6.5. Laws of Motion#
Newton’s 1st Law#
If no net force acts on a body, the body’s velocity cannot change; that is, the body cannot accelerate. If the body is at rest, it stays at rest. If it is moving, it continues to move with constant velocity.
Newton’s first law states:
“A body at rest will remain at rest unless acted on by an unbalanced force. A body in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force.”
It is important to note that the net force is what will cause a change in velocity. The net force is the sum of all forces acting on the body. For example, we can imagine gently pushing on the rock in the figure above and observing that the rock does not move. This is because we will have a friction force equal in magnitude and opposite in direction opposing our gentle pushing force. The sum of these two forces will be equal to zero, therefore the net force is zero and the change in velocity is zero.
Newton’s 2nd Law#
Newton’s second law states:
“When a net force acts on any body with mass, it produces an acceleration of that body. The net force will be equal to the mass of the body times the acceleration of the body”
The net force on a body is equal to the product of the body’s mass and its acceleration.
Notice that the force (\(\vec{F}\)) and the acceleration (\(\vec{a}\)) are vector quantities, having both a magnitude and a direction. Mass (\(m\)) on the other hand is a scalar quantity having only a magnitude. Based on the above equation, you can infer that the magnitude of the net force acting on the body will be equal to the mass of the body times the magnitude of the acceleration, and that the direction of the net force on the body will be equal to the direction of the acceleration of the body.
Newton’s 3rd Law#
Newton’s third law states:
“For any action, there is an equal and opposite reaction.”
When two bodies interact, the forces on the bodies from each other are always equal in magnitude and opposite in direction.