Friction & Gravity

1. Friction & Gravity 8-5.3 Analyze the effects of forces (including friction & gravity) on the speed and direction of an object.

2. Friction • What is friction? • What does friction affect? • Two factors determine the amount of friction: • a force that occurs when one object rubs against another object • an object’s speed & direction • the kinds of surfaces • the force pressing the surfaces together

3. How Friction Affects Speed & Direction • What does friction do? • How does friction affect an object’s speed? • In what direction is friction force? • acts to resist sliding between two surfaces that are touching • slows down or stops the motion of an object • always acts in the direction opposite to the force causing the motion

4. Examples of Friction • Machine parts-slow/stops • Friction slows or stops the motion of moving parts of machines. • Athletic shoes vs. dress shoes • with tread grooves to increase friction have better traction for starting or stopping motion than smooth-soled dress shoes.

5. Many receivers wear gloves that either have a sticky rubber palm, called tact gloves, or are covered with a sticky substance like rosin or a sticky spray. Linemen also wear gloves -- they don't have to catch the ball, but they do have to use their hands to fight off opposing linemen. The gloves worn by linemen usually have thick padding in them to better protect their fingers and hands, which can sometimes get caught up in another player's face mask or be stepped on in a pile of players. Players are not permitted to put any type of gel or "stick 'ems" on their gloves.

6. If a player wears the wrong shoe and it sticks to the turf, he gets knee injuries. An NFL team plays both on Astroturf and on natural grass, and the surface on game day can be dry, damp, wet or icy. Conditions can even change in the middle of a game. Each surface and condition requires a different shoe, and it's the equipment manager's job to anticipate the needs of the team and get the correct shoe on every foot. Boxes and boxes of shoes in a multitude of styles and sizes line the walls of a team's equipment room. Players with shoe contracts can also fill out a form and have their shoes flown in via overnight delivery, and many order a new suite of shoes every week. Amazingly, an NFL team can burn through 2,500 pairs of shoes in a single season! (Most teams donate used shoes to local high schools.)

7. Natural Grass Field Shoes

8. Astroturf Field Cleats

9. What animals can you think of that really need friction in order to survive? • Polar bears! • “Feet: Polar bears have large feet, which help propel them through the water as they swim.  When they are on the ice or snow their feet act as snowshoes by spreading out their weight and keeping the bears from breaking through (Stirling and Guravich 1990, p 24).  Their feet are flatter than those of other bears and are useful for shoveling snow when building dens (Brown 1993, p 73). The pads of the polar bear are covered with small papillae, which increase friction between the foot and the ice.  There are also small depressions in the pad that function as little suction cups and increase the grip of the polar bear's paws on the ice when it runs (Stirling and Guravich 1990, p 25). • http://www.bear.org/website/bear-pages/polar-bear/22/70-polar-bear-facts.html

10. Starting & Slowing with Friction • Friction can make it difficult to… • What must be applied to a nonmoving object to overcome the friction between the touching surfaces? • The smoother the two surfaces are… • start an object moving • enough force to start it moving • the less friction there is between them; therefore, the moving object won’t slow down as quickly

11. Reducing/Eliminating Friction • How can you make objects move more easily? • What are some ways to reduce friction? • If friction could be removed, • reduce friction between surfaces • smooth the surfaces • using wheels or rollers between the surfaces • lubricate/oil the surfaces • an object would continue to move.

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

13. Gravity • This chair is closest to zero-gravity you can get (presumably without actually going into space and experiencing it there, together with freeze-dried food and suits that invariably make you look fat). The Gravity chair can rest in a variety of positions depending on your mood or task; from upright as an office chair to rocked right back for those noon-time naps. The price for it is $2310 from Stokke. designed to support the human body extremely well, creating a sense of weightlessness and comfort.

14. Gravity • What Is gravity? • Gravity affects what? • Gravitational attraction depends on what? • Gravity causes objects to be pulled where? • a force that always attracts or pulls objects toward each other without direct contact or impact • speed & direction of objects • the mass of the two objects & the distance they are apart • toward the center of Earth

15. What is Gravity?

16. How Gravity Affects Speed • Gravity, like all forces… • As an object falls… • When air resistance is ignored… • can change the speed of objects • its speed will continually increase as Earth’s gravity continually pulls it downward • all objects will speed up at the same rate they fall

17. How Gravity Affects Direction • How does gravity affect an object that is thrown into the air’s speed & direction? • The pull of Earth’s gravity causes the moon to do what 2 things? • It goes up fast, slows down, & falls back toward Earth’s surface quickly • stay in orbit & constantly change direction

18. 1. The man applies a force to the box, but the box does not move. The force of friction is acting on the box to cancel the force of his push. What is the most likely reason for this friction? • The floor is polished to a smooth finish. • The weight of the box is greater than the force of the man’s push. • The rough surfaces of the box and the floor are sticking together. • The man is not pushing the box with enough force.

19. 2. According to the diagram, which statement best describes what happens when two astronauts collide in space? • Their force increases. • Their velocity increases. • Their momentum is conserved. • Their momentum increases.

20. 3. The gravitational force between two masses ___. a. decreases rapidly as the distance between the masses increases b. decreases rapidly as the distance between the masses decreases c. increases rapidly as the distance between the masses increases d. remains constant 4. Earth’s gravity causes projectiles to ____. a. travel vertically only b. travel horizontally only c. follow a curved path d. accelerate with a forward motion

21. 5. Felicia wanted to pull out a particular issue from a pile of magazines. First, she tried pulling slowly, and the whole pile began to move. Then she tried pulling quickly, and the issue she wanted came out without the pile moving. The pile did not move when she pulled quickly because of the ____. • smooth surface of the magazines b. inertia of the pile and friction c. larger force Felicia applied d. friction between the magazines

22. 6. What are two different forces acting on the ball in the diagram above? • gravity and air resistance b. gravity and acceleration c. air resistance and friction d. mass and acceleration

23. 7. Air resistance is a form of friction that makes a moving object slow down. Which of these drawings shows a skier whose shape would allow him or her to move through the air with the LEAST resistance?

24. 8. Which of the following best describes force? a. a push or a pull always causing motion b. a push or a pull always causing acceleration c. a push or a pull acting without an object d. a push or a pull acting on an object 9. What is a force that opposes motion between two surfaces that are in contact? a. friction c. velocity b. motion d. acceleration 10. When the net force on an object equals 0 N, the forces are which of the following? a. balanced c. a push b. unbalanced d. a pull

25. 11. Two forces act on an object. One force has a magnitude of 10 N and is directed toward the north. The other has a magnitude of 5 N directed toward the north. The object experiences a net force of ____. a. 5 N south. c. 50 N north. b. 15 N north. d. 5 N north. 12. Which of the following best determines the net force when more than one force is acting on an object? a. a combination of all motions of an object b. a combination of all motions of an object in two directions c. a combination of all forces acting on an object d. a combination of forces acting on an object in two directions

26. Friction Interactive http://www.sciencekids.co.nz/gamesactivities/friction.html

27. BrainPop: 5 facts