Revision summary

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

F_resultant = ma, so a = F_resultant / 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 and inertia are not extra forces.

Balanced forces or an interaction pair?

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. Greater mass also means greater inertia: more resistance to a change in motion.

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.

Three-force equilibrium

1. Draw the three forces on the stationary point mass.
2. Choose a paper scale and draw the known force.
3. Use the other force directions to construct a closed head-to-tail triangle.
4. Check arrow directions, measure lengths and convert them to forces.

The triangle closes because the resultant is zero. Use the stated scale on paper; a resized screen does not preserve centimetres.

Falling bodies

• Negligible air resistance: weight is the only force and a = g in a uniform field.
• Air resistance below weight during downward motion: the body speeds up downwards. As resistance grows, the downward acceleration decreases.
• Air resistance equal to weight: zero resultant and zero acceleration; terminal motion has a constant, non-zero downward velocity.
• Air resistance above weight: upward acceleration slows a downward-moving body.

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.