Newton’s Laws 2nd

1. Newton’s Laws 2nd • acceleration of an object is directly proportional to the net force on it and inversely proportional to its mass • (defines the newton)

2. Newton’s second law of Motion • F=ma • Force = mass x acceleration • Mass = kilogram • Acceleration = m/s2 • Force = 1 N (Newton) • Force = 1 kg-m/s2

3. A larger force acting on an object causes a greater acceleration A larger mass requires a greater force than a smaller mass would require to achieve the same acceleration. Hitting a ball harder causes a greater acceleration If you hit a ping pong ball and a tennis ball with the same force, would they have the same acceleration? Newton’s second law of Motion

4. The greater the mass, the greater the force must be for a given acceleration. • the amount of acceleration depends not only on the force but on the mass being pushed. • For a given force, the acceleration produced is inversely proportional to the mass.

5. More Mass, Less Acceleration The greater the mass of an object, the less it accelerates when acted on by a force. Physics 1 (Garcia) SJSU

7. Newton’s second law of Motion • A net force acting on an object causes the object to accelerate in the direction of the force. • Acceleration is determined by the size of the force and the mass of the object. • F= ma

8. implies implies implies Mass & Acceleration For a given force, greater mass, smaller the acceleration Double the Mass Triple the Mass Half the Mass Half the Acceleration Third of the Acceleration Double the Acceleration Acceleration goes as the inverse of mass. Mathematically, we write, Acceleration ~ 1/(Mass). Physics 1 (Garcia) SJSU

9. Demo: Hammer Head Hammer a nail into a piece of wood placed on top of massive object (gold brick or huge book). Inertia of massive object keeps it from moving; can place on top of head and not feel it. Physics 1 (Garcia) SJSU

10. Force is a scalar property depends on 2 things; Sizeor strength of the force and the Directionof the force. • Vector. • A quantity with size and direction • Scalars • are described by size only

11. Force • You can measure force with a Spring scale.

12. Mass and Weight should not be confused with... • Volume • the quantity of space an object occupies • Density • the quantity mass per unit volume • Mass • the quantity of matter in an object • the measurement of the inertia • measured in kilograms (kg)

13. Mass & Weight Mass: Quantity of matter in an object Weight: Force of gravity on an object Mass is a universal property. Weight depends on gravity (different on Moon). Mass is the measure of inertia. Metric unit for mass is the kilogram. Metric unit for weight is Newton (since it’s a force) Weight Earth Moon Physics 1 (Garcia) SJSU

14. Mass Vs. Weightmass vs. force Mass Weight is a measure of the force of the Earth’s gravity on the mass of the object. • is a measure of the total amount of matter contained within an object.

15. In metric units, the unit of force is the newton, • . A 1-kilogram brick weighs about 10 newtons (more precisely, 9.8 N). • The books offer the same resistance to speeding up or slowing down regardless of whether it’s on Earth, on the Moon, or on any other body attracting it. • You’d have to provide the same amount of force to accelerate a huge truck to a given speed on a level surface on the Moon as on Earth.

16. An anvil in outer may be weightless, but it is not massless. • The astronaut in space finds that it is just as difficult to shake the “weightless” anvil as it would be on Earth. • If the anvil were more massive than the astronaut, which would shake more—the anvil or the astronaut?

17. Newton’s 2ndLaw of Motion • F=ma • N = Kg*m/s2 • Newton was the first to discover the relationship between—acceleration, force, and mass. • It states; an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object.

18. m m m m m M M M m NEWTON'S 2nd LAW OF MOTION a F F a F a F a F a F a

19. Acceleration is directly proportional to force.

20. 0 N 15 N Demo: Elevator Cable Tension in elevator cable depends on acceleration 5 m/s2 upward ( ½ g upward) Zero acceleration 5 m/s2 downward ( ½ g downward) 10 m/s2 downward (Free fall) 10 N 5 N 1 kg 1 kg 1 kg 1 kg Physics 1 (Garcia) SJSU

21. 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

22. 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).

23. Location Mass Weight Earth 18.4 kg 180 N Moon 18.4 kg 30 N Space 18.4 kg 0 N http://observe.phy.sfasu.edu/images/KC135-Summer2001/

24. The weight of a 10 kg brick is... • A) 98 N • B) 10 kg • C) 9.8 kg • D) 10 N • E) 98 kg

25. Newton’s Cradle

26. How much force is need to accelerate a 70 kg rider and her 200 kg motorcycle at 4 m/s2? • Mass of rider = 70 kg • Mass of motorcycle =200 kg • Acceleration = 4 m/s2 • Force = unknown • Equation F=ma

27. 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? • A) 747 N • B) 4 kg • C) 1/4 kg • D) 4 m/s2 • E) 30,000 kg times 9.8 m/s2

28. F F Example Questions • 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? • A) Blue • B) Yellow

29. F F Example Questions • 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?

30. Inertia • Friction increases with greater force • Friction is in the opposite direction of the force applied • Doesn’t want to change what it is doing • Momentum

31. Frictionreduces net force & resulting acceleration

32. Force that opposes the motion between 2 surfaces that are in contact Friction

33. Depends on kinds of surfaces Force pressing surfaces together Life without Friction Couldn’t stand Clothes wouldn’t stay on Friction

34. Friction • is a result of irregularities in the surfaces of objects. • The force required to overcome friction is called the static frictionforce. • The force needed to keep a constant speed is called the kinetic frictionforce.

35. Friction Origin of friction is molecular interaction between solid surfaces. Friction is complicated. Friction depends on support force and on properties of the surface. Basic properties of friction first established by Leonardo da Vinci. Physics 1 (Garcia) SJSU

36. Friction depends on 3 things: • The friction force depends on whether or not the surfaces are moving. • The friction force depends on the materials of which the surfaces are made of. • The friction force depends on how hard the surfaces are pressed together. This is called the Normal Forceand depends on mass and gravity also known as Weight

37. Imagine your car broke down and you have to push it. Which takes more force, to get it started rolling or to keep it rolling? To get it started Static friction is greater than kinetic friction.

38. How is a car affected by friction? Types of Friction Static Sliding Rolling fluid

39. sliding friction is less than static friction • Once tires start to slide, the frictional force is reduced and off you go • While the tire is rolling, its surface does not slide -it is static friction—and therefore greater than sliding friction.

40. static friction and sliding friction • It is very important that you not jam on the brakes in an emergency stop. (the tires lock in place), sliding, providing less friction than if they are made to roll to a stop.

41. Fluid Friction • Occurs when moving through Air, water & oil • includes falling through air

42. Fluid friction is called drag. • Friction occurs also in liquids and gases, collectively called fluids (because they flow) • drag in a fluid depends on the nature of the fluid • drag does depend on speed and area of contact.

43. the amount of fluid pushed aside by a boat or airplane depends on the size and the shape of the craft. • A slow-moving boat or airplane encounters less drag than faster boats or airplanes. • wide boats and airplanes must push aside more fluid than narrow crafts • For slow motion through water, drag is proportional to the speed of the object.

44. Air Drag • In air, drag at most speeds is proportional to the square of the speed • So if an airplane doubles its speed it encounters four times as much drag • At very high speed, however, the simple rules break down when the fluid flow becomes erratic and such things as vortices and shock waves develop.

45. Friction between the tire and the ground is nearly the same whether the tire is wide or narrow. • The purpose of the greater contact area is to reduce heating and wear.

46. Check Yourself • 1. What net force does a sliding crate experience when you exert a force of 110 N and friction between the crate and the floor is 100 N? • 2. A jumbo jet cruises at constant velocity of 1000 km/h when the thrusting force of its engines is a constant 100,000 N. What is the acceleration of the jet? What is the force of air resistance on the jet?

47. Lubricants • reduce friction by keeping the two sliding surfaces apart with a thin layer of fluid. • Friction is no longer rubbing the surfaces but instead rubbing the lubricant.

49. Another way to reduce friction is to roll an object over a surface. • This is called rolling frictional force instead of kinetic frictional force.

50. Friction and Air Resistance Friction and air resistance are forces opposing motion. Physics 1 (Garcia) SJSU