AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Newton's Second Law: resultant force = mass × acceleration
Newton's second law links the resultant force on an object to its mass and acceleration.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 2 of 11
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Newton's second law links the resultant force on an object to its mass and acceleration. The equation is resultant force = mass × acceleration, often written F = m × a. Force is measured in newtons (N), mass in kilograms (kg) and acceleration in metres per second squared (m/s²). A larger resultant force produces a larger acceleration for the same mass, and a larger mass needs a larger force for the same acceleration. For example, a 1200 kg car accelerating at 2 m/s² needs a resultant force of 1200 × 2 = 2400 N. If the forces on an object are balanced, the resultant force is zero and acceleration is zero. Rearranged, acceleration = force ÷ mass and mass = force ÷ acceleration.
What earns the marks
- State the equation resultant force = mass × acceleration and recall the correct units: newtons, kilograms and metres per second squared.
- Substitute known values of mass and acceleration into the equation correctly.
- Calculate the resultant force, or rearrange the equation to find mass or acceleration when required.
- Interpret a zero resultant force as producing zero acceleration, and relate larger forces or masses to changes in acceleration.
- Give the final answer with the correct unit and a sensible size for the situation described.
Common mistakes and how to put them right
Using grams instead of kilograms for mass.
Put it right by converting grams to kilograms by dividing by 1000 before substituting.
Confusing resultant force with a single applied force when several forces act.
Put it right by finding the vector sum of all forces first.
Rearranging the equation incorrectly, for example dividing force by acceleration to find force.
Put it right by using acceleration = force ÷ mass or mass = force ÷ acceleration as appropriate.
Exam tips
- Write the equation, then substitute values with units before calculating to reduce errors.
- Check that the unit of the answer matches the quantity asked for, such as N for force or m/s² for acceleration.
- If forces are given in different directions, resolve them to find the resultant force before using the equation.
Try the lesson
The start of the Newton's Second Law: resultant force = mass × acceleration lesson, made from checked teaching notes for this specification point.
Have a go first
A box of mass 5 kg accelerates at 2 m/s². Using the equation resultant force = mass × acceleration, what is the resultant force on the box?
Listen
Newton's second law links the resultant force on an object to its mass and acceleration. The equation is resultant force = mass × acceleration, often written F = m × a. Force is measured in newtons (N), mass in kilograms (kg) and acceleration in metres per second squared (m/s²). A larger resultant force produces a larger acceleration for the same mass, and a larger mass needs a larger force for the same acceleration. For example, a 1200 kg car accelerating at 2 m/s² needs a resultant force of 1200 × 2 = 2400 N. If the forces on an object are balanced, the resultant force is zero and acceleration is zero. Rearranged, acceleration = force ÷ mass and mass = force ÷ acceleration. A strong answer earns marks for these points. State the equation resultant force = mass × acceleration and recall the correct units: newtons, kilograms and metres per second squared. Substitute known values of mass and acceleration into the equation correctly. Calculate the resultant force, or rearrange the equation to find mass or acceleration when required. Interpret a zero resultant force as producing zero acceleration, and relate larger forces or masses to changes in acceleration. A common slip: Using grams instead of kilograms for mass. Instead, converting grams to kilograms by dividing by 1000 before substituting. Exam tip: Write the equation, then substitute values with units before calculating to reduce errors.
Key words
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resultant force
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newton
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acceleration
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balanced forces
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See every definition
- resultant force
- The vector sum of all forces acting on an object, which equals mass × acceleration.
- newton
- The unit in which force is measured, written as N.
- acceleration
- Measured in metres per second squared (m/s²), it equals force ÷ mass when the equation is rearranged.
- balanced forces
- A situation where the forces on an object give a resultant force of zero, so acceleration is zero.
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