Newton's Laws Chapter 2 Joseph F. Alward, PhD Department of Physics University of the Pacific

  Acceleration is Independent of Mass

Until Galileo Galilei, "common sense" told people that heavier objects fall faster. Galileo showed the power of experiment over logic.

   Inertia Trick

Inertia is the tendency of an object to remain at rest if it's already at rest, or to keep moving if it's already moving. ---------------------------------------------- Tablecloth trick: Too little force, too little time to overcome "inertia" of tableware.

  Inertia is Mass

Inertia is also called mass. Mass is measured in kilograms. One kilogram is the amount of mass in a 2.2 pound weight.

  Three Examples of Inertia

  Another Inertia Example

Anvil is weightless. Can it easily be pushed away?

  Newton's Laws of Motion

Isaac Newton (1642-1727)

One newton is the approximate weight of a cube of butter.

  Newton's First Law

Motion tends to continue unchanged. The elephant at rest tends to remain at rest.

Forces are pushes or pulls Force is measured in newtons. A newton is the weight of a cube of butter.

  Newton's Second Law:  Acceleration = Force / mass

a = F / m

2a = 2 F / m

  Acceleration is Inversely Proportional to Mass  

a = F / m

a  / 2= F / 2m

a / 3 = F / 3m

  Adding and Subtracting Forces

  Equilibrium Means "Zero Acceleration"

Forces in balance:  Equilibrium

  Equilibrium

Forces are in balance, so block is in equilibrium, moving at constant speed.

 Acceleration Due to Gravity

All falling objects near the earth accelerate at the rate of about 10 m/s per second: g = 10 m/s2

   Acceleration Due to Gravity

Gravitational force (weight) is proportional to mass. Double the mass and the gravitational force will be doubled also. Ratio of weight to mass is always the same:  g This is explained further, below.

 

Gravitational force is proportional to mass. Ratio F / m is always the same:  g ----------------------------- Circumference is proportional to the diameter.   Ratio C/D is always the same:  p

 Weight

The weight of an object is the force of the earth's pull and is given by the equation Weight = m g

Example: A cube of butter has a mass of about 0.1 kg. weight = mg            = 0.1 x 10            = 1 N

  Terminal Speed

Air resistance increases as the speed increases. Eventually, the force R of air resistance becomes equal to the force exerted by the earth, and the object reaches equilibrium.

 

Why does the heavier person fall faster?

   Newton's Third Law of Motion:  Action-Reaction 

Whenever one object exerts a force on another object, the other object exerts an equal but opposite force o Note:  each of the two forces in the pair acts on a different object. Hammer pushes on stake. Stake pushes on hammer. The hammer acts, the stake re-acts.

  Action-Reaction Pair Example

Yellow ball moves because the blue ball exerts a force on it. Blue ball stops because the yellow ball exerts a force on it. ------------------------ Would the blue ball have stopped if it had more mass than the yellow ball?

  More Action-Reaction Pair Examples

  Action-Reaction

If action-reaction forces are equal but opposite, why don't they cancel?

   Recoiling Gun

Expanding gas pushes bullet out of gun barrel.  Why does the gun recoil?

  More Recoil Examples

Gas pushes against the projectile; projectile pushes on the gas.

Wings push down on air, air pushes up on wings. Air pushed down swirls back up, providing lift to the bird behind.