An object in motion will remain in motion unless acted on by an external force.
The acceleration of an object is directly proportional to the sum of the forces
acting in the object, and inversely proportional to the mass of the object.
For every force, there is an equal and opposite force.
Newton's first law states that unless there are forces present, an objects motion will
keep moving the way it is currently moving. It will not change direction, and it will not
Discussion question - Does this make sense? Assume for a second that Newton was right.
Gently toss a ball back and forth between yourself and another student, or between your two
hands. Does this agree with Newton's first law? Slide a book gently across a table. Does it
slow down? Does this agree with Newton's first law? If Newton was right, what can you
say about these two examples?
Newton's second law states that more massive objects take more of a push to get moving,
and that the harder you push, the more movement you will get. This can be written mathematically
a = F/m.
Sometimes this is rearranged and written as
F = ma.
(Note that F and a are both vectors. Not only is the acceleration
proportional to the net force, it is also in the same direction as the net force.)
Discussion question - The gravitational pull of the Earth is proportional
to the product of the mass of the Earth and the mass of the object that is being
pulled by the Earth. If the acceleration of an object pulled by that gravity
is inversely proportional to the object's mass, what does that say about the
acceleration due to gravity of any two objects of different mass near the Earth's surface?
Newton's third law states that when you push on something it pushes back.
Discussion question - Lean on a sturdy wall nearby. Note that when you
lean on the wall, you are pushing on the wall. What would happen if the wall did
not also push back on you?