5.2 Newton's First Law

Newton's_First_Law_0### and Inertia are foundational concepts in physics. They explain how objects behave when no forces act on them, introducing the idea of inertia as resistance to changes in motion.

These concepts set the stage for understanding motion and forces. They're crucial for grasping more complex ideas in mechanics, like equilibrium and acceleration, which build upon this fundamental principle.

Newton's First Law and Inertia

Newton's first law of motion

• States an object at rest stays at rest and an object in motion stays in motion with constant velocity unless acted upon by an unbalanced force
• Implies objects at rest remain at rest unless a net external force acts on them (book on a table)
• Implies objects in motion continue moving at a constant velocity unless a net external force acts on them (hockey puck sliding on ice)
• Describes the relationship between force and motion

Inertia and object mass

• Inertia resists any change in an object's motion, including changes to its speed and direction
• Greater mass leads to greater inertia
  • Higher mass objects require a greater force to change their state of motion (boulder vs. pebble)
  • Lower mass objects require a smaller force to change their state of motion (feather vs. brick)
• Inertia measures an object's tendency to resist changes in its motion
• Closely related to an object's momentum

Friction's effect on Newton's law

• Friction opposes the motion of an object and can cause it to slow down or stop
• With friction, an object in motion will not continue at a constant velocity indefinitely
  • Friction causes the object to slow down and eventually stop unless a force is continuously applied to counteract it (sliding a box across a rough floor)
• With friction, an object at rest will start moving when the applied force overcomes the frictional force (pushing a heavy crate)
• Frictional force magnitude depends on the surfaces in contact and the normal force between the objects

Inertial Reference Frames and Equilibrium

Inertial reference frames

• A frame of reference where Newton's first law holds true
  • In inertial frames, an object at rest stays at rest and an object in motion stays in motion with constant velocity unless acted upon by an external force (stationary observer watching a moving car)
• Non-inertial reference frames are accelerating frames
  • Rotating platforms or accelerating elevators
  • Fictitious forces (centrifugal force) appear to act on objects, causing their motion to deviate from Newton's first law (apparent force pushing you back in an accelerating car)
• Reference frame choice is crucial when observing and analyzing motion
  • Motion can appear different when observed from different reference frames (dropping a ball while on a moving train vs. stationary platform)
  • Identify whether a reference frame is inertial or non-inertial when applying Newton's laws

Equilibrium conditions in systems

• An object is in equilibrium when the net force acting on it is zero
  • For equilibrium, the vector sum of all forces acting on the object must be zero: $\sum \vec{F} = 0$
• Types of equilibrium:
  1. Static equilibrium: An object at rest remains at rest if the net force is zero (a book lying on a table)
  2. Dynamic equilibrium: An object in motion continues with constant velocity if the net force is zero (a car moving at a constant speed on a flat road)
• To determine equilibrium conditions:
  1. Identify all forces acting on the object
  2. Draw a free-body diagram representing the object and the forces
  3. Set the sum of the forces equal to zero and solve for the unknown quantities
• Objects in equilibrium can be balanced or under the action of opposing forces that cancel each other out (a tug-of-war with equal forces on both sides)

Newton's Laws of Motion and Acceleration

Newton's laws of motion

• First law: Describes the behavior of objects when no net force acts on them
• Second law: Relates force, mass, and acceleration (F = ma)
• Third law: States that for every action, there is an equal and opposite reaction
• These laws form the foundation for understanding the relationship between forces and motion

Acceleration and force

• Acceleration occurs when there is a net force acting on an object
• The magnitude of acceleration depends on the net force and the object's mass
• Direction of acceleration is the same as the direction of the net force