Physics, visually · Motion & Forces · Concept 02
Newton's Three Laws
Why do you lurch forward when a bus brakes, and why does a heavy truck take longer to get going than a bike?
Three laws explain almost every push and pull you will ever see. Use the three tabs: one law each.
- Cart tab. Keep the force at 40 N and change the mass: 5 kg, 10 kg, 20 kg. Double the mass and the acceleration halves. Now double the force instead.
- Cart tab. Set friction to None and watch what happens after the push ends. The cart does not stop. Nothing is pushing it, so nothing changes its speed.
- Skaters tab. Make one skater light (30 kg) and the other heavy (90 kg), then Push. The forces are equal, so who flies away faster, and by how much?
Lazy, Loud, Linked.
Law 1 (Lazy): things keep doing what they are doing unless a force acts. Law 2 (Loud): a bigger push makes a bigger change, a heavier thing changes less: F = m a. Law 3 (Linked): forces come in pairs, equal and opposite, on different objects.
Sizing a motor for a trolley
To get a 500 kg trolley to 1 m/s² you need F = 500 N plus whatever friction takes. Every drive, cylinder and motor selection starts with F = m a.
Why conveyors have soft starts
Boxes on a belt stay put by inertia and friction. Start the belt too sharply and the boxes slide backwards; stop it too fast and they slide forward. The drive ramp limits acceleration.
Robot bases and recoil
A robot arm swinging a load pushes back on its base with an equal force (Law 3). That reaction is why bases are bolted to heavy pedestals.