Forces on one body

Draw the forces on the chosen body, then combine them to find the resultant force.

A force comes from an interaction, such as a string pulling or a surface pushing. Review force types and weight if you need help naming the forces.

A free-body diagram shows the forces acting on one selected body. Represent the body with a dot or a simple outline. Draw labelled arrows from it: the arrowhead gives direction, and the length can represent magnitude using a chosen scale.

1. Choose the body: for example, the book, rather than the book and table together.
2. Identify its interactions: Earth attracts the book, and the table pushes on it.
3. Draw only forces on that body: the book's force on the table belongs on a different diagram.
4. Check the directions and labels: weight is downwards; the normal contact force is perpendicular to the table.

The resultant is the single force equal to the vector sum of all the forces on the body. Along one line, forces in the same direction add; forces in opposite directions subtract. Keep their directions in the answer.

Balanced forces do not require the object to be at rest

Forces are balanced when their resultant is zero. The body's acceleration is then zero, so its velocity stays constant. A body already at rest remains at rest; a moving body continues at the same speed in the same direction.

This is Newton's first law: a body stays at rest, or keeps moving at constant velocity, unless a resultant force acts on it. Your syllabus does not ask you to state Newton's laws, but school mark schemes often name them.

The trolley has forces acting on it, but their resultant is zero. The string's forward pull balances the resistance. If resistance were absent, a forward pull would no longer be needed to maintain the same velocity.

An unbalanced force changes velocity

A non-zero resultant force produces acceleration in its direction. Since velocity includes direction, several changes are possible:

• An object at rest can start moving.
• A resultant in the direction of motion can increase its speed.
• A resultant opposite to its motion can reduce its speed.
• A resultant can turn its motion. An object following a curved path has changing velocity even if its speed stays constant.

For example, increasing the trolley's string pull to 7 N while resistance remains 3 N gives a resultant of 4 N forwards. Its velocity now changes. We calculate how quickly it changes in resultant force and acceleration.

Do not add a "force of motion". Motion is what the body does; each force must come from an interaction. If a separate velocity arrow helps, label it as velocity and keep it distinct from the force diagram.