Newton’s Laws

1. Newton’s Laws Three Laws of Motion

2. Aristotle’s Motion • Natural Motion is up or down • Down for falling objects • Up for smoke • Circular for heavenly bodies since without end • Violent Motion • Due to imposed forces such as wind pushing a ship or someone pulling a cart • Natural state of motion is rest • A force is needed to keep something moving

3. Aristotle’s Basic Error • Friction not understood as a force

4. Galileo’s Motion • Force is a push or a pull • Friction is a force that occurs when objects move past each other • Friction due to tiny irregularities • Only when friction is present is a force required to keep something moving

5. Ball rolling downhill speeds up Ball rolling uphill slows down He asked about ball on smooth level surface Concluded it would roll forever in absence of friction Galileo’s Inclined Planes

6. Inertia • Resistance to change in state of motion • Galileo concluded all objects have inertia • Contradicted Aristotle’s theory of motion • No force required to keep Earth in motion around sun because no friction

7. Newton • Born 1665 • Built on Galileo’s ideas • Proposed three laws of motion at age of 23

8. Newton’s First Law Ourtesy www.lakeheadu.ca/~alumni/ hockey.gif • Every object continues in its state of rest, or of motion in a straight line at constant speed, unless compelled to change that state by forces exerted on it. • Also called Law of Inertia: things move according to their own inertia • Things keep on doing what they are doing • Examples: Hockey puck on ice, rolling ball, ball in space

9. Mass • Amount of inertia depends on amount of mass…or amount of material (number and kind of atoms) • Measured in kilograms • Question: Which has more mass, a kilogram of lead or a kilogram of feathers? • Mass vs. Volume: volume is how much space something occupies

10. Experiencing Inertia • Inertia is resistance to shaking • Which is easier to shake, a pen or a person? • Why is it so hard to stop a heavy boat?

11. Mass vs. Weight • Mass is intrinsic property of any object • Weight measures gravitational force on an object, usually due to a planet • Weight depends on location of object • Question 1: How does mass of a rock compare when on Earth and on moon? • Question 2: How does its weight compare?

12. Review Mass vs. Weight • What is mass? • Answer: quantity of matter in something or a measure of its inertia • What is weight? • Answer: Force on a body due to gravity

13. Weight of 1 Kilogram • 9.8 Newtons • About 2.2 pounds • Compare the weight of 1 kg nails with 1 kg styrofoam • Answer: Same

14. Weight Examples • What does a 70 kg person weigh? Weight = mass x g(acceleration due to gravity) W = mg = 70 kg x 9.80 N/m2 = 686 N • An object weighs 9800 n on Earth. What is its mass? • m = W/g = 9800 / 9.8 m/s2 =1000 kg

15. Inertia in a Car • Discuss three examples of inertia in a car • Car hitting a wall • Car hit from behind by a truck • Car going around a corner

16. Newton’s Second Law • Law of Acceleration • The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, and is inversely proportional to the mass of the body. • Acceleration = net force ÷mass • F =ma • Acceleration is in direction of net force

17. Net Force • Net Force means sum of all forces acting • Sum is Vector sum F2 F1 Resultant force

18. Understanding the Second Law Force • The cause of acceleration is… • _________ resists acceleration • The greater the force, the ________ the ______________ • The greater the mass, the _________ the acceleration. Mass or inertia greater acceleration less

19. Units • F = ma • Unit of force is the Newton (N) • 1 N = 1 kg m/s2

20. F = ma is Three Equations • F and a are vectors • So F = ma equation is really three SFx = max SFy = may SFz = maz

21. Examples • What force is required to accelerate a 1000 kg car at 2.0 m/s2 ? Answer: F = ma = 1000 kg x 2.0 m/s2 = 2000 N. • What is the acceleration of a 145 g baseball thrown with a force of 20.0 N? a = F/m = 20.0 N/0.145kg = 138 m/s2

22. F = ma Example; m unknown • An astronaut puts a 500.0 N force on an object of unknown mass producing an accelerations of 0.462 m/s2 . What was the mass? • M = F/a = 500.0N/0.462 m/s2 = 1082 Kg = 1.08 x 103 Kg

23. Newton’s Third Law • Forces always come in pairs • Two forces on different objects • Whenever one object exerts a force on a second object, the second exerts an equal and opposite force on the first • Example: hammer hits nail

24. Example: pushing on wall • What are the forces when you push on a wall? • You exert force on wall • You accelerate in the opposite direction • Wall must have exerted a force on you in the direction you accelerated (by 2nd Law)

25. Example: person walking • Foot exerts force backward on ground • Ground exerts force forward on foot

26. Example: Throwing ball • Pitcher exerts force on ball • Ball exerts equal and opposite force on pitcher • Why doesn’t pitcher move?

27. Example: Rocket • Rocket engine exerts rearward force on gas molecules • Molecules exert forward force on rocket.

28. Horse and Cart • Horse exerts force on cart • Cart exerts equal and opposite force on horse • Net force = zero • Cart can’t move • Huh?

29. Book on Table • The mass of the book is one kg. What is the force (magnitude and direction) on the book? • 9.8 N upward

30. More Examples • Can you think of some more examples of Newton’s Third Law in Action?

31. Review: Newton’s Laws of Motion • Newton’s First Law: Every object continues in its state of rest, or of motion in a straight line at constant speed, unless compelled to change that state by forces exerted on it. • Newton’s Second Law: The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, and is inversely proportional to the mass of the body. • Newton’s Third Law: Whenever one object exerts a force on a second object, the second exerts an equal & opposite force on the first