EXPERIMENTAL MEASUREMENTS TO STUDY SLIDING FRICTION, THE DECELERATION
OF OBJECTS AND OBJECTS THROWN HORIZONTALLY
Janos Kosztyu
In Hungarian secondary grammar schools physics is a compulsory subject for four years.
These experiments are usually carried out by Year 9 pupils in classes of about 30.
These experiments only last a few minutes and you can do them easily using simple
instruments. They were developed by myself and pupils in an extracurricular science club
and have been performed many times by my colleagues and I. In our experience pupils have
always enjoyed taking the experimental measurements, participating with great enthusiasm
and often improving the experiments with their own ideas. They also enjoy doing
calculations with their own data and drawing conclusions from the experiments.
SLIDING FRICTION AND DECELERATING OBJECTS
Physical quantities to be measured:
initial velocity: v0
acceleration: a
coefficient of slide friction: m
force of friction: Ffr.
The questions we are going to put to the students:
What is the acceleration ?
What is the frictional force ?
What is the coefficient of sliding friction ?
Instruments and requirements:
A long table with a smooth surface, an object to be thrown, a tape measure, a balance
to measure the mass of the sample and some stop-watches.
Experiment:
We throw the object to slide it across the table. It stops on the table (the final
velocity vt = 0) after travelling some distance S.
Measurements:
the distance of the slide S
the time of slide t
the mass of the object m
Calculations:
To do the calculations the students have to remember these formulas:
the distance and the final velocity for the accelerated motions
S = v0.t + a t2 / 2
vt = v0 + a t ;
the force for sliding friction on the horizontal surface
Ffr = m m g .
These need to be rearranged to give
a = 2 ( vt t – S) / t2
v0 = vt – a t
m = a / g
Ffr = m m g
THROWING OBJECTS HORIZONTALLY
Physical quantities to measure:
initial velocity: v0x
initial velocity: v0y ;
the final velocity: vy ;
the final velocity: v .
The questions we are going to put to the students:
How does the motion change vertically?
How does the motion change horizontally?
What are the horizontal and vertical accelerations?
Instruments and requirements:
A long table with a smooth surface, an object to be thrown, a tape measure, a balance
to measure the mass of the sample and some stop-watches.
Experiment:
We throw the object to slide it across the table and have it fall off the end. The
distance is reaches from the end of the table is measured.
Measurements:
The students measure: the distance that the object slides on the table S2, the
time it slides for t2, the height of the table h and
the distance from the edge of the table S3 .
Calculations:
To do the calculations the students have to remember these formulae
the height of the fall
h = g t2 / 2
the formulas for the velocities
vx = vox t
vy = g t2 / 2
the initial velocities
v0y = 0
v0x = vt2
where vt2 is the final velocity of the sample before falling from the
table in the second experiment ;
the distance
S3 = v0x t
the final velocity of the sample at the moment of falling
v = ÷ (vx2+ vy2
)
These need to be rearranged to give
t = ÷ (2 h / g )
v0x = S3 / t = vx - the velocity of
the sample in the direction 0X;
vy = g t - the velocity of the sample in the direction 0Y;
v = ÷ (vx2+ vy2
)
vt2 = v0x ;
v02 = ( 2 S2 – vt2 t2 ) / t2 ;
a2 = ( vt2 – v02 ) / t2 ;
m 2 = a2 / g .
The two last data a2 and m 2 have
to be compared with a and m measured earlier in order to allow
the possibility of making a conclusion that the acceleration does not depend upon the
initial velocity of throwing as the coefficient of friction and the force of friction.
In conclusion, I hope that the experiments that I have written about will make
you consider using such experiments in your physics lessons also. Our results should be
the students who enjoy the study of physics, and realize its great beauty.
