Grade 11

Newton's Laws

Six lessons of mechanics in strict order: forces as vectors first, then each of the three laws in turn, and finally the friction and tension problems that need all three of them at once.

6 lessons

  1. 1Why does every mechanics problem start with a free-body diagram?Because Newton's laws are about the net force, and you cannot work that out before you know which forces are acting. The diagram forces you to answer the most important question in any problem: which bodies are touching or pulling this one, and what exactly is each of them doing to it.35m
  2. 2If there is no net force, why does a body keep moving?Because motion needs no cause. Newton's first law says that a body with zero net force stays at rest if it was at rest, and carries on in a straight line at constant speed if it was already moving. What requires a force is a change in motion — slowing, speeding up or turning — not the motion itself.35m
  3. 3Does the net force set a body's speed, or its acceleration?Its acceleration. The net force equals mass times acceleration, and the acceleration always points along the net force rather than along the velocity. So a constant force does not maintain a constant speed — it changes the speed at a constant rate — and a force perpendicular to the motion changes direction without changing speed.45m
  4. 4Why do a heavy body and a light one fall at the same rate?Because the same mass appears twice and cancels. The gravitational force on a body is its mass times the gravitational field strength, and the acceleration that force produces is the force divided by that same mass. The mass cancels, and what is left is the field strength alone — the same number for every body, whatever its weight.40m
  5. 5If every force has an equal and opposite partner, how does anything move?Because the two forces act on different bodies. The force I exert on the wall acts on the wall, and the force the wall exerts on me acts on me. Forces only cancel when they act on the same body, which is why an action-reaction pair never cancels out.40m
  6. 6Why does it take a bigger push to start a crate moving than to keep it moving?Because there are two kinds of friction. Static friction acts while the body is not moving and grows to match the push, up to a maximum. The instant the body starts to move, kinetic friction takes over, and its value is constant and lower. So the first shove is the hardest one.45m