1. Agenda FF EQ Notes on Friction & Gravity I Bet You Can’t Activity
Present HW
Closure

2. 2 Volunteers
Socks vs. Shoe Friction Demontration

3. Questions does it make a difference sliding in socks? explain.
Is it easier or harder on carpet than wood or tiles?
how do you think friction affects motion based on what you observed?

4. FRICTION AND GRAVITY NOTES
Performance Indicator
8.P.2A.5 Analyze and interpret data to describe and predict the effects of forces (including gravitational and friction) on the speed and direction of an object.

5. What is a force?
A force is a push or pull

6. 2 more Types of Forces 1. Gravity 2. Friction
Forces such as gravity and friction can affect the speed and direction of an object.

7. Friction & Gravity Video
What is gravity?
Why is gravity considered a force?

8. Gravity (force that pulls objects down)
Gravity is a force that always attracts or pulls objects toward each other without direct contact or impact.
Gravity is a force of attraction, or a pull, between objects.
Gravity pulls objects down.
Do gravity affects everything? ____________

9. What Position Does Gravity Affect?1 2 3
All

10. Gravitational attraction depends on the mass of the two objects and the distance they are apart.
Objects on Earth are pulled toward the center of Earth.

11. The force of gravity, like all other forces, can cause changes in the speed of objects.
As an object falls, its speed will continually increase as Earth’s gravity continually pulls it downward. When air resistance is ignored, all objects will speed up at the same rate as they fall.

12. Gravity can also cause an object that is thrown into the air to change its upward motion, slow down, and fall back toward Earth’s surface.
The pull of Earth’s gravity keeps the Moon in orbit; the moon is constantly changing direction because of gravity.

14. How Does Mass Affect Gravity?
Imagine an elephant and a cat. Because the elephant has a larger mass than the cat does, gravity between the elephant and Earth is larger. So, the cat is much easier to pick up than the elephant. The gravitational force between objects depends on the masses of the objects.
Objects with Larger or Heavier Mass has a larger gravitational attraction?
Objects with a smaller mass has a smaller gravitational attraction.

15. How Does Distance Affect Gravity?
The mass of the sun is 300,000 times bigger than that of Earth. However, if you jump up, you return to Earth every time you jump rather than flying toward the sun. If the sun has more mass, then why doesn’t it have a larger gravitational pull on you?
This is because the gravitational force also depends on the distance between the objects.
As the distance between two objects gets larger or further, the force of gravity gets much smaller.
And as the distance between objects gets smaller or shorter, the force of gravity gets much greater.
Although the sun has tremendous mass, it is also very far away. This means that it has very little gravitational force on your body or on small objects around you. The sun does have a large gravitational force on planets because the masses of planets are very large.

16. Air Resistance
When air resistance is ignored, all objects will speed up at the same rate as they fall.
What is air resistance? Air resistance opposes the downward motion of a falling object.
Any falling object meets air resistance.

17. Questions WHAT TWO THINGS DETERMINE GRAVITATIONAL FORCE?
do all objects have a gravitational force? explain your answer.

18. Questions
IS THERE MORE GRAVITATIONAL FORCE BETWEEN OBJECTS WITH SMALL MASSES OR OBJECTS WITH LARGE MASSES?
THE SUN IS MUCH MORE MASSIVE THAN EARTH. WHY IS THE FORCE OF GRAVITY BETWEEN YOU AND THE SUN SO MUCH LESS THAN EARTH’S GRAVITY AND YOU?
HOW DOES AIR RESISTANCE AFFECT AN OBJECT’S MOTION?

19. Friction
Friction is a force that occurs when one object rubs against another object.
Friction will cause a moving object to slow down and finally stop.
Friction is the Force that one surface exerts on another surface when they are moving across each other.

20. Two factors determine the amount of friction –
(1) the kinds of surfaces, and
(2) the force pressing the surfaces together. (how hard the surfaces are pushed together during the motion )
Friction can effect the speed and direction of an object.

21. Friction is the force that acts to resist sliding between two surfaces that are touching. It can slow down or stop the motion of an object.
The slowing force of friction always acts in the direction opposite to the force causing the motion.

22. Friction Opposes motion
The slowing force of friction always acts in the direction opposite to the force causing the motion.
Friction Opposes motion
Examples: rubbing hands together, gym sneakers allowing you to stop better on the gym floor (compared to dress shoes)

23. For example, friction slows or stops the motion of moving parts of machines.
 Another example would be athletic shoes with tread grooves to increase friction have better traction for starting or stopping motion than smooth-soled dress shoes.

24. Friction can also be the force that makes it difficult to start an object moving.
Enough force must be applied to a nonmoving object to overcome the friction between the touching surfaces.

25. The smoother the two surfaces are, the less friction there is between them; therefore, the moving object will not slow down as quickly.

26. Friction between surfaces can be reduced, in order for objects to move more easily, by smoothing the surfaces, using wheels or rollers between the surfaces, or lubricating/oiling the surfaces.
If friction could be removed, an object would continue to move.

27. The greater the force pushing the two surfaces together, the stronger friction prevents the surfaces from moving.
As an object gets heavier, the force of friction between the surfaces becomes greater.
To move a heavy object, a greater force must be applied to overcome the friction between the surfaces.

28. Less or More Friction

29. Less or More

30. Less or More

31. Less or More

32. Types of Friction
Static friction: (acts between objects that aren’t moving) the frictional force that opposes any attempt to move a stationary object along a surface.

33. Sliding friction: (occurs when 2 solid surfaces slide over each other) (sometimes called "kinetic" friction): this frictional force opposes the sliding motion of two surfaces rubbing together.

34. Rolling friction(occurs when an object rolls across a surface)
is a special case of static friction.

35. Fluid Friction is the movement of solid objects through a fluid ex: oil, water and air. (occurs when a solid object moves through a fluid)

36. Comparing Gravity and Friction

37. Weight Weight is the pull of gravity.
An object placed near the surface of Earth experiences the pull downward of gravity. This pull, or force, is called the weight of the object. The weight of an object is the pull of gravity on that object.

38. Weight is a force.
Since weight is a force, it is measured in Newtons. A common symbol for it is:
Fg or W
'F sub g' is often read 'the force of gravity' or 'the weight'.

39. Weight is what causes an object to fall.
The downward pull of the weight of an object is the force that causes the downward acceleration of that object. Objects fall because of their weight.
Without air resistance an object on Earth will accelerate at a rate of 9.8 m/s2 as it falls.

40. Normally, falling bodies experience air resistance
Normally, falling bodies experience air resistance. With air resistance, this acceleration would be decreased as the object fell. Under these conditions the object could reach a terminal velocity and speed up no more.

41. Weight happens on any planet or moon.
An object would experience weight on any planet or moon. On the surface of the Earth's moon, for example, you would have a weight, you would feel the pull of the Moon's gravity.
Your weight on the Moon, though, would be one sixth of your weight on Earth. That is because the gravitational field strength on the Moon is much less than on Earth.

42. Formula for weight

44. Acceleration due to gravity
When an object falls, it is pulled down by the force of gravity. This constant force, called the weight, causes the object to accelerate as it moves downward. This acceleration is the same for all bodies (all bodies on the same planet or moon), and it is called the acceleration due to gravity.

45. The acceleration due to gravity (on Earth) has a size of 9. 8 m/s2
The acceleration due to gravity (on Earth) has a size of 9.8 m/s2. The symbol ag is often used for this acceleration, so we have:
ag = 9.8 m/s/s or 9.8 m/s2
Lower case g is also often used for this acceleration, so we also have:
g = 9.8 m/s/s

46. Question
Weight formula was derived from which Newton’s law of motion?

47. Example problems
On Earth
1. What is the weight of an object with a mass of 2.5 kg on planet Earth? Here is that calculation:
An object with a mass of 2.5 kg weighs 25 N on Earth. Note that mass and weight are different concepts.

48. Example

49. Practice Problems 2. What is the weight of a 9.24 kg mass on Earth?
A N
B90.55 N
C58.38 N
D0.94 N
E1.06 N

50. What is the gravitational acceleration on Planet X if an object with a mass of 9.7 kg weights N?
A m/s/s
B16.75 m/s/s
C5.61 m/s/s
D29.94 m/s/s
E0.06 m/s/s

51. Center of Gravity Lab “BET YOU CAN’T BALANCE” CHALLENGES*
Use what you've learned about balance to compete with your friends and family with following balance challenges.
HERE WE GO!!!!!!