Four Forces Get a mini whiteboard and marker

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What is a FORCE? Front page

What do you know about gravity?

Vote: Compared to the United States, the strength of gravity in Australia is: A. A little less B. Equal C. A little more US Aus

Gravity is a field that pulls things to the center of the Earth. • Caused by mass. • NOT caused by air pressure. • NOT caused by the Earth’s rotation.

Why does the tennis ball fall?

Is there a gravitational force between these two tennis balls sitting on the table?

Cavendish Experiment http: //www. youtube. com/watch? v=dy. LYbv. ZIYo. U

Newton’s View of Gravity • All masses exert a force of attraction between them, decreasing with distance. • Action at a distance – does not touch • "I have not yet been able to discover the cause of these properties of gravity from phenomena and I feign no hypotheses. . . It is enough that gravity does really exist and acts according to the laws I have explained, and that it abundantly serves to account for all the motions of celestial bodies. "

Einstein’s View of Gravity http: //video. pbs. org/viralplayer/2163057527/? chapter=3

If Atlas drops an object with the same mass as the Earth from one mile high, what will happen? Explain. “Earth” Earth

If Atlas drops an object with the same mass as the moon from one mile high, does the “Moon” exert a force on the Earth while it is falling? Explain. “Moon” F? Earth

If Atlas drops a tennis ball from one mile high, does the tennis ball exert a force on the Earth while it is falling? Explain. F? Earth

Gravity Summary • It acts at a distance. • It always attracts. • Gravity is a TWO WAY action. • It can act through matter and through a vacuum. • It acts between all masses, no matter how small. • No simple model to explain gravity.

Normal Force

Does the table exert an upward force on the book? Yes or No? How sure are you? 1 – Not at all sure 5 – Very sure

Do the ropes exert an upward force on the book? Yes or No? How sure are you? 1 – Not at all sure 5 – Very sure

Do the metersticks exert an upward force on the book? Yes or No? How sure are you? 1 – Not at all sure 5 – Very sure

Does the meterstick exert an upward force on the book? Yes or No? How sure are you? 1 – Not at all sure 5 – Very sure

Does the table exert an upward force on the book? Yes or No? How sure are you? 1 – Not at all sure 5 – Very sure

Pushing on Matter Model • How is this situation like the book on the table? • How is it different?

Springy model for solids

Does the table exert an upward force on the book? How does the springy model explain this?

Car pushing on fire hydrant • Which force is bigger… • Force of car on hydrant? • Force of hydrant on car? • Same?

Consider the car pushing on a rubber hydrant. How does this compare to the original car problem?

Normal Force Any force that PUSHES we will label as a Normal force. “Normal” means perpendicular.

Tension Force

VOTE : Which situation is more likely to rip the Levi’s jeans? ●Using two horses ●Using one horse and one tree

Tension – Spring Scales BE CAREFUL NOT TO OVERSTRETCH THE SPRING SCALE!!!! Hook your spring scale with your partner. One of you pull. What do you notice? Now, pull a little harder. What do you notice? How does it change if the other partner does the “pulling”?

VOTE: Which situation is more likely to rip the Levi’s jeans? ●Using two horses ●Using one horse and one tree ●Both are equally likely

Tension Force Any force that PULLS we will label as a Tension force.

Friction Force

You and your partner each have a toothbrush. Interlock the bristles and gently pull in opposite directions. Draw what you see. Draw an interaction diagram. Does the Top brush exert a force on the Bottom brush? In which direction? Does the Bottom brush exert a force on the Top brush? In which direction? Which Force is bigger: Force of TOP brush (dealer) on BOTTOM brush (feeler)? Force of BOTTOM brush (dealer) on TOP brush (feeler)?

Does the bottom brush exert a force on the top brush? What is the direction of that force? What forces does the bottom brush feel?

Two different brushes What are the directions of the opposing forces? How could they possibly be equal if one bump is stronger? Two rubber bands and ring

Bumps model… Consider a rough block that someone is trying to drag across a rough floor. The block is not moving. What is the direction of the friction force acting on each surface? Which force is larger, the force of the block pushing sideward on the floor, or the force of the floor pushing sideward on the block?

Friction Force When two surfaces slide against each other (or try to slide against each other), there is a friction force. The friction force always acts OPPOSITE the motion (or attempted motion).