Newton’s second law connects net force, mass and acceleration: F= ma. Use force in newtons, mass in kilograms and acceleration in metres per second squared. One newton is the force needed to accelerate one kilogram at one metre per second squared. Rearranging gives a=mF and m=aF. Include all relevant forces before substituting; an applied push is not necessarily the net force.
For a fixed mass, doubling net force doubles acceleration. For fixed net force, doubling mass halves acceleration. Acceleration also equals change in velocity divided by elapsed time. On a velocity–time graph, the gradient gives acceleration; a horizontal line shows zero acceleration, even if velocity is non-zero. State the direction when using signed values or describing the motion.
Example 1
A 5kg trolley has 18N forward and 3N backward forces. Find acceleration.
Net force =18−3=15N forward. Acceleration =515=3m/s2 forward.
Example 2
Velocity increases from 2m/s to 14m/s in 4s. Find acceleration.
Acceleration =4(14−2)=3m/s2. This is the gradient over the interval.
Watch out: F is the net force, the signed sum of forces acting on the object.