Elastic-Normal-Friction Forces Elastic-Normal-Friction

SPRINGS Why did we make a line of best fit for our data? Here’s our slope- what does it tell us about the red spring? 2nd = 3.737cm/N 3rd = 3.679 cm/N 4th= 3.403 cm/N 6th= 3.740 cm/N 7th= 3.868 cm/N When finding the slope, why do we choose the “circled” points to calculate it? What does this slope tell you about this spring? How much weight is hanging on the spring if it stretches 23.7 cm?

Robert Hooke’s finding F = k Dx F is the force applied x = the change in length of the spring k = a spring constant (usually expressed in N/cm) k depends on what it is made of, temperature thickness of the material etc. What does this mean? Force & change in spring length are PROPORTIONAL If you know k, you can predict x when a particular force is applied- like on bridges!

Elastic Materials Hooke’s Law applies to all elastic materials- This means that all elastic materials have a linear graph like what we got for the red spring. * What if we had a spring that was thinner than the red spring. What would it’s graph look like? * What if it were a thicker, more tightly coiled spring, what would it’s graph look like?

Draw a model of an atom & identify its 3 main parts with their charges

Which objects will apply an ELASTIC FORCE? Clay Rubber stopper Coal Popsicle stick String Spring Rubber band

Will this linear graph go on forever? change in length vs Fg What makes springs elastic? Change in spring length (cm) Fg (N)

The Big Spring Elastic Phase- can go back to its original shape Plastic Phase- is deformed & can’t go back to its original shape Breaking Point- is broken apart

The spring Press on spring between finger/thumb & keep spring still & in elastic phase Did the spring push back? How hard? F push = F el Would the same be true if it were a balloon or a dodgeball or a sponge? Now press a bit harder on the spring Does the spring still push back? How does it know to push back harder (copy your push)?

How MATTER is arranged All matter is made of atoms & molecules Atoms & molecules in solids are arranged & bonded in a pattern (magnet demo) Bonds between atoms are electromagnetic field forces that hold them together in their pattern.

Why do things stretch and break? Solid being pulled Molecules in a solid Atoms/molecules get pushed/pulled out of their pattern in the solid, if they can find their way back to their original spot- they are in the elastic phase *What happens in the plastic phase? *What happens when they break?

How does the spring know to push back more (copy your push)? The atoms want to go back to their original spot. The more you push, the more they push back because you are making them too “crowded”. That’s better! Now I’m not so crowded! Stop repelling & pushing on me!

What is the name of the force from the spring pushing on your finger? ELASTIC FORCE Push or pull from solids that are stretchy or springy Where does that elastic force come from?

Stapler on table Is stapler pushing on the table? What direction? How can we measure it? DO IT! Name that force Is table pushing back? How much? Will the table push on all things with that same number of Newtons?

How does the table know how hard to push up on the stapler? See model The NORMAL FORCE is from the atoms being pushed but they still stay in their pattern The atoms are repelling each other because they are too close- the closer they get, the more they repel & push back FN = Fw

NORMAL/SUPPORT FORCE What if we set the stapler on tissue? *WHY did that happen? The bonds between the tissue atoms are too weak- they can’t push back with enough Newtons so the stapler’s weight force rips them out of their place & the stapler falls The bonds between atoms in the tissue can’t support the stapler.

WHY IS IT NORMAL? Normal is a math term for PERPENDICULAR (at 90o angle) The normal force is a push from a HARD SURFACE and always pushes in the direction PERPENDICULAR to the surface of the agent

Which force is it? Name that force http://www.npr.org/templates/story/story.php?storyId=8997621

How MATTER is arranged All matter is made of atoms & molecules Atoms & molecules in solids are arranged & bonded in a pattern (magnet demo) Bonds between atoms are electromagnetic field forces that hold them together in their pattern. Surfaces of solids look smooth but they are really bumpy at the microscopic level.

Smooth solids are bumpy when magnified steel

Plastic surface- some bumps are only 100 atoms high

What is friction? FRICTION- a force that resists motion between 2 surfaces can cause things to heat up- rubbing hands stay still- wax & wood boxes on a ramp slow down- sliding to a stop move- walking

4 TYPES OF FRICTION STATIC friction- from atoms of each surface attracting each other; is why it’s tougher to get an object moving than to keep it moving Demo with logger pro

4 TYPES OF FRICTION SLIDING friction- from the bumps of each surface getting caught on each other http://www.nsf.gov/news/special_reports/winterolympics/slopestyleskiing.jsp Start at 4 min

4 TYPES OF FRICTION 3. ROLLING friction- from wheels or bearings 4. FLUID friction- from motion through a liquid or gas; Walking/diving in a swimming pool; hand outside of a moving car window; parachute falling http://www.nsf.gov/news/special_reports/winterolympics/compsuits.jsp We can change sliding friction into fluid friction if we put a fluid between 2 surfaces

Use friction to explain. MOVING DAY- Do you push harder to get the table to move or to keep it going?   Use friction to explain.

WHICH WAY? Friction force is a vector. Which direction does friction push?

HOW MUCH? Friction force is a vector. How much does it push? Speeding up friction force is LESS THAN the pushing force Slowing down friction force is greatest Moving at a constant speed friction force is EQUAL to the pushing force