Describe the effects of forces, distinguish mass from weight, and apply the relationship force equals mass times acceleration to simple situations

What this dot point is asking

SEAB wants you to describe what forces do, tell the difference between mass and weight, and use the relationship $F = ma$ in simple calculations. The central idea is that a force is a push or a pull, and an unbalanced (resultant) force changes how an object moves.

The answer

What a force does

A force is a push or a pull, measured in newtons ($\text{N}$). A force can:

• change the speed of an object (speed it up or slow it down),
• change the direction of motion,
• change the shape of an object (stretch or squash it).

Forces are measured with a newton meter (a spring balance).

Balanced and unbalanced forces

When the forces on an object are balanced, they cancel out, the resultant force is zero, and the object stays still or keeps moving at constant speed. When the forces are unbalanced, there is a resultant force, and the object accelerates in the direction of that resultant force.

Force, mass and acceleration

The resultant force, the mass and the acceleration are linked by:

where $F$ is the resultant force in newtons, $m$ is the mass in kilograms, and $a$ is the acceleration in $\text{m/s}^2$. For a fixed force, a larger mass gives a smaller acceleration.

Mass and weight are different

These two are often confused:

• Mass is the amount of matter in an object, measured in kilograms ($\text{kg}$). It is the same everywhere.
• Weight is the downward pull of gravity on the object, measured in newtons ($\text{N}$). It changes if the gravitational field strength changes.

Weight is found from:

where $g$ is the gravitational field strength, about $10\ \text{N/kg}$ on Earth.

Examples in context

Example 1. A parachutist reaching terminal velocity. As a parachutist falls, air resistance grows with speed until it balances their weight. The forces are now balanced, the resultant force is zero, and they fall at a steady speed called terminal velocity. No resultant force means no further acceleration.

Example 2. Why a loaded lorry accelerates slowly. A lorry with the same engine force but a much larger mass has a smaller acceleration, because $a = F/m$ and the mass is large. This is why heavy vehicles pull away from traffic lights more slowly than light cars.

Try this

• Cue. A $5\ \text{kg}$ object is pushed by a resultant force of $15\ \text{N}$. Find its acceleration: $a = \dfrac{15}{5} = 3\ \text{m/s}^2$.
• Cue. Find the weight of a $12\ \text{kg}$ bag on Earth where $g = 10\ \text{N/kg}$: $W = 12 \times 10 = 120\ \text{N}$.
• Cue. Explain why an object moving at constant speed has a resultant force of zero. The forces are balanced, so there is no net force to change its motion.