ch+2+Sec+7+KirnumB

What do you see? What do you think? I see something being much easer to move when there is less friction working on it.

Some sports require special shoes because of the way it reacts to the ground the players are on Cleats are helpful for soccer because they keep you balanced. Ice skates are best for the ice because you can glide on them.

Physics to Go
 * 1) You would want to increase friction in the snow so you can walk around and not slip
 * 2) You dont want alot of friction when you are ice skating, the ice is smoothed to lessen friction
 * No, the ground may have different amount of friction
 * 1) They don't have different shoes because there is a similar amount of friction on each terrain
 * 2) below
 * 3) below
 * 4) Forces of air resistance and water resistance do remain constant. When you swim, you may swim faster, but you still need to push as hard on the water because it pushes back on you.
 * 5) Yes, you can't start faster then the maximum frictional force allows but you can have more than a certain acceleration even if you have stronger mussles. This is dependent on the shoe you are wearing.
 * 6) Friction is important when you run because it keeps you from sliding. Cleats are used because it causes more friction when you run.
 * 7) "Here comes Johnson up to bat! he swings and dashes to first base, then second base! will he stop on the base or go over? If his shoes apply allot of friction he will slow down in time!
 * 1) "Here comes Johnson up to bat! he swings and dashes to first base, then second base! will he stop on the base or go over? If his shoes apply allot of friction he will slow down in time!

Physics Talk Analyzing the forces acting on the shoe
 * motion with a constant velocity happens only when there is no net force on the shoe
 * all the forces on the shoe must add up to zero
 * Friction: a force that resists relative motion between two bodies in contact
 * The downward force of gravity on the shoe must be equal to the upward force applied to the shoe by the surface
 * Normal force: the force acting perpendicularly or at right angles to a surface
 * Coefficient of sliding friction µ is a ratio of two forces
 * : a dimensionless quantity symbolized by the greek letter µ; its value depends on the properties of the two surfaces in contact and is used to calculate the force of friction
 * it doesn't have units
 * it is usually expressed in decimal form
 * is valid only for the pair of surfaces in contact when the value is measured; any significant change in either of the surfaces may cause it to change

Checking up
 * 1) ƒ = the force on the spring scale when it is pulling on a shoe because there is no acceleration and friction= force needed to pull it
 * 2) since it is N/N the units cancel out
 * 3) Wether or not the object is moving

Active Physics Plus

5)



6)

e) The driver lied, he said he was traveling at 29m/s but he was traveling at 32.34m/s

What do you think?

Some sports require special shoes because of the way it reacts to the ground the players are on; depending on the kind of surface they are on, the more, or less, friction they will need Cleats are helpful for soccer because they keep you balanced and add friction so you don't slip. Ice skates are best for the ice because you can glide on them due to their ability to lessen friction

Lab

Lab: Bowling with Blocks CP Physics

**Introduction**: In our last chapter we discussed forces and motion. Newton stated that an object in motion remains in motion unless acted upon by another force. One of the forces that changes motion is friction. Friction opposes motion.

In this experiment we will slide a block of wood across a floor observing both the time and distance in which the object comes to rest.

**Materials**:


 * * Piece of 2x4 (1) || * Stop watch (1) ||
 * * String (.5 m) || * 2.5 Newton scale (1) ||

**Purpose**

In this lab you will be measuring the coefficient of friction between a wooden block and the floor. You will work in pairs, but each individual must collect their own data.

=Procedure=

**Part I**: Measuring µ


 * 1) Find the weight of the block in Newtons.
 * 2) Place the block of wood on the floor and place a 1-kg mass on top of it. Use a spring scale to find the force needed to pull the block at constant speed. (Hint: make sure to pull the string parallel to the floor.)
 * 3) Repeat 2 more times.
 * 4) Use the problem solving method we went over in class to compute the coefficient of friction between the wood and the floor.
 * 5) Write your µ on the board.
 * 6) What is the percent difference between your value and that of the class average?

**Table 1** – Friction and Weight

 || //**% Difference**// ||
 * **Tension (N)** || **F****f** **(N)** || **Total Weight (N)** ||  || //**Class Average**//
 * **4** || 4.1 || 4 || 4.03 || 11 || 0.37 || 0.33 || 0.6 ||

//**Sample Calculations:**//

h=w=mg 4.03/11 **Part II**: Chucking the Block


 * 1) //__**Quietly**__// go into the hallway and stretch a 10 m tape along the hall.
 * 2) You are to stand at the “0” and of the tape and your partner is to stand towards the other end.
 * 3) You are to slide your block along the tape as if you were bowling. Be sure to release the block of wood as close to the starting point as possible.
 * 4) Your partner is to record the time that your block is moving and distance it has traveled. Do not throw so hard that it goes past the end of the tape!
 * 5) Repeat the experiment __at least__ three times. Each trial will have different time/distance combinations. This is okay.
 * 6) Switch roles so your partner can gather their data.
 * 7) Use Newton’s Second Law to compute the rate at which the block was slowing.
 * 8) Use the measured distance, calculated acceleration, and the final velocity to find your throwing speed (initial speed of the block).
 * 9) Use kinematics again to find the time it took the block to stop.
 * 10) Repeat for the other trials.
 * 11) Calculated percent error between your measured time and calculated time for each trial.

**Table** – Friction and Kinematics


 * **Mass (g)** || **Mass (kg)** || **Measured Time (s)** || **Measured Distance (m)** || **F****f** **(N)** || **Acceleration (m/s****2****)** || **Calculated v****i** **(m/s)** || **Calculated time (s)** || **% error** ||
 * **180** || 0.18 || 1.83 || 5.62 || 0.06 || -3.3 || 6.09 || 1.85 || 1.08 ||
 * **180** || 0.18 || 1.65 || 5.17 || 0.06 || -3.3 || 5.84 || 1.77 || 6.77 ||
 * **180** || 0.18 || 1.66 || 4.68 || 0.06 || -3.3 || 5.6 || 1.7 || 2.35 ||

Sample Calculations:

µ=f/N .332=f/mg

∑Fx=max -f=max -f=.332 (9.8) -f=-3.3

∑Fy=may N-w=0 N=mg

Vf^2=Vi^2+2ad 0=Vi^2+2ad -Vi^2=2ad vi^2=6.6

Vf=vi+at -vi/a=t

**Part III**: Questions/Conclusion

> The friction of the block on the floor > It is very close, they should be the same because we all had the same factors > My times were very close, the percent is very low Yes, it relates to the real world in many different ways including bowling
 * 1) What does the coefficient of friction in Part I mean?
 * 1) How does your µ compare to the µ of your classmates? Should your results be the same as everyone else’s in the class? Why or Why not?
 * 1) How well did your times agree? Was your percent error high or low?
 * 1) Does the theoretical physics we are doing in class seem to apply in the real world? Why or why not?

> 1. FRICTION OF THE FLOORS ARE DIFFERENT > 2. Slow reaction time > 3. measuring a diagonal > 4. the boxes are a little different
 * 1) Describe at least 3 sources of experimental error. Be specific about how these errors would throw off your results.