Newton's First Law
Newton’s first law states that an object remains at rest or
in motion unless acted on by a force. Thus, a swimmer
remains at rest or moves in constant velocity unless acted
on by a force. Water has resistive and propulsive forces,
and in order for a swimmer to be in motion the propulsive
forces must be greater than the resistive forces.
Newton's Second Law
Newton’s second law states that the sum
of the forces of an object must be equal to the object’s
mass times its acceleration. There are many different
forces acting upon every part of a swimmer’s body, the
arms, legs, and head. To simplify it, we will look at the
swimmer as a whole by focusing on the center of mass of
the swimmer. By doing this, we can look at the following
free body diagram which gives all the forces that act on a
swimmer. The propulsive force acts in the direction of
motion propelling the swimmer forward, while the resistive
force opposes the direction of motion having the same
effect as a friction force by slowing down the swimmer. B
stands for the buoyancy force, which acts upward from the
water. The buoyancy force is equal to W, which is the
force of gravity acting downwards. The force of gravity is
equal to the mass of the swimmer times the gravity
constant which is approximately 9.81 m/s^2. Since the
swimmer is only moving in the horizontal direction, F(net)
= propulsive force – resistive force.
Photo from indiana.edu
Newton's Third Law
Newton’s third law states that every action must have an equal and opposite reaction. In swimming, when the hands and feet push against the water, the water pushes back on the swimmer and propels the swimmer forwards.