Presentation Transcript

1. Chapter 12

2. What Starts an Object Moving? Materials: 5 pennies Procedure: On a flat surface, arrange 4 pennies in a row so they are touching one another. Using your index finger, slide a fifth penny across the surface line with the row of pennies so that it strikes a penny at one end of the row. Think about it (on looseleaf)…
3. What Starts an Object Moving? Observing – Describe how the row of pennies moves in response to the collision Hypothesize – What do you think caused the pennies to move after the collision? Why didn’t the pennies move before the collision?
4. 12.1 Forces Force-A ____ or____ that acts on an object. A force can cause a resting object to _____, or it can accelerate a moving object by changing the object’s ______ or ______________. The wind pushes against the man and his umbrella. The push from the wind is a force
5. What is a Force? Measuring ForceForce is measured using a ________. ______ is a type of force. Units of force Force is measured in __________, ___. A newton is the force that causes a 1kg mass to _________ at a rate equal to 1kg meter per second squared. 1N =
6. Representing Forces Representing Force An _____can be used to represent the direction and strength of a force. The direction of the arrow represents the ________ of the force. The length of the arrow represents the strength, or ___________, of the force.
7. Combining Forces You can combine force arrows to show the result of how forces _________. That is, forces in the same direction ___ together and forces in opposite directions __________ from one another. _________-The overall force acting on an object after the forces are combined.
8. Visualize Forming mental pictures Close your books. Listen to what I read and visualize (form mental pictures of) each object that I read about. I will read (pg. 358), you continue to visualize. As I pause, take a moment to draw an example of what you visualize (next slide) Turn to a partner and explain your picture
9. Your drawing…
10. Balanced vs. Unbalanced Balanced Forces Sometimes net forces are ____. Think of a tug-of-war that does ___ move. When the forces on an object are _________, the net force is zero and there is ___ change in the object’s motion.
11. A force that results when the net force acting on an object is ____________to zero. When an unbalanced force acts on an object, the object ________________ Unbalanced Forces
12. Unbalanced Forces Forces acting in opposite directions can also _________ to produce an unbalanced ______ Forces can add together or subtract from one another. A Two forces acting in the same direction ______ together. B Forces in opposite directions ___________ from each other. C Forces that are equal in size and opposite in direction result in _____ net force.
13. Friction Friction is a ___________________the motion of objects that ______ as they move past each other. Without friction, everything would be slicker than ice. There are four main types of friction: ______ friction, ________friction, _______friction, and______friction.
14. Static Friction- The friction force that acts on objects that are ____ moving. Static friction always acts in the direction _________ to that of the applied force. 2. Sliding Friction- A force that opposes the direction of motion of an object as it______over a surface. Because sliding friction is ____ than static friction, less force is needed to ____ an object moving than to ____ it moving.
15. Rolling Friction- The friction force that acts on _______ objects. When a round object rolls across a floor, the object and the floor are ______ slightly out of shape. This change in shape at the point of contact is the _____ of rolling friction. Rolling friction is ________ times less than the force of static or sliding friction. 4. Fluid Friction- A fluid friction opposes the moving of an object through a fluid. (______ or ___) Think of how hard it is to stir thick cake batter. Fluid friction acting on objects moving through the air is known as ____________________. At high speeds, fluid friction can be a _________force. This is why cyclists and speed skates wear special ____________
16. Observe the effects of Friction Materials 2 rubber erasers, sticky notes, scissors, metric ruler Procedure Attach a sticky note to the widest side of a rectangular eraser. The note must cover the entire side of the eraser. Trim off excess paper with scissors. Place the eraser from Step 1 note-side down, next to a second eraser so that one end of each eraser extends 2 cm over the edge of a table. Use a ruler to strike both erasers firmly and evenly at the same time. Record the distance each eraser slides. Repeat Steps 2 and 3 two more times. Calculate and record the average distance each eraser slides.
17. Observe the effects of Friction Analyze and Conclude Observing Which eraser slid farther? Why did the erasers slide different distances? How does friction affect the motion of a sliding object? What could you do to make the erasers slide even farther?
18. Gravity Why do leaves fall to the ground? ______- A force that acts between any two masses. It’s an ________ force. It pulls objects together. Earth's gravitational force exerts a force of attraction on _____ other object that is near Earth Unlike friction, gravity can act over _____ distances. Earth’s gravity acts ___________ toward the center of the Earth. Fortunately, an ________ force usually balances the downward force of gravity Because the forces on the boulder are balanced, it remains at rest as it has for thousands of years.
19. Gravity Earth exerts an attractive, downward force on this boulder Because the boulder is at rest, what do you know about the net force acting on it? Answer: The net force on the boulder is zero.
20. Falling Objects Falling Objects- Gravity causes objects to accelerate downward, whereas air resistance acts in the direction _______ to the motion and __________ acceleration. As objects fall to the ground, they accelerate and _____ speed. With increasing speed comes increasing air resistance. After a long time, the upward force of air resistance equals the downward force of gravity Forces are balanced. Acceleration is _____. Velocity is __________.
21. Terminal Velocity- Constant ________ of a falling object when the force of air resistance ________ the force of gravity.
22. Projectile Motion Projectile Motion- ________ path a ball takes when thrown. Then motion of a falling object (_________) after it is given an initial forward velocity. Balls of different masses fall at the __________. A projectile and an object dropped straight down also fall at the _____________.
23. A Gravity acts on falling objects. Although their masses are different, the blue and green balls fall at the same rate. B The yellow ball is a projectile, following a curved path.What forces act on each of the falling balls? Answer: Air resistance and gravity, in addition, the projectile in figure 9B was briefly acted on by the force that gave it its initial horizontal velocity
24. Gravity - Connecting Concepts Relating velocity and acceleration to falling objects. Sketch Figures 9A and 9B. Add velocity and acceleration arrows at three locations on each sketch.
25. Gravity: A Force of Attraction Gravity Poster Pick a sport or activity affected by gravity Identify examples from the sport or activity in which gravity is beneficial and examples of when gravity is harmful Make a poster (drawing) illustrating your examples
26. Investigating Inertia Procedure (pg 364) 1. Place an index card on a flat table. Place a coin in the middle of the card. As quickly as you can, try to pull the card out from under the coin. Observe what happens to the coin. 2. Repeat Step 1 while moving the card slowly. 3. Repeat Step 1 again. This time, slowly accelerate the card, and then suddenly bring it to a stop.
27. Analyze and Conclude 1. Applying Concepts Use the concepts of inertia and friction to explain the behavior of the coin each time you moved the card. 2. Predicting How would your observations be different with a coin of greater mass? Test your predictions.
28. Aristotle- Greek philosopher (384-322 B.C.) ___________ proposed force is required to keep an object moving at a constant speed. Galileo- Italian (1564-1642) He rolled balls down wooden ramps to see how gravity produces constant ______________. He concluded moving objects not subject to ________ would continue to move _______________. 12.2 Newton’s 1st & 2nd Laws of Motion
29. Newton Newton- (1665) Defined mass and force and introduced the laws of motion in a book called Principia Newton’s First Law of Motion The state of motion of an object does ___ change as long as the net force acting on the object is ____. Newton's first law of motion is sometimes called the law of _______. Inertia is the tendency of an object to ______ a change in its motion. Objects at rest remain at _____. Objects in motion remain in ________ with the same speed and direction unless an unbalanced force acts on it.
30. Inertia ________-Tendency of an object to resist change in its motion. Example: You continue to move ________ after a front end collision due to inertia. Weight is the force of gravity acting on an object. An object’s weight is the product of the object’s _____ and acceleration due to _____ acting on it. Weight = mass x acceleration due to gravity W =
31. Example Problem: If an astronaut has a mass of 112 kg, what is his weight on Earth where acceleration due to gravity is 9.8 m/s2? W = W = If you look at the formula, doubling the mass _______ the weight. Your mass is 88 kg. What is your weight. Weight on the moon is 141 N, what is gravity? Gravity on Mars is 3.7 m/s2, what is your weight?
32. F = ma Why would one cart accelerate eight times as fast as the chain of eight carts with the same force applied in each case? (Hint: Newton’s 2nd law of motion, pg 368)
33. Weight and Mass Weight = Mass x Acceleration due to gravity W = mg Visualize – The pound = 4.448 newtons (pg. 368)
34. Example Problem: If an astronaut has a mass of 112 kg, what is his weight on Earth where acceleration due to gravity is 9.8 m/s2? W = 112 kg x 9.8 m/s2 W = 1100 kg m/s2 or 1100 N If you look at the formula, doubling the mass doubles the weight. Your mass is 88 kg. What is your weight. Weight on the moon is 141 N, what is gravity? Gravity on Mars is 3.7 m/s2, what is your weight?
35. 12.3 Newton’s 3rd Law of Motion & Momentum A force cannot exist alone – they exist in________. Newton’s 3rd Law of Motion: Whenever one object exerts a force on a second object, the second object exerts an __________________ force on the first object. These two forces are called __________________ forces. They are always equal but opposite Examples Your bumper car is the action force, the other car is the ____________ force Pressing hand against the wall is the action force, the wall exerting a force on your hand is the ____________ force.
36. Action-Reaction Forces and Motion The swimmer uses her arms to push against the water and create an _______ force. The action force causes the water to move in the ___________ of the action force. However, the water also exerts its equal and ___________________ reaction force on the swimmer. That force pushes her forward through the water.
37. Action-Reaction Forces Do Not Cancel Why isn’t the net force of zero? Action and reaction forces do not act on the ____ _________. The action force acts on the ______, and the reaction force acts on the _____________. Only when equal and opposite forces act on the ____________do they result in a net force of zero.
38. Momentum If a loaded shopping cart and a marble are rolling towards you, the marble is________to stop. But what is the marble was moving 100 times faster than the cart? Momentum – Product of an object’s____and its ___________ Momentum = Mass x Velocity ρ= An object with_______momentum is hard to stop. Objects at rest have a momentum of _____.
39. Conservation of Momentum Under certain conditions, collisions obey the law of conservation of ___________________. Momentum does not increase or decrease in a ______ system. Objects _________ the system, however, can exert forces on one another. Law of Conservation of Momentum: if no net force acts on a system, then the total momentum of the system does _____ change. In a closed system, the____of momentum of one object equals the _____ in momentum of another object—momentum is ______________.
40. Momentum Figure 17 (pg 376) Three different collisions between equal-mass train cars. The different collisions between equal-mass train cars are shown above. In each collision, the total momentum of the train cars does not change—momentum is conserved.
41. A class studied the speed and momentum of a 0.25-kilogram ball dropped from a bridge. The graph shows the momentum of the ball from the time it was dropped until the time it hit the river flowing below the bridge. At what time did the ball have zero momentum? Describe this point in the ball's motion. At what time did the ball have the greatest momentum? What was the peak momentum value? What is the ball's speed after 1.25 seconds? (Hint: Use the graph and the momentum formula.)
42. Writing in Science Explain why it is impossible to identify a single isolated force. State in your first sentence the main idea of Newton’s third law of motion. Answer: Students must mention that forces always exist in pairs, and that according to Newton’s third law, every action force has an equal and opposite reaction force.
43. 12.4 Universal Forces There are _____ different forces that exist throughout the universe. All the universal forces act over a distance between particles of matter, which means that the particles need not be in _________ in order to affect one another.
44. 1st Universal Force 1. Electromagnetic Forces- These are the only forces that can both ______ and ____. It’s associated with _____________particles. Electric Forces- Act between charged objects or particles. Opposites attract, like repels. Examples: ________ and _______, static cling Magnetic Forces- Act on certain ______, the poles of ________, and on moving ___________. Magnets have a __ and __ pole. Opposites attract, like repels.
45. 2 & 3. Universal Forces Two forces, the _______ nuclear force and the ____ nuclear force, act within the _______ to hold it together. These forces are strong enough to _________ the electric force of repulsion that acts among the protons. 2. Strong Nuclear Force- Acts only on the neutrons and protons in the nucleus holding them together. It’s ___ times stronger than the electric force. 3. Weak Nuclear Force- Attractive force that affects all _______________.
46. 4th Universal Force 4. Gravitational Forces- It’s the _______ universal force. It’s an attractive force that acts between ____ two objects. Newton's law of ___________ gravitation states that every object in the universe _______ every other object. As mass increases, gravitational force ____________. As distance increases, gravitational force ___________.
47. Gravitational Forces
48. The Earth, Moon, and Tides The moon stays in orbit because it has______. It will continue to travel in a straight ____ until a force acts on it. That force is Earth’s gravitational force, which acts continuously to pull the ______ toward it.
49. Centripetal Force Centripetal force- A ____________ force that continuously changes the direction of an object to make it move in a _______. The gravitational pull from the moon produces two _____ in the Earth’s oceans. It takes the Earth ___ hours to rotate between the two bulges, causing___high tides a day. Satellites in Orbit Artificial orbits are launched into orbit. They use ________ and __________ force of gravity to stay in orbit.