Transcript 1 st Law
Chapter 2 Newton’s First Law of Motion Aristotle on Motion (350 BC) Aristotle attempted to understand motion by classifying motion as either • (a) natural motion • forces acting at a distance • (b) or violent motion • contact forces Large object tend to 'strive harder'. “The Earth remains at rest.” Geocentric Model - Earth Centered Universe Copernicus (1500's) "The Earth and planets orbit the Sun.” He reasoned this from his astronomical observations. Galileo (1600's) Scientist who supported Copernicus Dropped objects with different weights from the Leaning Tower of Pisa Found that all objects fall at the same rate if you can account for air resistance Friction - a force that resists motion • e.g. air resistance and sliding on rough surfaces Inertia - the resistance of an object to change in its state of motion Demo: Ball and incline plane Galileo’s Incline Planes Isaac Newton (1642-1727) His three laws of motion first appeared in his book called Principia. Newton’s First Law a.k.a “Law of Inertia” A body remains at rest or moves in a straight line at a constant speed unless acted upon by a force. Newton’s First Law Examples Weight and string Card, cup, and coin Figure 2.4 Fixing a Hammer Demo - Coins on elbow Demo - Lead Brick and Hammer Demo - Table setting Mass the quantity of matter in an object the measurement of the inertia measured in kilograms (kg) Weight the force upon an object due to gravity Weight = Mass Acceleration of gravity W = mg measured in Newtons (N) in the metric system or pounds (lb) in the British system The weight of a 10 kg brick is... • • • • • A) B) C) D) E) 98 N 10 kg 9.8 kg 10 N 98 kg Mass and Weight should not be confused with... Volume • the quantity of space an object occupies Density • the quantity mass per unit volume Mass and Weight On the Moon the gravitational force is only 1/6 as strong as on the Earth. In space you are “weightless” but not “massless”. Your mass does not depend on where your are. • (e.g. Earth, Moon, or space). Location Mass Weight Earth 18.4 kg 180 N Moon 18.4 kg 30 N Space 18.4 kg 0N Chapter 3 Linear Motion See Homework 5 Chapter 4 Newton’s Second Law of Motion NEWTON'S 2nd LAW OF MOTION F a m F M a m F F a m M a m m F m 1 a m F a aF or M a F ma Newtons’ Second Law F=ma The acceleration of an object is directly proportional to the net force acting on the object… …and inversely proportional to the mass of the object. Example Questions How much acceleration does a 747 jumbo jet of mass 30,000kg experience in takeoff when the thrust of all of the engines is 120,000N? Example Questions F F The same net force is applied to two blocks. If the blue one has a smaller mass than the yellow one, which one will have the larger acceleration? If the net force is parallel to the velocity, then the speed of the object increases. If the net force is anti-parallel to the velocity, then the speed of the object decreases. If the net force is perpendicular to the velocity, the direction of the velocity changes. Force and acceleration are vector quantities. If v is parallel to F, speed increases. If v is antiparallel to F, speed decreases. If v perpendicular to F, direction of v changes. When Acceleration Is Zero - Equilibrium Scales pushing up Static Equilibrium Velocity is zero Examples: Computer setting on a table Normal up Weight down Weighing yourself on a set of scales Hanging from a tree Car parked on an incline Friction Tree pulling up Weight down Normal Weight down Weight down When Acceleration Is Zero... …we say the object is in Mechanical Equilibrium. …the net force is zero. For Static Equilibrium the velocity is zero. For Dynamic Equilibrium the velocity is constant. Dynamic Equilibrium Velocity is nonzero and constant Examples: Driving at constant velocity Normal up Air resistance Force from road Friction Weight down Terminal velocity in parachuting Weight down When the Acceleration is g... …the object is in Free Fall. Consider a 1kg rock and a 1gram feather. – Which object weighs more? • Answer: The rock – On which is the gravitation force stronger? • Answer: The rock – Which has a greater acceleration when dropped from rest? • Answer: Both have the same acceleration, g. When the Acceleration Is Less Than g... …the object is not in Free Fall. In this case there is a force other than gravity. That force is air resistance. Air resistance depends on size and speed. Example: A heavy parachutists will fall faster than a light one. When the force of air resistance is equal to weight of the falling object, the object will reach a Terminal Velocity. See Questions on page 66,67 and 69. After jumping from an airplane a skydiver will fall until the air resistance equals her weight. At that point... * – A) she will fall with constant speed – B) she will fall no farther – C) she will fall faster – D) she opens her parachute – E) she will hit the ground