Chapter revision
Revision summary
Key ideas, equations and common mistakes. Open any topic below for the full explanation.
Choose the body, identify its interactions and find the resultant before predicting its motion.
- Weight and gravitational field
- W = mg. Weight W in N; mass m in kg; gravitational field strength g in N/kg. Near Earth, g is approximately 10 N/kg. Use the value given.
- Resultant force and acceleration
- Fresultant = ma, so a = Fresultant / m. Acceleration is in the resultant's direction. Use all the relevant forces, not just the applied pull.
From forces to motion
- Zero resultant: zero acceleration. The body stays at rest or continues at constant velocity.
- Non-zero resultant: velocity changes. The body can start, speed up, slow down or change direction.
- A fast-moving object need not have a large resultant force. A slowing object still has acceleration.
- Draw only forces on the chosen body. Identify the interaction behind each arrow; motion is not an extra force.
Balanced forces or an interaction pair?
| Balanced forces | Action-reaction pair |
|---|---|
| Act on the same body and sum to zero. | Act on different bodies in the same interaction. |
| Example: table on book upwards and Earth on book downwards. | Example: table on book upwards and book on table downwards. |
Mass, weight and force types
Mass is a measure of matter and is measured in kg; weight is gravitational force and is measured in N. An unchanged object's mass stays the same when its gravitational field changes. For the same resultant force, a larger mass has a smaller acceleration.
Contact examples: normal force, tension, friction and air resistance. Non-contact examples: gravitational, electrostatic and magnetic forces. Normal force is perpendicular to the contact surface; tension pulls along a string.
Friction direction
Identify the contact and the relative sliding or tendency to slide. Friction can slow a sliding box, prevent a stationary box from sliding, or push a walking person forwards during push-off.
Check a practical explanation
State which mass or force you measured and its unit. For a force-motion comparison, keep total mass controlled, account for resistance and check that a slope has not introduced an extra force along the track. Name a limitation and explain its effect.
Back to forces on one body