Forces and Motion: Combining Forces

Forces are vectors, meaning they have both magnitude (size) and direction. When combining forces, both aspects must be considered.
When two forces act along the same line (whether in the same or opposite directions), they can be easily combined by adding or subtracting the forces respectively.
The resultant force is the single force which would have the same effect as all the original forces acting together. Calculating the resultant force involves adding or subtracting forces properly taking into account their directions.
If forces are acting at an angle to each other, use parallelogram or triangle method to find the resultant force. Break down forces into components along axes (often x and y), add or subtract as required, and recombine using Pythagoras’ theorem.
The net force or balanced forces have a resultant force of zero, meaning there’s no acceleration in the object. It is either stationary or moving at a constant velocity.
Unbalanced forces (where the resultant force is not zero) will cause an object to accelerate in the direction of the resultant force.
For moving objects, friction is always acting in the opposite direction to motion. If an object is moving at a constant speed, the driving force must be balancing the frictional forces.
Air resistance, or drag, is a type of frictional force that acts in the opposite direction to an object moving through the air. It increases as the speed of the object increases.
The principle of moments states that for an object in equilibrium (not rotating), the sum of clockwise moments about any point equals the sum of anti-clockwise moments about that point.
Forces have units of Newtons (N) and are measured using a force-metre.
Applying a force over a distance is known as doing work, which is related to energy transfer. Work done = Force applied x Distance moved in the direction of the force. It is measured in Joules (J).
Newton’s Second Law of Motion states that Force = Mass x Acceleration (F=ma).

Remember to practise applying these points to different physical situations, as understanding the principles is key.