IGCSE Physics / Forces and movement
Force, mass and acceleration
Edexcel International GCSE Physics (4PH1), chapter 3: Forces and movement, specification point 1.17. Free lesson.
Read the revision notes · IGCSE Physics past papers
A resultant force changes an object's velocity, and the relationship between force, mass and acceleration is F = m × a. This equation lets you calculate any one of the three quantities, as long as you use the resultant force and mass in kilograms.
What happens when there is no resultant force
A resultant force is the overall force left after forces in opposite directions are added and subtracted. With no resultant force, an object's velocity does not change. It stays at rest or continues moving at a constant velocity. This is Newton's first law. A moving object does not need a resultant force to keep moving. A resultant force is needed to change an object's velocity, so the object accelerates.
What F = m × a means
When a resultant force acts, it causes acceleration. Use the equation F = m × a to link the three quantities. The acceleration is in the same direction as the resultant force. F is the resultant force, measured in newtons (N). m is the mass, measured in kilograms (kg). Convert a mass in grams to kilograms first. a is the acceleration, measured in metres per second squared (m/s²). The equation is on the equation list, but you need to know what each symbol and unit means.
How to calculate force, mass or acceleration
Choose the version of the equation that gives the quantity you need. Substitute the values, then rearrange if necessary. For force, the mass and acceleration must be in the stated units.
How to find the resultant force first
If several forces act, do not use just the largest force. Work out the resultant force by subtracting forces in opposite directions, then use that result in F = m × a. For a 1000 kg car, a 3000 N driving force forwards and 1000 N friction backwards give a resultant of 2000 N forwards. This gives a steady acceleration of 2 m/s². Forces in the same direction add together. Forces in opposite directions subtract from each other. Only the resultant force causes acceleration. Friction and air resistance can act backwards throughout the motion.