Newton's First Law of Motion

Natural Motion

•  Every object in the universe has a proper place determined bya combination of four elements:

  •  Any object not in its proper place will strive to get there.

•  Examples:

  • Stones fall.

  •  Puffs of smoke rise.

  •  Straight up or straight down for all things on Earth

  • Beyond Earth, motion is circular. Example: The Sun and Moon continually circle Earth

Violent Motion

• Produced by external pushes or pulls on objects

  •  Example: Wind imposes motion on ships.

• Applying forces to objects

Galileo’s concept of Inertia

• Galileo demolished Aristotle's assertions in the 15 00s. Galileo's discovery:

  • Objects of different weight fall to the ground at the same rate in the absence of air resistance.

• A moving object needs no force to keep it moving in the absence of friction.

• Just before Newton, Galileo died in 1642

  • Newton was born in the same year

  • Galileo is the father of scientific thinking

Force

is a push or a pull

Inertia

•  is a property of matter to resist changes in motion.

• depends on the amount of matter in an object (its mass)

• Balls rolling on downward-sloping planes pick up speed.

•  Balls rolling on upward-sloping planes lose speed.

• SO a ball on a horizontal plane maintains its speed indefinitely.

• If the ball comes to rest, it is not due to its nature, But due to friction

Newton’s first law of motion

• Every object continues in a state of rest or at uniform speed in a straight line unless acted on by a non zero net force

• The key word in this law is continues:

  • An object continues to do whatever it happens to be doing unless a force is exerted upon it.

  • If it is at rest, it continues in a state of rest.

  • If an object is moving, it continues to move without turning or changing its speed.

vector quantity

• a quantity whose description requires both Magnitude (how much) and direction(which way)

• can be represented by arrows drawn to scale, (this quantity is) called vectors

  • length of arrow represents magnitude and arrowhead shows direction

  • Examples: force, velocity, acceleration

Net force

• Net force is the combination of all forces that act on an object.

  • Example Two 5-N pulls in the same direction produce a 10-N pull (net force of 10 N). If the pair of 5-N pulls are in opposite directions, the net force is zero.

Applied force

• The first object is pulled to the right by two forces of 5 newtons each. The net force is 10 newtons to the right.

• The second object is pulled left with 5 newtons and pulled right with 5 newtons. The net force is 0 newtons.

vector quantity 2

• has magnitude and direction.

• is represented by an arrow.

• Example: velocity, force, acceleration

Scalar Quantity

• has magnitude but not direction.

• Example: mass, volume, speed

resultant force

• The sum of two or more vectors 

  • For vectors in the same direction, add arithmetically.

  • For vectors in opposite directions, subtract arithmetically.

  • Two vectors that don't act in the same or opposite direction: use parallelogram rule.

  • Two vectors at right angles to each other use pythagorean theorem

The equilibrium rule

• A string holding up a bag of flour

• Two forces act on the bag of flour:

  • Tension force in string acts upward.

  • Force due to gravity acts downward.

•  Both are equal in magnitude and opposite in direction.

•  When added, they cancel to zero.

• So, the bag of flour remains at rest.

• The equilibrium rule: the vector sum of forces acting on a nonaccelerating object equals zero

Support Force

• Support force (normal force) is an upward force on an object that is opposite to the force of gravity.

• Example: A book on a table compresses atoms in the table, and the compressed atoms produce the support force.

understanding support force

• When you push down on a spring, the spring pushes back up on you.

• Similarly, when a book pushes down on a table, the table pushes back up on the book.

equilibrium

a state of no change with no net force acting

static equilibrium

Example: hockey puck at rest on slippery ice

dynamic equilibrium

example: hocky puck sliding on slippery ice

equilibrium test

• whether something undergoes change in motion

  • Example: A crate at rest is in static equilibrium (no change in motion) 

  • Example: When pushed ,at a steady speed, it is in dynamic equilibrium (no change in motion)

The moving Earth

• Copernicus proposed that Earth was moving cirulating the sun

• this idea was refuted by people

  • Example: If Earth moved, how could a bird swoop (jump/dive) from a branch to catch a worm?

  • Solution: As it swoops, due to inertia, it continues to move sideways at the speed of Earth along with the tree, worm, etc.

conceptual example