Adding Up the Forces
- Physics
- Ages 11–14 · Grades 6–8
- Hands-on experiment
What you’ll find out
- An object’s motion changes only when the forces on it are unbalanced; the total is called the net force.
- Balanced forces mean the motion does not change — not that the object must be still.
- The same net force gives a larger mass a smaller acceleration (F = ma).
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 total is what counts
Almost nothing has only one force on it. A book on a table is pulled down by gravity and pushed up by the table. What decides how an object’s motion changes is all the forces added together, with their directions. That total is the net force. In a tug of war, if one team pulls with 300 newtons and the other with 250, the forces subtract, leaving 50 newtons towards the stronger team, and everything starts moving that way. If both pull with 300 newtons, the net force is zero and nothing changes. These forces are balanced. Balanced forces do not mean an object must be still; they mean its motion does not change. A car at a steady 50 kilometres an hour has balanced forces: the engine’s push matches the drag and friction. Press the accelerator and the car speeds up until the forces balance again.
Same push, more mass
An unbalanced force changes an object’s motion, but how much depends on the object. Push an empty trolley and it shoots forward; give a loaded one the same push and it barely picks up speed. How quickly velocity changes is called acceleration. Double the net force on the same object and its acceleration doubles. Keep the force the same but double the mass and the acceleration halves. Isaac Newton wrote this as F = ma: force equals mass times acceleration. A net force of 10 newtons on a 2 kilogram cart gives an acceleration of 5 metres per second every second; on a 4 kilogram cart, 2.5. Friction belongs in the sum too. Push with 10 newtons against 4 newtons of friction and the net force is 6. That is why racing cars are light and why a lorry needs a far bigger engine than a scooter.
Curriculum links: NGSS MS-PS2-2