Newton’s First Law

  • An object at rest, or in constant motion, will remain in rest or in constant motion unless an unbalanced force acts on it.

Newton’s first law comes from the work of Galileo, who was the first to understand that friction is a force. If you could remove friction, an object would continue sliding forever. Thus, a force is needed to speed it up, slow it down, or turn it. In other words, a force is needed to cause an acceleration.

Newton’s Second Law

  • The acceleration of a mass is directly proportional to the net force acting on it, and inversely proportional to the mass.

This is much easier to state as an equation:

but it is usually given in the form

Fnet = ma

This formula also tells us the units for force. Since it is mass times acceleration, its units will be the units of mass, kg, times the units for acceleration, m/s2. The unit for force is appropriately called a newton (N).

1 N = 1 kg m/s2

One newton is the force required to accelerate a 1-kg mass at a rate of 1 m/s2.

Newton’s Third Law

  • If object A exerts a force on object B, then object B exerts an equal force on A, acting in the opposite direction.

What this means is that if you are holding a weight in your hand, then the weight is pushing down on your hand but at the same time your hand is pushing up on the weight. Note that these two forces are acting on different objects: the weight pushes on your hand, and your hand pushes on the weight.

Mass vs. Weight

Although it may seem that the word mass and weight mean the same thing, there is actually a technical difference. Mass is an intrinsic property of an object, but weight is the force of gravity acting on the object. On the surface of the Earth, the acceleration due to gravity is g, about 9.8 m/s2, and so the weight in newtons of a mass m (in kilograms) would be given by

Weight = mg

Weight and mass will always be proportional as long as the strength of gravity is the same. If you went to another planet, gravity would be different and so would your weight. Your mass, however, would still be the same. In short,

  • Mass measures the quantity of matter
  • Weight measures the force of gravity on a mass

If you’re curious, you can see Newton’s original statement of the 3 laws.