4.3 Newton's second law of motion: concept of a system  (Page 7/15)

Newton’s second law of motion is more than a definition; it is a relationship among acceleration, force, and mass. It can help us make predictions. Each of those physical quantities can be defined independently, so the second law tells us something basic and universal about nature. The next section introduces the third and final law of motion.

Section summary

• Acceleration, $\mathbf{\text{a}}$ , is defined as a change in velocity, meaning a change in its magnitude or direction, or both.
• An external force is one acting on a system from outside the system, as opposed to internal forces, which act between components within the system.
• Newton’s second law of motion states that the acceleration of a system is directly proportional to and in the same direction as the net external force acting on the system, and inversely proportional to its mass.
• In equation form, Newton’s second law of motion is $\mathbf{\text{a}}=\frac{{\mathbf{\text{F}}}_{\text{net}}}{m}$ .
• This is often written in the more familiar form: ${\mathbf{\text{F}}}_{\text{net}}=m\mathbf{\text{a}}$ .
• The weight $\mathbf{\text{w}}$ of an object is defined as the force of gravity acting on an object of mass $m$ . The object experiences an acceleration due to gravity $\mathbf{\text{g}}$ :
  $\mathbf{\text{w}}=m\mathbf{\text{g}}.$

• If the only force acting on an object is due to gravity, the object is in free fall.
• Friction is a force that opposes the motion past each other of objects that are touching.

Conceptual questions