cahier physique - physics workbook

INTRODUCTION

Friction opposes the motion. It depends on the surfaces in contact. We are going to find out if there is a relationship (Linear ? quadratic ? ..) between the weight of an object and the frictional force. We are also going to find out if the surface area of an object affect the friction? and why not ?

PART I : Is there a relationship between weight and friction?
PROCEDURE
You need a block, weights of 100g, spring scale in Newtons, a large smooth table.

step1: Attach a spring balance to a rectangular block and record its weight in the TABLE (first row, 3rd column, no weight added)

step2: Place the block at the edge of a table, without weights added and pull the spring along the surface. (see picture).
Ask you instructor for help. Pull at a constant speed. Don't accelerate. The pull = kinetic or sliding friction. Record the spring sale reading (N) in the TABLE below
(first row, 4th column).

step3: Add a mass of 100g on the block. the weight added is 0.1 x 10m/s/s. (weight (N) = mass(kg) x 10 )
Record in column 2, row2. Record in column 3 the total weight of the block = weight of block + weight added .
Slide the block at a constant speed and record the sliding friction in column 4.

step4: Repeat step3 for all the masses added in col1. Fill the table.

DATA TABLE: Does weight affect friction?
mass added (g)	mass in kg	weight added (N) = mass(kg) x 10	total weight (N) = weight of block + total mass added x 10	Sliding friction (N) reading from spring scale
0	______	_______________	W1 = weight of block = _____N	______________
1 x 100g	________	_________________	W2 = W1 + 1 N = _______	_______________
2 x 100g	_______	________________	total weight _________	_______________
4 x 100g	________	______________	_________	_______________
5x 100g	_______	_____________	__________	_____________
7 x 100g	_______	_____________	____________	_____________
9 x 100g	_______	____________	_________________	_________

you can keep going if you the data don't seem good enough

ANALYSIS.

1) Let the total weight (N) be the independent variable and the sliding friction (N) be the dependent variable.
Fill the table (X,Y) below . Get a graph paper and a RULER. I said a RULER. Plot the points. don' t connect the points.
DONT GRAPH ANY LINE YET !

X=total weight (N)	Y= sliding friction (N)
weight block = ____	_____________
____________	____________
_______________	__________
____________	__________
____________	__________

KEEP GOING, ADD ROWS

2) Your graph shows that the relationship between the friction and the weight is ___________

3) Do you think the best fit line should go through the origin ? _____ why ? _____________
so if b is the y-intercept, b = ________. In this particular example/ Find the equation of the best finr line. (y = ax + b , find a and b)
Y = ____________, a = slope = ________, b= ___
THE EQUATION TELLS YOU: sliding friction (that's your y) = _______ weight (that's your x )
You must find a slope smaller than 1. So the sliding friction is pro________________ to the weight and the constant of proportionality
is ___________________ (slope) , the sliding coefficient. THE SLIDING COEFFICIENT IS THE SLOPE !!!

PART II: Does surface affect friction?
procedure

Using the same method, measure the kinetic friction of a block resting on different faces.
Based upon your observations, does surface area affect friction ?

ANALYSIS

1) Why do you think that, surpsringly, the friction DOES not depend on the surface area ?
hint: if you decrease the area, you increase the pressure

GOING FURTHER

1) The simple acts of walking and running would not be possible without friction. Explain.
(think action-reaction, Newtons third law)

2) Why do you need to change oil in a car ?

3) What element part in a car uses friction ? (to stop)

4) How native american used to start fire ? (has to do without friction).

5) The first friction matches were invented by ____________________ in 1827. These matches were tipped with ______ ___________ and ____________________ (chemicals) and were ignited when rubbed against a rough substance.

6) Any object that moves through the air is subject to air resistance, commonly called "drag ". Drag increases with speed and the density of the surrounding fluid. In outer space, there is no air and hence no friction on moving objects but when a meteor enters the atmosphere, friction between the meteor and ____.
rapidly generating heat. The heat is so large that part of the meteorite will vaporize before hitting the EArth.
What would happen without friction between air and the meteorites ?

Sometime, the meteorite are so large that they do make their way to EArth.
Go over this article to answer these questions :
A) How long ago most dinosaurs went extinct ?

B) where did the scientists find the meteorite ?

C) What is the estimated diameter of the meteorite ?

D) What happened after the meteorite fall on EArth ? (Sun)

E) This extinction was called the ___________________

7) You don't want a space shuttle , entering the atmosphere, to be destroyed the same way meteorites are destroyed.
How scientists cope with this problem ? (didn't work with Columbia )