Newton's Second Law: The Law That Connects Force and Motion
Imagine you're pushing a shopping cart. If you push gently, it moves slowly. Push harder, and it speeds up faster. Now imagine the cart is full of groceries — even with the same push, it accelerates much more slowly than an empty cart. This everyday experience is exactly what Newton's Second Law captures.
The Intuition First
Two things matter when you push something:
How hard you push — the force you apply.
How heavy the object is — its mass.
The harder you push, the more the object speeds up. The heavier the object, the less it speeds up for the same push. So acceleration depends on both force and mass — and in opposite ways.
Note
"Acceleration" here means any change in velocity — speeding up, slowing down, or changing direction. It's not just "going faster."
The Precise Statement
Newton's Second Law says:
The acceleration of an object is directly proportional to the net force acting on it, and inversely proportional to its mass. The acceleration is in the same direction as the net force.
In one equation:
a=mFnet
Or more commonly:
Fnet=ma
Where:
Fnet is the net force (the vector sum of all forces acting on the object) — measured in newtons (N)
m is the mass of the object — measured in kilograms (kg)
a is the acceleration — measured in metres per second squared (m/s2)
Fnet=ma
What This Really Means
Force causes acceleration, not velocity. A constant net force produces constant acceleration — meaning the velocity keeps changing at a steady rate. If you stop pushing, the net force becomes zero, and acceleration becomes zero (the object continues at constant velocity — that's Newton's First Law).
Mass is a measure of inertia. The more mass an object has, the harder it is to change its motion. A truck needs a much larger force than a bicycle to achieve the same acceleration.
Direction matters. Force and acceleration are vectors — they point the same way. If you push north, the acceleration is north. If multiple forces act, you must add them as vectors to find the net force.
A Simple Example
A 2 kg block is pushed with a net force of 10 N to the right.
a=mFnet=2kg10N=5m/s2
The block accelerates at 5m/s2 to the right. Every second, its velocity increases by 5 m/s in that direction. …
Air drag on the large parachute canopy grows with speed until it balances gravity, after which the descent continues at a constant (small) terminal velocity rather than accelerating. …
Step 1. As the parachutist falls, two forces act: the downward gravitational force (mg, constant) and the upward air-resistance (drag) force, which increases as speed increases and also increases greatly because of the parachute's large surface area.
Step 2. Initially (low speed, or before the canopy opens), drag is small compared to gravity, so there is a large net downward force and the person accelerates downward.
Step 3. As speed builds up, drag grows and eventually becomes large enough to equal the weight; at that point the net force is zero, and by Newton's second law the acceleration becomes zero too.
Step 4. From then on the parachutist descends at a CONSTANT (terminal) velocity — not zero velocity, just zero acceleration — exactly analogous to the raindrop example in this chapter (zero net force, but nonzero velocity). …