Transcript NewtonsFirstLawx

OBJECTIVE: LEARN ABOUT NEWTON’S FIRST
LAW AND UNDERSTAND THE VOCABULARY
AGENDA:
START POWERPOINT NOTES
CHAPTER 2: NEWTON’S FIRST
LAW OF MOTION-THE LAW OF
INERTIA
ARISTOTLE’S CLASSIFICATION OF MOTION
• Motion requires a force
• He classified motion in natural motion and unnatural
motion
• Natural: (he believed) occurred without force
• EX: Motions of Sun, Moon, and other objects in the
sky
• Did them on their own
• Unnatural: requires force such as push or pull by
animals or people
GALILEO’S CONCEPT OF INERTIA
• Aristotle's idea was taken for 2000 years, but Galileo
proved this false
• He found: Force is required to start an object moving, but
one moving, no force is required to keep It moving
• Except for force needed to overcome friction (Chp. 3)
• Inertia: If moving, tend to remain moving. If at rest, tend
to stay at rest.
• Property of matter to resist changes in motion
GALILEO’S CONCEPTS OF SPEED AND VELOCITY
• Speed: Distance covered per unit time
𝑆𝑝𝑒𝑒𝑑 = 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 ÷ 𝑡𝑖𝑚𝑒
• Average speed: speed at any instant
• Instantaneous speed: may be different
• Speed is measures m/s
VELOCITY
• Velocity: know speed and direction of an object
• Speed is a description of how fast, velocity is how fast and in
what direction
• Vector Quantity: specifies direction and magnitude
• Constant Speed: steady speed
• Constant Velocity: constant speed and constant direction
• Constant Direction: Object moves in a straight line and
doesn’t change direction
CONCEPT CHECK
• What is the average speed of a cheetah that sprints 100m in 4
seconds? What about if it sprints 50m in 2s?
• The speedometer on a bicycle moving east reads 50 km/h. It
passes another bike moving west at 50 km/h. Do both bikes have
the same speed? Do they have the same velocity?
• “The bird flies at constant speed in a constant direction.” Say the
same sentence in fewer words.
MOTION IS RELATIVE
• Always moving!
• Relative motion: motion relative to something else
• EX: car is moving 25 mph relative to the road
• Galileo was concerned with how things move rather than
why they move
NEWTON'S FIRST LAW OF MOTION
• Isaac Newton, at age 24, extended Galileo’s concept of inertia
• Newton’s laws were influenced by Galileo’s findings
• Newton’s First Law (law of inertia): “Every object continues in its
state of rest or uniform speed in a straight line unless a net force
acts on it”
• Rest stays at rest!
• Motion stays in motion!
CONCEPT CHECK
• When the space shuttle moves in a nearly circular
orbit around Earth, is a force needed to maintain its
high speed? If suddenly the force of gravity were
cut off, what type of path would the shuttle follow?
NET FORCE: THE COMBINATION OF ALL FORCES THAT
ACT ON AN OBJECT
• Often more than one force acts on an object
• EX: Throwing a basketball, the force of gravity, air friction, and the pushing
force you apply with your muscles all act on the ball
• Net Force: ball is the combination of all these factors
• Net force changes an object’s state of motion
• EX: pull a 5 lb. weight, if a friend also pulls with 5 lbs., than the total net
force would be 10 lbs.
• Force is in the unit of N (newton)
• You show the amount of force by the length of arrows
EQUILIBRIUM FOR OBJECTS AT REST
• Newton is a unit of weight
• Hang a sugar bag upside down:
• 2 forces acting on it:
1. Tension force: acting upward on the bag
2. Weight: acting downward
• Equal but opposite forces = 0
• Mechanical Equilibrium: state where no physical changes occur; it
is a state of steadiness
• Net force = 0
• Equilibrium Rule: When an object is said to be in mechanical
equilibrium
EQUILIBRIUM RULE
Forces
The vector sum of
𝐹=0
CONCEPT CHECK
A gymnast is hanging from the rings:
1. If she hangs with her weight evenly divided between the two
rings, how would scale readings in both supporting ropes compare
with her weight?
2. Suppose she hangs with slightly more of her weight supported by
left ring. How would a scale on the right read?
SUPPORT FORCE: WHY WE DON’T FALL THROUGH
THE FLOOR
• Support force: the upward force that balances the weight of an object on a
surface
• An object at rest on a horizontal surface, the support force must equal the
object’s weight
• Ex: Spring on hand
• Ex: Measuring your weight on a scale
CONCEPT CHECK
1.What is the net force on a bathroom scale when a 110
pound person stands on it?
2.Suppose you stand on two bathroom scales with your
weight evenly distributed between the two scales. What
will each scale read? How about if you learn to put more
of your weight on one scale than the other?
EQUILIBRIUM FOR MOVING OBJECTS
• An object moving at constant speed in a straight-line path is also in
equilibrium
• Sum still equals 0 (Newton’s First Law)
• Must not be changed by any force or else the object is not in
equilibrium
• Objects at rest are said to be in static equilibrium; objects moving
at constant velocity are said to be in dynamic equilibrium.
• Types of mechanical equilibrium
CONCEPT CHECK
• An airplane flies at constant velocity. In other
words, it is in dynamic equilibrium. Two horizontal
forces act on the plane. One is the thrust of the
propeller that pushes it forward. The other is in the
force of air resistance (drag) that acts in the
opposite direction. Which force is greater?
EARTH MOVES AROUND THE SUN
• The law of inertia states that objects in motion
remain in motion whenever no net force acts.
• EX: Stand up