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Balanced forces

Forces and motion · level 3, lesson 2 of 6

A question to start

Two teams pull on a rope in a tug of war, and the flag in the middle does not move. Is nobody pulling? Or is something else going on?

The idea

Forces on something are balanced when they cancel out: as much push one way as the other. In the tug of war, both teams pull hard, but equally hard in opposite directions, so the rope stays put.

Balanced forces have a surprising result. They do not change how something moves:

  • If it is still, it stays still. A book on a table.
  • If it is moving, it keeps moving at the same speed in the same direction. A car cruising on a flat road, with the engine's push exactly balancing friction and the air.
pull: 500 unitspull: 500 unitsoverall: zero

That second point surprises most people. We expect that something moving needs a force to keep it going. But a force is only needed to fight friction and air. In space, with nothing to fight, a spacecraft coasts for years with its engines off.

On Mars, a parachute coming down at a steady speed has balanced forces too: the air pushing up on it as hard as gravity pulls it down.

Worked examples

1. A hovering helicopter. The rotors push air down, the air pushes the helicopter up, as hard as its weight pulls down. Balanced: it stays in place. Ingenuity, the helicopter on Mars, hovered this way on its first flight.

2. A falling skydiver at a steady speed. Air resistance up equals weight down. Balanced: the speed stays the same.

Try it

  1. What does "balanced forces" mean?
  2. A ball is lying still on the ground. Are the forces on it balanced?
  3. A train moves at a steady speed along a straight track. Are the forces on it balanced?
  4. A car's engine push is bigger than friction and air together. Are the forces balanced?
Answers
  1. The forces cancel out: as much one way as the opposite way.
  2. Yes: its weight down and the ground's push up.
  3. Yes: steady speed in a straight line means balanced forces.
  4. No. The car will speed up.

Common mistake

"If something is moving, there must be an overall force pushing it forward." Something moving at a steady speed in a straight line has balanced forces. An overall force is needed only to change its speed or direction.

Where it leads