OCR GCSE Physics A (J249) — Forces
Every time you watch a rocket launch or a car brake suddenly, you are seeing the combined effect of multiple invisible pushes and pulls. To understand exactly how these objects will move, physicists use a free body diagram.
This is a simplified drawing that represents a single, isolated object and shows only the forces acting on it. A key feature of a free body diagram is what it leaves out: it does not show any forces the object exerts on its surroundings. Because forces are vector quantities, they possess both a magnitude (size) and a direction, which are represented by arrows on the diagram starting from the object's centre of mass.
To correctly describe and draw the forces on an object, you must follow a strict sequence of steps:
A resultant force is a single force that has the exact same effect as all the individual forces acting on an object combined. You can calculate the resultant force by adding or subtracting forces that act along the same mathematical line (in one dimension).
If the resultant force is exactly zero, the object is in a state of equilibrium. In this state, the forces are perfectly balanced, and the object will either remain stationary or continue moving at a constant velocity.
A drone is hovering in the air before accelerating directly upwards. Its motors produce an upward lift of . The downward force of its weight is . Calculate the resultant force on the drone.
Step 1: Identify the forces and their directions.
Step 2: Describe the Free Body Diagram. In a free body diagram for this drone, the object would be represented by a central dot. There would be a long arrow pointing directly upwards labeled 'Lift' and a noticeably shorter arrow pointing directly downwards labeled 'Weight'.
Step 3: Calculate the final magnitude and state the direction.
A cyclist is travelling along a flat road at a constant speed. They provide a forward thrust of . The combined forces of air resistance and friction acting backwards total . Calculate the resultant force.
Step 1: Identify the forces and their directions.
Step 2: Describe the Free Body Diagram. In the free body diagram, the cyclist is represented as a single point. The forward arrow (Thrust) and the backward arrow (Friction/Drag) must be drawn to exactly the same length to show the forces are balanced.
Step 3: Calculate the final magnitude.
Students often draw force arrows pointing towards the object; you must always start the arrow at the object's centre and point it outwards.
Always state the direction of a resultant force in your final answer, as a magnitude alone will not score full marks (e.g., write '150 N forwards' instead of just '150 N').
In diagram labelling questions, examiners expect precise OCR-approved terms; use 'Weight' instead of 'gravity' and 'Normal Contact Force' instead of 'reaction' to guarantee the marks.
Higher Tier: If a question asks you to 'describe' an example using a free body diagram, make sure to mention the relative lengths of the arrows (e.g., 'the thrust arrow is longer than the drag arrow').
Free body diagram
A simplified diagram representing an object as a point or simple box, showing only the forces acting on that specific object.
Vector
A physical quantity that has both magnitude (size) and direction.
Magnitude
The numerical size or strength of a physical quantity, such as a force measured in Newtons (N).
Resultant force
A single force that represents the combined effect of all individual forces acting on an object.
Equilibrium
A state where the resultant force acting on an object is zero, meaning all forces are perfectly balanced.
Normal Contact Force
A force exerted by a solid surface on an object in contact with it, always acting perpendicular (at 90 degrees) to the surface.
Drag
A frictional force that opposes the motion of an object as it moves through a fluid, such as a liquid or gas.
Weight
The downward force acting on an object due to gravity.
Thrust
A forward driving force produced by an engine, rocket, or propeller.
Tension
A pulling force transmitted through a string, rope, or wire when it is pulled tight by forces from opposite ends.
Centre of mass
The single point through which the entire weight of an object is considered to act.
Lift
An upward force exerted on an object moving through a fluid (usually air) due to pressure differences.
Friction
A resistive force that opposes motion between two surfaces that are sliding, or trying to slide, across each other.
Put your knowledge into practice — try past paper questions for Physics A
Free body diagram
A simplified diagram representing an object as a point or simple box, showing only the forces acting on that specific object.
Vector
A physical quantity that has both magnitude (size) and direction.
Magnitude
The numerical size or strength of a physical quantity, such as a force measured in Newtons (N).
Resultant force
A single force that represents the combined effect of all individual forces acting on an object.
Equilibrium
A state where the resultant force acting on an object is zero, meaning all forces are perfectly balanced.
Normal Contact Force
A force exerted by a solid surface on an object in contact with it, always acting perpendicular (at 90 degrees) to the surface.
Drag
A frictional force that opposes the motion of an object as it moves through a fluid, such as a liquid or gas.
Weight
The downward force acting on an object due to gravity.
Thrust
A forward driving force produced by an engine, rocket, or propeller.
Tension
A pulling force transmitted through a string, rope, or wire when it is pulled tight by forces from opposite ends.
Centre of mass
The single point through which the entire weight of an object is considered to act.
Lift
An upward force exerted on an object moving through a fluid (usually air) due to pressure differences.
Friction
A resistive force that opposes motion between two surfaces that are sliding, or trying to slide, across each other.
