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05 Friction - lab report

lab report

Course

College Physics Lab (PHY 1621L)

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St. John's University

Academic year: 2023/2024

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Experiment 5: Coefficients of Friction

Purpose

(1) To become familiar with the concepts of static and kinetic friction.

(2) To measure the coefficients of static and kinetic friction for a plane.

Theory

I. Static Friction on a Horizontal Board

If an object, resting on a horizontal surface is pulled by a horizontal force F (Fig 1), the

surface will exert a force of friction f s

(force of static friction) which exactly balances F until F

reaches a critical value F crit

. F

crit

is the maximum value of the static force of friction f max

.

Until this point, the net force is zero and the object remains at rest. Above this point the net force

is not zero and the object will move. Experiments show that f max

is proportional to the normal

force N:

f

max

= F

crit

= 

s

N

where  s

is the coefficient of static friction.

In your experiment, an object of mass M will be sitting on an inclined plane in a

horizontal position. A force F will be applied by hanging a mass m from a string attached to the

mass. (Fig. 2) You will gradually increase the hanging mass until the force from its weight mg

equals the maximum force of static friction f max

. Beyond that point the object would slide since

the net force is no longer zero.

II. Static Friction on an Inclined Board

On an inclined plane the force of gravity Mg is downward. There are components of the force of

gravity parallel and perpendicular to the surface of the plane. Figure 3 shows the components

parallel and perpendicular to the surface of the inclined plane. The normal force N on the block

is equal to the component of the force of gravity that is perpendicular to the surface

M

block

g cos s

. The gravitational force parallel to the plane which can cause the block to slide

down is M block

g sin s

. If the block doesn’t move, the force of friction is equal in magnitude

and opposite in direction to the force of gravity down the slope (parallel to the surface of the

plane). Equation 1 still applies: f max

= 

N = M

block

g cos s

.

Part III: Determination of 

k

with horizontal board

(7) Place the board in horizontal position. Put the block on the board and adjust the pulley so

that the string is horizontal.

(8) Put some weights on the hanger and tap the board with your finger so that the block slides

slowly at a constant speed. Your goal is the make the force from the weights equal the force of

kinetic friction. If you do not have enough weight, the block will stop. If you have too much

weight, the block will accelerate. Determine the smallest load m weights

which will make the

block slide very slowly without stopping.

(9) Repeat this procedure with extra masses M extra

of 1, 2, 3 and 4 kg put on top of the block.

Record in a table the smallest loads for each which will result in a slow uniform motion.

Remember that we must add the mass of the hanger to the mass of the weights to get the total

load.

M

extra

(g)

m weights

(g)

m load

= m weights

m hanger

(g)

0 1000

2000

3000

4000

Part IV: Determination of 

k

with inclined board

(10) Remove the block and disengage the hanger. Raise the board to about 5 degrees from the

horizontal. Place the block on the board. Tap the board slightly. If the block moves and then

stops the force of gravity from the block’s mass is too small to overcome the force of friction.

The angle is too small.

(11) Gradually increase the angle and determine the angle  k

for which the block will

consistently creep down (after tapping the board) without acceleration. Record  k

with an

accuracy of 0 degrees.

Before you leave the lab

Make sure you know how to draw a graph of your results in Part III and how to analyze it.

Lab Report

Calculation of 

s

Calculate  s

from the equation



M

block

g = m load

g using your data.

Calculate  s

from your data for

Part II using Equation 3 from

right.

Display the accuracy of your work by using the following formula:



s

(Part I) - 

s

(Part II)

.5 [

s

(Part I) + 

s

(Part II)]

Notice that the denominator is the average of the Part I and Part II values for  s

.

Calculation of 

k

Make a plot of m load

vs M extra

from your data table for Part III and draw a line of best fit.

Don’t forget to include the value for M extra

= 0. Calculate the slope and find the y intercept of

your best fit line.

From Equation 1 , we know that f max

= 

N. We

also know f max

= m load

g. Therefore,

f max

= 

N = 

M

block

f max

= m load



M

block

g = m load

In Part II, the maximum force of friction is equal to the maximum

force of gravity down the slope (ie parallel to the surface of the

plane) for which the block doesn’t slide.

f max

= F

gravity



M

block

g cos  s

= M

block

g sin  s



cos  s

= sin  s

Equation: 3



= sin  s

= tan  s

cos s

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