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TEACHER MASTER
Forces - Science 8
Concept of Force
Learning Outcomes Addressed
C5 Explain the concept of force
Define force (push or pull of one object on another)
List different types of forces (eg magnetic, friction, gravitational, elastic, electrical)
Differentiate between mass and weight
Describe the movement of objects in terms of balanced and unbalanced forces
C8 Explain the relationship between pressure, temperature, area and force in fluids
It is important to note the difference between a textbook definition and a working operational
definition. The concept of mass, force, and energy are all very elusive and are not fully
understood by modern science. “Action at a distance” is a very difficult phenomenon. In
Science 10 on a cd there is an excellent activity that utilizes an excerpt from Richard
Feynman’s book “Surely you must be joking Mr. Feynman?” He explains the danger of
equating naming something and thinking you understand it.
Mass
Start by letting a pen drop from your hand and asking why did the pen fall. Students will likely
use a mysterious term called “gravity”. A more logical answer would include –Because you
let it go … but that is another discussion–
Use the glossary in your textbook to define mass:
Now use a glossary to define matter:
We encounter mass and its properties every day yet we still do not know what it is. There are
several strong theories however. In terms of density; mass is what is contained in a
certain volume.
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Do demonstration: Which brick has more mass?
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The inertial concept of mass can be defined as:
The resistance to change in motion
Anything that has mass will require a force applied to it to change its motion. So a pen that
is caused to go from stationary (moving at zero) to moving at a different speed has had a
change in motion. Therefore a force must be applied to it to cause it to ‘fall’.
Notes and Activities – Science 8
TEACHER MASTER
Forces - Science 8
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Do demonstration: Accelerometer
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A bucket on a rope being swung around in a circle has a force being applied to it along the
direction of the rope towards the center where the hand is holding the rope. This felt by
the person swinging the bucket as a constant pulling or tension in the rope. Do not mistake
that there is a force outwards on the rope. The only force is inwards.
Mystery Question
A young dude goes out on a Friday night but does not have a date. So he draws a happy
face on a helium balloon and tapes its string to the passenger seat. As he comes to a stop
and his head continues to go forward; the helium balloon goes backward. He momentarily
confused by this opposite behaviour. When the light turns green he accelerates the car
forward leaving his head to apparently swing backwards. Yet the balloon goes forward.
Explain how the happy balloon head goes in the opposite direction of confused young dude’s
head.
Answer
The balloon is less dense than the air around it. Consequently it floats above
the denser air. When the car accelerates forward the air inside it is stationary. As the back
window moves forward it pushes the air forward and this causes the air to be denser at the
back of the car compared to the front. Thus the balloon goes forward. This is the same
reason why when a car turns a sharp corner there is no force that pushes you against the
door. Rather the door is pushing on you to turn you from forward motion to a curved motion.
This is a change in motion and therefore a force is being applied.
[This can be demonstrated by filling a garbage bag ¾ full and tying off the end. Then pull the
bag by the tied end. The other saggy end will fill with air.]
Weight
There is a force of attraction between any objects that have mass. This is called
“gravitational force”. So the pen that falls from a hand is attracted to the earth and the earth
is attracted to the pen. The amount of attraction is weight.
If the pen is dropped on the moon (which has a lot less mass than earth) the attraction is not
as strong. So the pen will have less weight and will not fall at the same rate. But it will still
have the same mass.
Notes and Activities – Science 8
TEACHER MASTER
Forces - Science 8
Questions
1) If you stub your toe on a brick on earth it hurts quite a bit. If there was no difference in
footwear would it hurt more, less or the same if you kicked it on the moon?
Answer
It will hurt the same. Since the mass is the same and the change in motion is
the same it will require the same force.
2) If things are virtually weightless in space why does the Canada Arm being used to
assemble the International Space Station have to be such a strong crane?
Answer
The objects being moved have huge amounts of mass. And it requires a great
force to change their current motion.
3) List 5 types of forces and give an example of each:
Answer
Magnetic – compass
Friction – air resistance(drag)
Gravitational – waterfall
Elastic – slingshot
Electrical – Static cling
Note that there are other forces. For example: Nuclear
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Do activity: Types of Forces
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Notes and Activities – Science 8