sce 3105 - pract 2
Post on 22-Feb-2015
710 Views
Preview:
TRANSCRIPT
PRACTICAL 1
TOPIC: FORCE AND MOTION
AIM/ OBJECTIVE:
1) To identify the different types of force in different contexts.
2) To measure the acceleration of a freely falling object.
HYPOTHESIS:
There are forces acts on any objects in any situation or contexts. There can
be different types of force acting on the objects in their different contexts
where a situation context is not only involving just one type of force but also
can be more than one type.
The acceleration of freely falling objects on this earth will give the same value
of acceleration due to the gravity that is 9.8 ms-2, where the acceleration is
independent of mass.
1
Activity 1: Identifying Different Types Of Force In Different Contexts .
STATEMENT OF PROBLEM:
What kind of different types of force identified in different context?
THEORETICAL KNOWLEDGE:
1. Force is a vector quantity. Force has both magnitude and direction. There are
six types of forces. They are friction, normal, spring, tension, thrust and
weight.
Force Definition
Friction, Ff The contact force that acts to oppose sliding
motion between surfaces.
Normal, FN The contact force exerted by a surface on an
object.
Spring, Fsp A restoring force, that is, the push or pull a spring
exerts on an object.
Tension, Ff The pull exerted by a string, rope, or cable when
attached to a body and pulled taut.
Thrust, Fthrust A general term for the forces that move objects
such as rockets, planes, cars and people.
Weight, Fg A long- range force due to gravitational attraction
between two objects, generally Earth and an
object.
2
2. When a body is in equilibrium, the resultant force on it is zero. For example,
the Figure 1 below shows the force acting on the book and a box. The forces
that act on them are balanced. The forces cancel each other so that the
resultant force is zero. So the book and the box are said to be in equilibrium.
Figure 1
INSTRUMENTS AND MATERIALS:
Materials Instruments
A wooden block No
A ping-pong ball
A stretched wire on a wooden block
A beaker
A book
3
VARIABLES:
Manipulated: Contexts of situation
Responds: Types of force identified
Kept constant: Acceleration due to gravity (9.8 m s-2)
PROCEDURE:
1. A wooden block is placed on the floor. The wooden block is pushed and the
observations are recorded.
2. A ping-pong ball is thrown up into the air. The observations are recorded.
3. A piece of wire is tied tightly between 2 nails hammered into a piece of wood.
Then, the wire is touched. The observations are recorded
4. A ping pong ball is placed into a beaker of water. The ball is tried to be
pushed into the water. The observations are recorded.
5. A book is placed on the palm of the hand. The observations are recorded.
6. The inferences are stated for each of these observations.
4
RESULTS:
Procedure Observation Inferences
Procedure 1 The wooden block is moving after
being pushing and then stop.
Presence of normal force (FN),
weight force (Fg), friction force
(Ff) and thrust force (Fthrust).
Procedure 2 The ping-pong ball is moving up into
the air and then falling down.
Presence of weight force (Fg),
thrust force (Fthrust) and friction
force (Ff).
Procedure 3 The wire is stretched or moved
slightly from its initial position and
then turn back to the initial position.
Presence of tension force (FT)
Procedure 4 The ping-pong ball resists from the
water or cannot sink into the water.
Presence of weight force (Fg) and
thrust force (Fthrust).
Procedure 5 The book stays static or immobile on
the palm of hand.
Presence of weight force (Fg) and
thrust force (Fthrust).
Table 1
5
DISCUSSION:
1. Activity 1 shows me that there are forces acting on any objects in any
situations. There can be many types of forces acting on an object depend on
its situation context. There is also situation that just involving a force action.
The forces can either make the objects moves or stay static (equilibrium
state).
2. I notice that an object is being acted by forces to be in its situation. For
example, Procedure 1 that makes the wooden block move after being
pushing and then stop is because of the presence of normal force (FN), weight
force (Fg), friction force (Ff) and thrust force (Fthrust) which acting on it. Here,
the wooden block is moving because of the thrust force and then slows down
because of the friction force.
3. The ping-pong ball is moving up into the air and then falling down because of
the presence of weight force (Fg), thrust force (Fthrust) and friction force (Ff).
The ping-pong ball is moving up into the air by the thrust force, and then
falling down because of the Earth’s gravity is pulling it downwards with a
weight force. At the same time, the air resistance that acts again gravity
causes a friction force which pulls the ball downwards.
4. The wire is stretched or moved slightly from its initial position and then turn
back to the initial position due to the presence of tension force (FT) along the
wire.
5. The ping-pong ball resists from the water or cannot sink into the water
because of the presence of weight force (Fg) in the ball itself and thrust force
(Fthrust) in the medium of water.
6
6. For the last procedure, the book stays static or immobile on the palm of hand.
Why? It is because of the presence of weight force (Fg) and thrust force
(Fthrust). The book is said to be in equilibrium. The forces of weight and thrust
that act on them are balanced. The forces cancel each other so that the
resultant force is zero.
Activity 2: Measuring The Acceleration Of A Freely Falling Object
STATEMENT OF PROBLEM:
How to measure the acceleration of a freely falling object
THEORETICAL KNOWLEDGE:
1. Gravitational field is a region around the earth in which an object
experiences a force towards the centre of the earth. This force is the
gravitational attraction between the object and the earth. The gravitational
field strength at the surface of the earth is 9.8 N kg-1. This means that an
object of mass 1 kg will experience a gravitational force of 9.8 N.
2. Measurement with more accurate instruments shows that the acceleration
due to the gravity at the surface is also 9.8 m s-2. The acceleration due to
gravity does not depend on the mass of the falling object. All objects falling
freely fall with the same acceleration.
7
3. In Diagram 1 below, all the masses are falling freely (gravity is the only force
acting). From F = ma, it follows that all the masses have the same downward
acceleration, g. This is the acceleration of free fall.
Diagram 1
4. Acceleration tells us how fast the speed of something in changing.
Acceleration can be shown very clearly on a speed- time graph. We calculate
the acceleration by working out how much the speed changes in one second.
5. We can say the units as ‘metres per second, per second’. This is because the
acceleration tells us how much the speed has changed (in metre per second)
in each second.
Acceleration = Change in speed
Time taken
8
INSTRUMENTS AND MATERIALS:
Materials Instruments
A ticker-timer No
Ticker tape
Clamp
Power pack
Connecting wires
Masses of 300 g, 450 g and 600 g
Cellotape
VARIABLES:
Manipulated: Masses of the objects
Responds: Accelerations of the freely falling objects
Kept constant: Frequency of ticker timer (50 Hz)
PROCEDURE:
1. A chair is placed on top of a table. A ticker-time is clamped onto the upper
side of the chair as shown in Picture 1.
2. A piece of ticker tape is fixed through the ticker-time and a 300g mass is
cellotaped to the tape.
3. The ticker time is switched on and the mass is left to be falled freely
downwards.
9
4. The tape is cut into 2 dots strips and they are arranged to form a graph.
5. Step 3 is repeated by using masses of 450 g and 600 g.
6. For each case, a graph is build from their strips and then their accelerations
are determined.
Picture 1
10
RESULTS:
Mass (g) Acceleration (ms-1)
300 7.361 ms-1
450 7.963 ms-1
600 8.242 ms-1
Table 2
A) Calculating acceleration for mass 300 g:
a) Time covered for the 3- tick strip:
= 3 x 0.02
= 0.06 s
b) The average initial velocity over 3- tick strip, u:
u = 2.0
0.06s
= 33.33 cms-1
c) The average final velocity over 3- tick strip, v:
v = 19.2
0.06s
= 320 cms-1
d) Time taken for the change in velocity, t:
t = T7 - T1
11
= 0.42 – 0.06
= 0.36 s
e) Acceleration of the freely falling 300g load, a300:
a = v – u
t
a = 320 - 33.33
0.36 s
= 796.31 cms-2
B) Calculating acceleration for mass 450 g:
a) Time covered for the 3- tick strip:
= 3 x 0.02
= 0.06 s
b) The average initial velocity over 3- tick strip, u:
u = 2.1
0.06 s
= 35 cms-1
c) The average final velocity over 3- tick strip, v:
v = 18.0
0.06 s
= 300 cms-1
d) Time taken for the change in velocity, t:
t = T7 - T1
12
= 0.42 – 0.06
= 0.36 s
e) Acceleration of the freely falling 300g load, a450:
a = v – u
t
a = 300-35
0.36 s
= 736.1 cms-2
C) Calculating acceleration for mass 600 g:
a) Time covered for the 3- tick strip:
= 3 x 0.02
= 0.06 s
b) The average initial velocity over 3- tick strip, u:
u = 6.6
0.06 s
= 110 cms-1
c) The average final velocity over 3- tick strip, v:
v = 24.4
0.06 s
= 406.67cms-1
d) Time taken for the change in velocity, t:
t = T7 – T1
13
= 0.42 – 0.06
= 0.36 s
e) Acceleration of the freely falling 300g load, a450:
a = v – u
t
a = 406.67-110
0.36 s
= 824.08 cms-2
► Average of acceleration value:
= 796.31cms-2 + 736.1cms-2 + 824.08cms-2
3
= 785.5 cms-2
= 7.86 ms-2
DISCUSSION:
14
1. In activity 2, I calculate the value of acceleration for each mass as recorded
by using the below formulas in Table 1. The time for a tick is 0.02 s is due to
the main voltage in Malaysia that is an alternating current (AC) with a
frequency of 50 Hz. This provides a very reliable means of measuring time
because the interval between each cycle is exactly 1 s (i.e 0.02 s).
50
Find 1st strip 7th strip
The displacement of the 10-
tick strip, x
x1 X7
The time covered for the 3- tick
strip
t1 = 0.06 s T7 = 0.06 s
Average velocity over the 3- tick
strip, u
u = x1
t1
v = x7
t7
Change in velocity between the
two 3- tick strips, v - u
Final velocity – Initial velocity
= v - u
Time taken for the change in
velocity, t
t = Time taken for the six strips – Time taken for
the first strip
Acceleration, a a = Change in velocity (v – u)
Time taken (t)
Table 1
2. Then, I get the average of the acceleration values of freely falling objects. The
average is 7.86 ms-2. This value is not accurate enough compared to the
actual value of gravity acceleration that is 9.8 ms-2 because of some small
errors made while doing the experiment.
3. An object is falling freely when it is falling under the force of gravity only.
However, the air resistance can affect the fall either being smoothly in freely
15
falling or not. The objects of masses are considered to fall freely because the
air resistance is small compared to the pull of gravity and therefore is
negligible. So, the experiment is considered successful although the
acceleration value is not so accurate because of the air resistance is
everywhere on this Earth except in vacuum.
4. Another error while doing the experiment is the angle between the ticker- time
and the mass which is going to fall freely is not in 90° degree. Besides, there
is friction between the ticker- timer and the ticker tape also. These all affect
the last result because the masses are not freely falling smoothly.
5. During the experiment, the time of the mass fall is not synchronizing with the
time to switch on the power pack. This is also one of the small reasons of why
the acceleration values of different masses are not so accurate. Anyhow, the
experiment is still considered successful.
PRECAUTIONS:
1. Make sure the ticker timer is in good condition before connecting it to the
power supply.
2. We must be careful of the environment during the experiment. The fan should
be switched off because it can interrupt the falling of the object either may be
freely or not.
3. We must take at least three strips to get a more accurate acceleration in
average.
4. The ticker-time must be clamped onto the upper side of the chair wisely so
that it results accurately.
5. The ticker tape should be fixed through the ticker-time thoroughly because it
will influence the smoothness of freely falling downwards of the objects.
16
QUESTIONS AND ANSWERS:
1. Are the values of acceleration of the different masses obtained equal to g
(acceleration due to gravity on earth)? Explain.
The value of acceleration of the different masses obtained is not so equal to g
(acceleration due to gravity on earth). It is because of some errors done in the
experiment. The air resistance affects the freely falling of the masses from being
smoothly. However, the experiment is considered successful although the
acceleration value is not so accurate because of the air resistance is everywhere
on this Earth except in vacuum. Then, the angle between the ticker- time and the
mass which is going to fall freely is not in 90° degree. Besides, there is friction
between the ticker- timer and the ticker tape also. These all affect the last result
of not being so equal enough because the masses are not freely falling smoothly.
Lastly, the time of the mass fall is not synchronizing with the time to switch on the
power pack producing the acceleration values of different masses are not so
accurate.
2. What conclusion can be made about the force acting on a freely falling
object?
The force acting on a freely falling object is a weight force or gravity due to Earth.
Object is exerted by air resistance, which opposes its motion The acceleration of
freely falling objects on this earth gives the same value of acceleration due to
gravity that is 9.8 ms-2, where the acceleration is independent of mass. The small
air resistance interrupts the acceleration a bit but still shows their equality
because air resistance is everywhere except in vacuum.
17
3. If this experiment was done on the moon, will the value of acceleration of
the different masses obtained be equal to g? Explain.
No. If this experiment was done on the moon, the value of acceleration of the
different masses obtained will not be equal to g. It is because the value of gravity
on the moon is 1/6 to the value of gravity on the earth. If the experiment is done
in vacuum on Earth, yes, they will be equality in acceleration although different in
mass because the objects will fall freely as same as the Earth gravity without any
disturbance or air resistance. It is due to the gravity acceleration that is the same
everywhere on the Earth. Yes, being at the moon is just being in vacuum but its
gravity is not the same as the Earth.
Mastery Of Knowledge And Skills Questions:
(a) What have you learnt from the practical?
This experiment has taught me the different types of forces exist in different
contexts of situation. The objects involved in any situations either in moving or
static are because of the forces action. Besides, the types of force involved can
be more than one. I learn how to make such correct observations in identifying
the forces. I then can explain scientifically about the reasons towards the
observations.
I also learn how to handle the instruments and materials involved in the
experiment correctly and thoroughly. I have learned how to measure the
acceleration of the objects especially free fall objects using the formulas. Lastly, I
know the relation between the forces and the free fall object. I know why their
accelerations are the same although different in mass that is due to the gravity
acceleration.
18
(b) Which part of the primary science curriculum teaches this particular topic?
The part is in Investigation Force and Energy theme, Year 5.
(c) Discuss how you can use the above practical in your teaching and
learning?
First of all, I will teach the student about the concept related to this experiment in
the class before doing experiment in the laboratory. I will make sure that they are
understood first before doing the experiment. How to teach them are many ways.
I can do demonstration or relate the situation with their experiences or daily life.
Then, when turning to the hands on application for meaningful learning, I will
make them do the experiment. I will show them how to do first. I will show the
skills required. I will give a clear briefing to the students regarding the experiment
while demonstrate it first. I will show how to use the apparatus well in order to get
an accurate result and record the result. Then, I will let them do themselves in
group. Lastly, I will be with them in the calculation process for the last result.
(c) What was the role of the teacher in the above practical?
Teacher acts as a facilitator, guider and demonstrator in the above experiment.
Teacher guides students how to do the experiment and being together with the
students along the experiment in order to facilitate them. Indeed, teacher always
demonstrate how to do first before let the students do themselves. Teacher
should encourage the students in their application of scientific skills such as
solving problem, measuring and analyzing. Here, the support from the teacher is
important for the students to elaborate their answers. Yes, they will learn the best
from the hands on activity in the experiment.
19
20
top related