Kinetic friction happens when the two surfaces ARE sliding across each other. It is a smaller force.
The HW is to finish the vector side of the handout for tomorrow. And you may do the ABS question for bonus points.
Also, remember to bring the book work that was due today (see way below for the problems). We will go over
Here is the handout:
Vectors and Friction Name ________________________
some common vectors: momentum, force, velocity, displacement, acceleration
Definitions:
A vector is a measurement that has size AND direction. Some common VECTORS are:
Displacement: where you are compared to where you started (including direction)
Velocity is how fast and what direction
Force is how big a push or pull AND what direction (which is why we use ARROWS)
Momentum, since it depends on mass and velocity
Measurements that do NOT depend on direction are called scalers. They include mass, speed, and distance.
1. Yuri runs 20 meters West and then runs 40 meters West.
a. How far did he run? What DISTANCE did he run? (same question)
b. What is his displacement (which includes direction)?
2. Lydia walks 100 yards South and then turns around and walks 30 yards North.
a. What distance did she walk?
b. How far is she from where she started?
c. Where is she compared to where she started?
d. What is her DISPLACEMENT (which includes how far and direction)?
3. Hector walks 3 meters North and then walks 4 meters South.
a. What DISTANCE did he travel?
b. Draw a picture to show his trip. Draw a line from his starting point to his end point.
c. Solve for the size of his displacement using the Pythagorean theorem.
4. Murray is following a treasure map. He travels 16 meters East, 22 meters South, 19 meters East, 4 meters North, and then 3 meters West.
a. Combine all of the East and West parts of the trip. What is his net displacement in the Eastern direction?
b. Combine all of his North and South parts of the trip. What is his net displacement in the Southern direction?
c. Draw a triangle showing his displacement. How far is he from where he started?
Friction: --is the force that happens between two surfaces that are in contact
--depends on how mn-slippery the two surfaces are (rubber on asphalt big, plastic on ice small)
--depends on how hard the two surfaces are being squished together (called Normal force)
--comes in two different types STATIC (not moving) and KINETIC/SLIDING (moving)
FORMULA: Force of friction = coefficient of friction X normal force or Ff = m Fnormal
Activity:
1. Use a string, a book and a spring scale to measure the amount of force needed to start a textbook moving across the top of a lab desk. Write down the answer.
_______________ N
2. Make a guess about what will happen if we put more mass on the book. Test it and write down your answer.
________________ N
3. Measure how much force you need to keep it moving at a constant velocity. Compare your answer to the answer you got on part 1, to get it to start moving. _______________ N Is this force Bigger or Smaller than the force to Start it moving? (circle your choice)
4. Now, move the book onto a different surface (the floor, a wooden board, etc.). Make a guess about how much force to get it to start to move now, and test it.
________________ N
5. Go back to the second surface and move the book across that new surface at a constant speed. Compared to getting the book to move across this surface, did you need to apply a bigger force to keep it moving at a constant speed, or a smaller force?
6. Make a free body diagram of the forces that are acting on the book while it is moving at a constant speed. Label all FOUR of the forces that are acting on the book.
7. Which is usually bigger, the force of static friction (not moving), or the force of kinetic friction (surfaces sliding across each other)?
EXTRA: Why are ABS brakes important?