Newton's Second Law (Oxford AQA IGCSE Physics)
Revision Note
Written by: Leander Oates
Reviewed by: Caroline Carroll
Newton's Second Law
What is Newton's second law of motion?
Newton's second law of motion explains what happens when a non-zero resultant force acts on an object
A resultant force occurs when the forces acting on an object are not balanced
A resultant force acting on an object will cause a change in the object's motion
This change in motion is an acceleration:
Speeding up
Slowing down
Changing direction
If the resultant force on an object is not zero, the object will accelerate in the direction of the resultant force
Examples of Newton's second law
Newton's second law of motion states:
The acceleration of an object is proportional to the resultant force acting upon it and inversely proportional to the object's mass
Where:
F = resultant force measured in newtons (N)
m = mass of object measured in kilograms (kg)
a = acceleration measured in metres per second squared ( m/s2)
The bigger this resultant force, the larger the acceleration
For a given force, the greater the object's mass, the smaller the acceleration experienced
Worked Example
A car salesperson says that their best car has a mass of 900 kg and can accelerate from 0 to 27 m/s in 3 seconds.
Calculate:
(a) The acceleration of the car in the first 3 seconds.
(b) The force required to produce this acceleration.
Answer:
Part (a)
Step 1: List the known quantities
Initial velocity = 0 m/s
Final velocity = 27 m/s
Time, t = 3 s
Step 2: Calculate the change in velocity
Step 3: State the equation for acceleration
Step 4: Calculate the acceleration
Part (b)
Step 1: List the known quantities
Mass of the car, m = 900 kg
Acceleration, a = 9 m/s2
Step 2: State the equation for Newton's second law
Step 3: Calculate the force required to accelerate the car
Worked Example
Three shopping trolleys, A, B and C, are being pushed using the same force. This force causes each trolley to accelerate.
State which trolley would have the smallest acceleration. Explain your answer.
Answer:
Step 1: Identify which law of motion to apply
The question involves quantities of force and acceleration, and the image shows trolleys of different masses, so Newton's second law is required:
Step 2: Re-arrange the equation to make acceleration the subject
Step 3: Explain the inverse proportionality between acceleration and mass
Acceleration is inversely proportional to mass
This means for the same amount of force, a large mass will experience a small acceleration
Therefore, trolley C will have the smallest acceleration because it has the largest mass
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