Motion, Heat and Waves
Forces & Newton's Laws
13 minStudent plan
Why things move, why they stop, and why a robot that accelerates hard needs a motor sized for far more than its steady speed.
After this lesson you can
- Apply F = ma to a resultant force, not just any force
- Draw the forces on a body and find the resultant
- Say which body each force in a third-law pair acts on
The three laws
The first law: a body continues at rest or at constant velocity unless a resultant force acts on it. Constant velocity needs no force at all, only a change of velocity does.
The second law: F = ma. A resultant force produces an acceleration proportional to it and inversely proportional to mass. Note "resultant", it is the vector sum of every force acting, which is why the vectors lesson comes first.
The third law: forces come in equal and opposite pairs acting on different bodies. A wheel pushes backwards on the ground and the ground pushes forwards on the wheel, and it is that second force that drives the robot. Confusing which body each force acts on is the most common source of error in mechanics.
In the full lesson
- The three laws
- Sizing for acceleration
- Friction and normal force
This lesson comes with the Student plan. The first module of Physics for Engineers is free.