UNIT 1 - ENERGY & MOTION
Physics
Lab 1.3 Forces, Friction & Integrated Forces
This lesson deals with
friction forces and forces used to store energy.
The video for this lesson
illustrates the following experiments:
- Measure static and
dynamic friction between two surfaces
- Measure the elastic
potential energy stored in an archers’ bow
Get
Started
After watching the video presentation, answer the
questions in the exercises below and review the solutions.
Experiments
Carry out the experiments described below.
Record the results using
the data tables provided. Enter the experimental results
in the appropriate columns and calculate the values
needed to complete the data tables.
Answer the questions that
are associated with the experiments.
- When the tension in a
spring balance is slowly increased in order to
start pulling a block of wood that is attached to
it across a smooth surface, the block jumps when
it starts to move. Why does the block jump as it
starts to move?
- Spring balances are
usually calibrated to measure mass but actually
measure the force of gravity on the object being
weighed. If the force of gravity on 1 kilogram is
9.81 Newtons, what is the downward force (in
Newtons) that is acting on a spring balance if it
gives a weight reading of 0.5 kg?
- If a spring balance
gives a mass reading of 100 grams, what is the
downward force in Newtons?
- A block of wood with
a mass of 330 grams is resting on a table. What
force does it exert on the table top?
- If a force of 0.5
Newtons is needed to keep the block sliding
horizontally across the table, what is the
dynamic coefficient of friction between the
wooden block and the surface of the table?
Answers to Questions
Experiment
1.3.1 Friction Forces
The purpose of this
experiment is to measure frictional forces and to
demonstrate the difference between static and dynamic
friction.
Materials and Equipment:
- Two block of wood,
one with a hook at the end
- Pulley
- Piece of string
- Spring balance
Procedure:
Place the block of wood on
the flat surface with one of its largest surfaces in
contact with the smooth surface.
Attach the piece of string
to the block, thread the string throught the pulley so
that an upward movement of the end of the string results
in a horizontal movement of the block.
Attach the spring balance
to the end of the string.
Gently pull the spring
balance upwards until the block just starts to move.
Record the tension force
in the string needed to get the block to start moving.
This is an indication of
the static friction between the block and the smooth
surface.
Keep the block moving by
moving the spring balance upwards.
Record the tension force
in the string needed to keep the block moving slowly
forward.
This is an indication of
the dynamic friction between the block and the smooth
surface.
Test the effect of
pressure and area on the amount of friction between the
two surfaces.
Turn the block of wood on
its side so that the area in contact with the other
surface is smaller.
Turn the block back onto
it’s wider side. Place a metal weight with a mass of
between 200g and 800g on the block of wood.
Results – Static
Friction
| |
Mass of block (grams)
|
Equivalent force between surfaces (N)
|
Surface area in contact |
Maximum weight reading BEFORE movement (grams)
|
Equivalent force (N)
|
Static Coefficient of friction |
| 1 |
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| 2 |
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| 6 |
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Results – Dynamic
/ Sliding Friction
| |
Mass of block (grams)
|
Equivalent force between surfaces (N)
|
Surface area in contact |
Maximum weight reading AFTER movement (grams)
|
Equivalent force (N)
|
Dynamic Coefficient
of friction
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| 1 |
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| 2 |
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| 3 |
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| 6 |
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Questions
- Does the area in
contact between the surfaces affect the amount of
static and/or
dynamic friction?
- Does the pressure
between the two surfaces affect the amount of
friction?
Experiment 1.3.2 Bows and
Forces
Purpose: The
purpose of this experiment is to measure the energy
stored in an archer’s bow as the string is pulled
away from the bow.
Materials and Equipment:
- Bow
- Spring
balance: 2 – 5 kg scale
- Clamp
- Tape measure:
1 – 2 m
Procedure:
Important
Safety Note: Wear
safety glasses while working with the spring and bow
under tension.
- Clamp the bow onto a
surface or board on which the distance of the
string from its normal position can be marked.
- Use the spring
balance to draw the string 2 cm away from the bow.
- Record the reading on
the spring balance.
- Mark the position of
the point where the hook of the balance connects
with the string on a piece of paper attached to
the board underneath the bow. The spring balance
reading can be written next to this mark.
- Draw the string a
further 2 cm and repeat the process of recording
the tension and the position of the string.
- Record the tension in
the spring balance another 12 times.
Results
| |
Spring balance reading
(grams)
|
Force
(N)
|
Total distance
(mm)
|
Increase in distance
(meters)
|
Force x distance
(J)
|
| 1 |
200
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| 2 |
400
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| 3 |
600
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| 4 |
800
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| 5 |
1000
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| 6 |
1200
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| 7 |
1400
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| 8 |
1600
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| 9 |
1800
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| 10 |
2000
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TOTAL
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J
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Calculations
The work done on the bow – and hence the energy
stored in the bow – is the sum of the products of
force x incremental distance. If the variation of force
with distance were plotted, this would be equivalent to
the area under the graph.
A good approximation could be obtained by taking the
average force between two successive readings and
multiplying this by 0.02m for each incremental distance
of 2 cm moved.
Question
If a line graph is plotted with the force needed to move
the string on the y-axis and the distance moved by the
string on the x-axis, how would the area under the line
be related to the energy stored in the bow?
Answer
Answers
to Questions on Friction
- The block jumps
because the force needed to overcome dynamic
friction is less than the force needed to
overcome static friction. Friction opposes the
movement of the block across the surface. Before
the block can start to move, the force that is
acting to cause movement needs to be greater than
the force of friction that opposes movement. When
there is no relative movement between the two
surfaces that are in contact, the force of
friction that opposes movement is known as static
friction. When the block starts to move and there
is relative movement between the two surfaces,
the force of friction that opposes movement is
known as dynamic or sliding friction. Static
friction is usually larger than dynamic friction.
If a spring is used in the process of applying a
force to oppose friction, the object being moved
jumps because, once movement starts, the force
applied is greater than the force needed to
overcome dynamic friction.
- 4.905 N: 0.5 kg x 9.81
N/kg = 4.905 N
- 0.981 N: 0.1 kg x 9.81
N/kg = 0.981 N
- 3.24 N: 0.33 kg x 9.81
N/kg = 3.24 N
- 0.154: The
coefficient of friction is the (sliding) force
needed to cause relative movement of the two
surfaces divided by the force that keeps the
surfaces in contact. The force keeping the two
surfaces in contact is the weight of the block
which is 3.24 N. (See question 4 above)
Coefficient of friction = 0.5 N ¸ 3.24 N = 0.154.
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