Mass is a scalar measured in kg. Weight is a gravitational force, measured in N and acting in the field's direction.
- A gravitational field is a region where mass experiences gravitational attraction. Its strength is gravitational force per unit mass.
- Contact forces include normal force, tension, friction and air resistance. Non-contact examples include gravitational, electrostatic and magnetic forces.
- Normal force is perpendicular to a surface. Tension pulls along a string. Name both interacting objects.
Choose a density method
| Sample | Method |
|---|---|
| Regular solid | Weigh it and calculate volume from its measured dimensions and known shape. |
| Suitable irregular solid | Weigh it and use the increase in cylinder reading when fully submerged. Avoid trapped air, water loss, absorption and dissolving. |
| Liquid | Measure volume and subtract the dry empty container's mass from its filled mass, or tare correctly. Keep the measured mass and volume matched. |
1 g/cm3 = 1000 kg/m3. Convert both mass and volume units. A heavier sample is not necessarily denser if its volume is also larger.
Compare pressures carefully
- Identify the actual contact area, not the object's total surface area or volume.
- Convert square units: 1 cm2 = 0.0001 m2.
- For a fixed force, reducing contact area increases pressure.
- If force and area both change, compare their ratio. Pressure is not another force to add.