The Effect of Weight on Sliding Friction Between a Block and Surface
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In this investigation we are going to find out what effect weight has
on sliding friction. Friction is the force that is between two objects
that are in contact with each other. The force is always present even
though people only think it is there when the object is moving.
Leonardo da Vinci was one of the first people to experiment with the
idea of friction. He came up with two types of friction; static
friction, which is the friction, which is present when the object is
stationary and kinetic friction, this type of friction is present when
the object is moving. Static friction is greater than kinetic
friction. Kinetic friction is the type we are going to be
investigating. Kinetic friction occurs when the surfaces of the two
objects move against each other. Even if the surface appears
completely smooth there will always be minute bumps and lumps on it.
The lumps on the one surface move against the lumps on the other
surface and produce the force of friction.
I predict that in this experiment if you double the weight the
friction on the object will also double. I have based this prediction
on the formula shown below.
Normal Force (Fn)
This formula shows that the force of friction is proportional to the
original force/ weight of the object. The formula is the work of
Leonardo da Vinci. He said
"Friction produces double the amount of effort if the weight is
This should be because if the force on an object is increased then the
forced needed to move it would also have to increase.
In this experiment I will make sure that I keep everything the same
except the weight of the block, as the weight is my variable. The
things that I must keep the same are:
Ø Surface the block is sliding on
Ø The wooden block- because the grain of the wood would change and
also the initial starting weight would be different.
Ø The Newton meter
Ø The slope of the surface
The reason that everything other than the variable must be kept the
same is so that is makes it a fair test. A fair test means that we
only change the one thing, called the variable, so that we can see
that it is only the variable that is affecting the results.
Before we conducted our investigation we carried out a preliminary
experiment. We did this so that we could test which surface would be
best to use in our experiment based on the readings that we gained
from the preliminary investigation. These results were as followed:
Force of Friction (N)
Type of surface
From the preliminary results it's clear to see that the best surface
to use would be foam, therefore I have chosen to use foam in my main
experiment. I have chosen foam because it gave me the best range of
results. From a good range of results I will be able to draw a clearer
conclusion. As well as foam, polystyrene gave a good range of results
but the piece of polystyrene I had was too small, because just as I
got the wooden block at a constant speed it reached the end of the
material. This made recording results very difficult and more likely
to be inaccurate as the block may not be moving at a steady speed and
therefore the results would be inaccurate.
Ø Foam surface
Ø Wooden block
Ø Newton meter
Ø Nine 100gram weights
Ø Flat surface that will stay continuously at the same angle
1) Firstly I am going to set up the equipment as shown in the diagram
2) I will then pull the wooden block until it is moving at a steady
speed. I will do this because according to Newton's first law of
motion Newton said
This means that when an object is moving at a steady speed the two
forces acting on the object are equal. I will use this information to
record the force of friction because when I pull the block my pulling
force will be measured on the Newton meter, so therefore if the block
is moving at a steady speed the force of friction will be the same as
my pulling force.
3) Once the block is moving at a steady speed I will record the
4) I will then repeat this but place one of the 100g weights on top of
the block, so as to increase the weight.
5) Once I have added the extra weight I will pull the block across the
foam until yet again it is moving at a steady speed.
6) I will again record the results.
7) I will continue adding the weights one at a time until all of the
weights have been used up and I will record the results each time.
The experiment will be carried out three times; this was done to make
the results more accurate. It makes the reading more accurate as it is
To ensure that my experiment is carried out in the safest way possible
I will conduct it in the middle of the table so that the weights do
not fall off the table and onto any part of my body.
Weight of Block (N)*
^ Adding together the three sets of results and dividing by three find
Average force (N) ^
* 100g = 1N
On the previous page was my graph to show the results from the
investigation. I showed that as the weight of the block increases the
force of friction acting on the block also increases. From first
looking at my graph it looks as though my prediction was incorrect
because the force does not double when the force is doubled. On a
closer inspection however you can see that the graph does not start at
zero. Therefore you need to rearrange the equation y = mx + c. you
must do this because is the graph was to continue backwards then the
line would eventually cross the x line. When it does the equation for
the line is y - c = mx. C stands for the frictional force present on
the block even when there are no weights on top of it. This force is
here because the block still has weight even though there are no
weights on top of it. The C must be taken away from the force that has
been recorded. Once this has been taken away it will be able to see if
my prediction that the weight is doubled then the force of friction is
doubled is right. Here are some examples to explain this:
[IMAGE][IMAGE]Y - C = mx
10.1 - 2.6
(The 2.6 is taken away because that is the original weight of the
5 - 0.9
(The 5 is the frictional force I recorded and the 0.9 is where the
line started on the y-axis.)
5 - 0.9 = 10.1 - 2.6
4.1 = 7.5
y - C = mx
2 - 0.9 = 4.6-2.6
1.1 = 2
These calculations are done using the figures from my graph. There are
lines on the graph to indicate where I got these statistics.
From these examples you can see that my prediction was in fact correct
as the weight or original force increases the force of friction
increases by the same amount.
I feel that, my results were relatively accurate though I do feel that
they could be improved by having a Newton meter that has more accurate
readings. For this experiment however I think that the results are
accurate enough to prove the theory and my prediction. The force of
friction did in fact double when the weight of the object was doubled.
This reverts back to my prediction and the workings of Leonardo da
Vince and another man called Guillaume Amontons. He said
"The friction made by the same weight will be of equal resistance
although the area of contact may be different lengths and breadths;
the friction produces double the amount of effort if the weight is
Amontons said that he noticed patterns of behaviour when two surfaces
moved against each other. He said that the roughness of the surfaces
was to blame for this. They were proved to consist of minuscule dip,
mountains and cavern that interlock with each other as they pass over
one another. By raising the upper object, in this case it is the
wooden block; you would reduce the amount of friction. So therefore by
pushing harder on the upper object you would increase the friction and
a greater force would be needed to pull the block and this proves my
results and my prediction.