Force, Mass and Acceleration
- Physics
- Ages 14–18 · Grades 9–12
- Hands-on experiment
What you’ll find out
- Acceleration is proportional to the net force and inversely proportional to mass.
- A free-body diagram shows every force on one object, and their sum decides the motion.
- Constant velocity means zero net force, not zero force.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
The equation behind every motion
Push a trolley and it speeds up. Push twice as hard and it speeds up twice as quickly. Load the trolley so it has twice the mass, and the same push gives only half the acceleration. Newton summed this up in one equation: force equals mass times acceleration, or F = ma. One newton is the force that accelerates one kilogram at one metre per second squared. So a 1,000 kg car pushed by a net force of 4,000 N accelerates at 4 m/s². The force in the equation is the net force, which means every push and pull added together, with direction taken into account.
Drawing the forces, then adding them
Before using the equation, draw the forces. Draw the object as a simple box, then one arrow for every force acting on it. A car has an engine force pushing it forwards, drag and friction pushing it backwards, its weight pulling it down, and the road pushing it up. The up and down forces cancel, which is why the car neither sinks nor flies. What is left, the net force along the road, decides the acceleration. A falling skydiver shows the same thing. Drag grows as speed grows, and when drag equals weight the net force is zero. The skydiver keeps falling, but at a steady speed, called terminal velocity.
Curriculum links: NGSS HS-PS2-1