1. FORCES Chapter 12.1

2. What is a Force?  A Force is a push or a pull.  Two things a force can cause: 1. a resting object to move 2. a moving object to accelerate by changing its speed or direction

3. Measuring Force  The type of force that can be measured with a spring scale is weight.

4. Units of Force  Force is measured in Newtons (N).  One Newton is defined as the force that causes:  a 1 kg mass to accelerate at a rate of 1 m/s 2  One Newton is equal to:  1 kg * m/s 2 (one kilogram meter per second squared)

5. Representing Force  Force is a vector, represented by an arrow.  Magnitude = strength of the force  Direction = direction of the force.

6. Combining Forces  You can combine forces by vector addition.  The Net Force is:  the overall force acting on an object after all the forces are combined (by using vector addition)

7. Balanced Forces  Balanced Forces are:  forces that combine to produce a net force of zero  This means there is no change in the object’s motion.  Example:  two equally matched people arm wrestling or playing tug-of-war  pushing on a wall

8. Unbalanced Forces  An Unbalanced Force is:  a force that results when the net force acting on an object is not equal to zero.  This means that the object accelerates.  Examples:  two unmatched people arm wrestling or playing tug-of-war  pushing something over

9. Friction  Friction is:  a force that oppose the motion of objects that touch as they move past each other.  Where does friction act?  at the surface where objects move past one another

10. Static Friction  What is Static Friction?  the friction force that acts on objects that are not moving It always acts in the direction opposite to that of the applied force. This is the strongest type of friction. It takes a lot of force to overcome it.  Examples:  walking- when your foot pushes off the ground

11. Sliding Friction  What is Sliding Friction?  a force that opposes the direction of motion of an object as it slides over a surface This type of friction is weaker than static friction. Less force is needed to keep the object sliding once it starts going.  Examples:  sliding a desk around the room

12. Rolling Friction  What is Rolling Friction?  the friction force that acts on rolling objects It takes A LOT less force to roll something than it does to overcome static friction or slide something.  Examples:  ball bearings  dollies  rollerblades

13. Fluid Friction  What is Fluid Friction?  the force that opposes the motion of an object through fluids (liquids and gasses) Fluid friction increases as the speed of the object increases.  Examples:  stirring a cake  an airplane flying in the sky  What is Air Resistance?  fluid friction acting on an object moving through air How can we lessen air resistance???

14. Gravity  Gravity is:  a force that acts between any two masses It pulls objects together even if they are not touching. It can act over large distances.  Earth’s gravity acts downward towards the center of the Earth.  What usually balances gravity?  an upward force (support) Like your neck supports your head.

15. Falling Objects- Contrast  always causes objects to accelerate downward  acts in the direction opposite of the motion. GravityAir Resistance  can increase an objects’ acceleration.  reduces an objects’ acceleration.

16. Terminal Velocity  What is Terminal Velocity?  the constant velocity of a falling object when the force of air resistance equals the force of gravity See page 370

17. Projectile Motion  What is Projectile Motion?  the motion of a falling object (the projectile) after it is given an initial forward velocity (a forward force)  What causes a projectile to follow a curved path?  the combination of an initial forward velocity and the downward vertical force of gravity

18. NEWTON’S 1 ST & 2 ND LAWS OF MOTION Chapter 12.2

19. Aristotle  When and where did this guy live?  Greece, 300’s B.C.  What did he wrongly conclude about motion?  Force is required to keep an object moving at a constant speed  What effect did this have on science?  It held back progress in the study of motion for almost 2,000 years. How would you like to be remembered like that?!

20. Galileo  When and where did this guy live?  Italy, 1564-1642  What did he study?  How gravity produces constant acceleration  What did he conclude?  Moving objects not subjected to friction or any other force will continue to move forever

21. Newton  When and where did this guy live?  England, 1665  What did he define?  Mass & Force  What did he introduce?  The Laws of Motion

22. Newton’s 1 st Law of Motion  State Newton’s 1 st Law of Motion:  An object at rest stays at rest and a moving object stays moving unless either one is acted upon by a force  What is it also called?  Law of Inertia  What is Inertia?  The tendency of an object to resist a change in motion If an object is still, it wants to stay still. If an object is moving, it doesn’t want to change speed or direction.

23. Newton’s 2 nd Law of Motion  How does an unbalanced force affect the motion of an object?  It causes an object to change its velocity (accelerate)  What is Mass?  A measure of the inertia of an object Depends on the amount of matter in an object  State Newton’s 2 nd Law of Motion:  The acceleration of an object is equal to the net force acting on it, divided by the object’s mass.

24.  What is the equation for Newton’s 2 nd Law?  A = F/M  Acceleration = Net Force divided by Mass  In which direction is the acceleration of an object compared to the net force acting on it?  In the same direction as the net force  Which units mean the same thing when using Newton’s 2 nd Law?  N/kg is the same as m/s 2

25.  What happens to an object when the net force is in the opposite direction of the object’s motion?  Deceleration  Negative acceleration  It slows down  What is the upward acceleration of a helicopter with a mass of 5,000 kg if a force or 10,000 N acts on it in an upward direction?  A = F/M  A = 10,000 N/ 5,000 kg  A = 2 N/kg or 2 m/s 2

26. Weight  What is the scientific definition of weight?  The force of gravity acting on an object  What is the formula for weight?  w = M * g  Weight = mass * acceleration due to gravity (9.8 m/s 2 )  How is this formula different from the equation for Newton’s 2 nd Law?  They are similar when A = F/M is switched around to F = MA Weight takes the place of force Acceleration due to gravity takes the place of acceleration

27. Mass  Define Mass:  A measure of the inertia of an object Remember: weight is a measure of the force of gravity acting on an object

28. NEWTON’S 3 RD LAW OF MOTION & MOMENTUM Chapter 12.3

29. Newton’s 3 rd Law of Motion & Momentum  Forces always exist in pairs.  State Newton’s 3 rd Law:  Whenever one object exerts a force on a 2 nd object, the 2 nd object exerts an equal and opposite force on the 1 st object.  What are the two forces called?  Action and Reaction Forces

30. Action & Reaction Forces  Think of an original example of action & reaction forces.  Kicking a soccer ball Foot = action Ball = reaction  Swimming Arm = action Water = reaction  Walking on the ground Foot = action Floor = reaction

31. Action-Reaction Forces & Motion  Do all action-reaction forces produce motion?  No  Think of an original action-reaction force that does not produce motion.  Walking on the ground  Pushing against a wall

32. Action-Reaction Forces Do Not Cancel  Why do action-reaction forces not have a net force of zero, even though they are opposite in direction and equal in force?  The action and reaction forces do not act on the same object Foot acts on ground Ground acts on foot

33. Momentum  What is the definition of momentum?  The product of an object’s mass and its velocity  What is harder to stop, something with a large or small momentum?  Large

34.  What causes something to have a large momentum?  When the product of its mass and velocity is large  What is the formula for momentum?  P = MV  Momentum = mass * velocity Kg*m/s = kg * m/s

35. Conservation of Momentum  In physics, what does the word conservation mean?  Something that has a constant value  What does conservation of momentum mean?  Momentum does not increase or decrease

36.  What is a “closed system?”  Other objects and forces cannot enter or leave a system  State the Law of Conservation of Momentum:  If no net force acts on a system, then the total momentum of the system does not change  Explain the conservation of momentum in a closed system:  The loss of momentum of one object equals the gain in momentum of another object Momentum is conserved.

37.  See trains on page 376

38. UNIVERSAL FORCES Chapter 12.4

39. What are the 4 Universal Forces? 1. Electromagnetic 2. Strong Nuclear 3. Weak Nuclear 4. Gravitational  With these forces, do the particles have to be in contact with one another?  No  Are they affected by the distance between the particles?  Yes

40. Electromagnetic Forces  What kind of particles are associated with electromagnetic force?  Charged particles.  What is unique about electric and magnetic forces?  They are the only forces that can both attract and repel.

41. Electric Forces  Describe electric forces in your own words:  They act between charged particles ( + and -) Opposite charges attract Similar charges repel

42. Magnetic Forces  Describe magnetic forces in your own words:  They act on some metals, magnetic poles, and moving charges North and south poles attract North/north and south/south repel

43. Nuclear Forces  What do scientists think holds the nucleus of an atom together?  Two forces: Strong nuclear force Weak nuclear force

44. Strong Nuclear Force  What is the strong nuclear force?  It is a powerful force of attraction that acts only on the neutrons and protons in the nucleus, holding them together.  Over what distances does this force act?  Extremely short distances 10 -15 m About the diameter of a proton

45. Weak Nuclear Force  What is the weak nuclear force?  It is an attractive force that acts only over a short range.  Over what distances does this force act?  10 -18 m  Less than strong nuclear

46. Gravitational Force  What is gravitational force?  It is an attractive force that acts between any two masses It is the weakest force  What is Newton’s Law of Gravitation?  Every object in the universe attracts every other object  Why can’t you feel the gravity that things such as your desk and your book have on you?  The masses are small, so their gravity is small.

47. Gravity Acts Over Large Distances  What determines the gravitational force between two objects?  Mass and the distance between objects The greater the mass, the greater the gravity The greater the distance between objects, the smaller the gravity  What is unique about gravity?  It is the weakest force, but it is the most effective over long distances.

48. The Earth, Moon, and Tides  What is centripetal force?  It is a center-directed force that continually changes the direction of an object to make it move in a circle.

49.  Explain ocean tides:  The moon’s gravity pulls on Earth just enough to affect the oceans.

50. Satellites in Orbit  Describe how satellites stay in orbit around the Earth:  Gravity causes them to have centripetal force, plus their inertia.

51. Uses of Satellites  Describe some of the many uses of satellites:  GPS  Weather  Satellite TV  Cell phones  Internet on cell phones