B. Berenger 12/99

2 Summary of Discussion: Newton’s First Law –inertia Mass vs. Weight Newton’s Second Law –F=ma –a=f/a Newton’s Third Law

B. Berenger 12/99 3 INERTIA The tendency of objects to remain in motion or stay at rest If it were not for friction, an object set in motion would continue to move forever Inertia

B. Berenger 12/99 4 FIRST LAW OF MOTION An object at rest remains at rest, or if in a state of motion, continues at constant velocity (same direction & speed) unless acted upon by an outside force Newton’s First Law

B. Berenger 12/99 5 Riding In A Car... When you are riding in a car and it stops suddenly, you keep moving forward if you did not have a safety belt to stop you, your inertia could send you through the windshield Inertia

B. Berenger 12/99 6 Standing On A Bus... When the bus starts to move forward you are thrown off balance and fall backward Your body has inertia –it is at rest and tends to stay at rest, even though the bus is moving Inertia

B. Berenger 12/ On The Same Bus When the moving bus stops, you fall forward Even though the bus stops, you do not –You are an object in motion Inertia

B. Berenger 12/99 8 MASS VS. WEIGHT Weight –a measure of the force of gravity on an object –weight = mass x acceleration due to gravity w = m x g –measured in Newtons Mass –a constant value; does not change –amount of matter –measured in Kilograms Mass vs. Weight

B. Berenger 12/99 9 CALCULATING WEIGHT measured in Newtons weight = mass x acceleration due to gravity w = m x g –acceleration due to gravity on Earth = 9.8 N/kg Example: m = 100 kg g = 9.8 N/kg Weight =100 kg x 9.8 N/kg = 980 N Weight

B. Berenger 12/99 10 WEIGHT Varies according to the force of gravity pulling on you a smaller mass means a smaller gravitational pull Weight EarthMoon Where would you weigh more? Why?

B. Berenger 12/99 11 MASS Remains constant You have the same amount of mass regardless of your location Mass EarthMoon Would your mass change? Why or why not?

B. Berenger 12/99 12 NEWTON’S SECOND LAW A force acting on an object will produce an acceleration of the object proportional to the force and in the direction of the applied force –if you double the force that you throw a ball, you will double its acceleration Newton’s Second Law

B. Berenger 12/99 13 Mathematically Speaking... Force = mass x acceleration F = ma Newton’s Second Law

B. Berenger 12/99 14 F = ma If the same force is applied to a bowling ball and a tennis ball, which ball will have the greater acceleration?If the same force is applied to a bowling ball and a tennis ball, which ball will have the greater acceleration? Newton’s Second Law WHY?

B. Berenger 12/99 15 Force Measured in Newtons 1 N = 1 kg x 1 m/sec/sec = kg m s 2 One newton equals the force required to accelerate one kilogram of mass at one meter/second/second Newton’s Second Law Remember: F = ma

B. Berenger 12/99 16 Better Gas Mileage? Small Car mass = 750 kg acceleration = 2 m/sec/sec Force required to accelerate car = 750 kg x 2 m/sec/sec = 1500 N Large Car mass = 1000kg acceleration = 2 m/sec/sec Force required to accelerate car = 1000 kg x 2 m/sec/sec = 2000 N Remember: F = ma Newton’s Second Law

B. Berenger 12/99 17 Not Just: F = ma Formula can also be written as: a = F/m Acceleration is directly proportional to the force Acceleration is indirectly proportional to the mass Newton’s Second Law

B. Berenger 12/99 18 REMEMBER: Acceleration does not only mean to go faster! Acceleration can mean: –direction of motion changed –motion started –motion stopped –speed increased –speed decreased Acceleration

B. Berenger 12/99 19 a = F/m If a person keeps adding snow to the snowball (increasing its mass), yet each time throws it with the same force, the snowball will accelerate less each time F m a = remains constant decreases increases Acceleration

B. Berenger 12/99 20 a = F/m If the snowball begins to melt (mass decreases), the same force applied to the snowball will cause it to accelerate more F m a = remains constant increases decreases Acceleration

B. Berenger 12/99 21 NEWTON’S THIRD LAW For every action, there is an equal and opposite reaction Every force must have an equal and opposite force –all forces come in pairs Newton’s Third Law

B. Berenger 12/99 22 As you walk, your feet push against the ground You move forward and the earth moves in the opposite direction! –Since the mass of the earth is so large, its motion is unobservable The ground pushes up against your feet with an equal and opposite force EQUAL AND OPPOSITE FORCES Equal and opposite forces Newton’s Third Law

B. Berenger 12/99 23 Space Shuttle Various fuels are burned in the engine, producing hot gases As the gases move downward, the rocket moves in the opposite direction, or upward Newton’s Third Law Equal and opposite forces

B. Berenger 12/99 24 TOPICS COVERED: Newton’s First Law –inertia Mass vs. Weight Newton’s Second Law –F=ma –a=f/a Newton’s Third Law

B. Berenger 12/99 25 Bibliography K. Batista notes Physical Science (1988). HBJ, pp ; pp