Lesson 16: Newton's 2nd Law (Motion)

To really appreciate Newton’s Laws, it sometimes helps to see how they build on each other.

The 2nd Law (The Law of Motion)

When an external, unbalanced force acts on an object,
the object will accelerate in the same direction as the force. The acceleration varies directly as the force, and inversely as the mass.

When an external, unbalanced force…
We are still talking about these external forces, but now we’ve added in the idea of “unbalanced”.

“…accelerates in the same direction as the force…”
Some people think this means that the object will move in the same direction as the force… not necessarily!

The acceleration varies directly as the force…”
This just means that if the force increases, the acceleration will increase. If the force decreases, the acceleration decreases.

“…and inversely as the mass.”
This means that if the mass is bigger, the acceleration is less. If the mass is less, the acceleration is more.

Mathematically this would be written as…

When you combine these two relations, you get one of the most basic and important formulas ever discovered in physics.

F = ma

F = force (Newtons)

m = mass (kg)

a = acceleration (m/s2)

The unit of force is called the Newton (equal to kg m/s2).

Example 1: Determine the force acting on a 5.46kg object if it is accelerated at 17m/s2.

F = ma
= (5.46 kg)(17m/s2)
F = 93N

Example 2: Determine the acceleration of a 1000 kg car if a 2.5e3 N force acts on it?