Download Terminal Velocity activity Basic Procedure

Terminal Velocity activity
Explore forces and motion using a parachute
Kit contents: scissors (students or teacher should have), string, tape, 2 pennies, metric ruler, timer
Volunteer should bring: non-cling plastic wrap or plastic grocery bags
Key concepts: speed, velocity, acceleration, balanced forces, unbalanced forces
Science Standards:
Student knows that the motion of an object is determined by the overall effect of all the forces acting on the object
(SC.C.2.2.4).
Student recognizes various forms of energy (SC.B.1.2.2).
Students knows that through the use of science processes and knowledge, people can solve problems, make
decisions, and form new ideas (SC.H.3.2.4).
Basic Procedure
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Use ruler and measure a piece of plastic wrap 15 cm x 15 cm. Cut out the square.
Cut four pieces of string 20 cm long and tape to the corners of the bag.
Bring the free ends of the four strings together and tape them to a penny.
Hold the parachute from the middle of the square and drop it from high over your head. DO NOT STAND ON
TABLE or CHAIR.
Describe how it falls.
Make another parachute 30 cm x 30 cm and strings 35 cm long.
Release both parachutes at the same time.
Do multiple trials and have students record results.
You can use the timer and have students record time.
This is a good activity for the scientific method and variables. Discuss potential and kinetic energy. Be sure to have
students form a hypothesis, discuss the variables in the experiment and make a conclusion.
Key Knowledge
Vocabulary
force - a push or a pull that causes changes in motion
gravity - force of attraction between an object and Earth
balanced forces - forces that act on an object but cancel each other
unbalanced forces - forces that act on an object and cause a change in motion
position - location of an object in space
speed - distance an object travels in a certain amount of time
velocity - measure of an object's speed in a particular direction
acceleration - rate at which velocity changes
terminal velocity - velocity an object reaches at the point when all forces acting on the object are balanced so the
object is no longer accelerating.
Concepts
Terminal Velocity. In this case of a dropped object, the object is accelerated downward by gravity, giving it a velocity
down. At a certain velocity, the "terminal velocity", the force upward from the air resistance is equal to the force
downward from gravity, and the object no longer accelerates. The object keeps going down, though, because it had a
velocity before the forces were balanced.
Measurement of motion. A given object's motion can be described in several ways. Its velocity is the current motion,
and the acceleration is the rate of change of velocity. The acceleration is due to an unbalanced force. There can also
be balanced forces acting on that object. For example, an object sliding across a frictionless surface on the ground,
similar to a sheet of ice, is being acted on by gravity and by an upward force from the ground, so the net force is zero
and there is no acceleration. If an object is travelling at constant velocity, there are no unbalanced forces.
Newton's first law: No acceleration can happen without a force. An object at rest tends to stay at rest; an object in
motion tends to stay in motion
Newton's second law: An object's acceleration depends on the object's mass and the force applied to it. Force =
mass x acceleration
Script idea
What we will talk about today--forces and motion.
[This activity, like Gravity of It All, deals with forces, motion, and gravity. Try to tie in lessons from the previous activity
as much as possible.]
Review motion vocabulary with students. Words that might come into the discussion will be distance, velocity,
acceleration, force, gravity.
Take two pieces of paper. Wad one up into a ball. Ask students to predict which will hit the ground first. Why do they
think the wadded one hit the ground first? [Less air resistance.]
What does air resistance mean? It is a force that resists motion through air due to the presence of air molecules in
the path of the object.
Show students a basic parachute with one paper clip. What do they predict will happen when you let go of the
parachute? Demonstrate it.
What happened? Try to elicit motion words such as accelerated, velocity, etc. What forces were acting on the
parachute?
Introduce the idea of terminal velocity, that at some point the forces were balanced.
What factors/variables might influence the terminal velocity of the parachutes? What can we measure to determine
the effect of these variables?
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Size of the parachute
Weight of the payload
Shape of the parachute
Height from which the parachutes were dropped
How the parachutes were dropped
Where the parachute was held as it was dropped
Once the students have come up with several possible variable, have the groups decide which variable they want to
test. Pass out plastic bags, rulers, scissors, tape, weights, timers.
Remind students to do multiple trials and record their data.
After students have had some time to test their variable, have them report their results to the class. Encourage them
to use scientific terms where possible. If time allows, let students try a second variable.
A possible extension might be to have students drop the helicopters at different heights to demonstrate that the
velocity does continue to stay constant. Have them drop it at a low height where they still see terminal velocity, then
at various other heights. Have them plot the difference in time (from the original drop height) versus the difference in
height (from the original drop height). They have in their books that speed = distance/time, so they can calculate the
terminal velocity.
Challenge questions for students: What would happen to the helicopters in a vacuum? [They would all fall at the
same speed, regardless of size or weight.] What would happen on the moon? [The moon has no atmosphere but a
lower gravity than Earth, so they would fall slower than in a vacuum on Earth, but the parachute wouldn't puff up.
Mass wouldn't matter.] What would happen to them in the space station? [Nothing--no gravity. Of course, if there
were a fan blowing, they might move.]
Parachute Science Background
Forces on the parachute
The two forces acting on the parachute are gravity and air resistance. A free body diagram of the parachute would
have a force up, the air resistance, and a force down, gravity.
The force of gravity causes a constant acceleration (9.8 m/s on Earth), regardless of mass. It can be written as:
Fg=mg
With no air resistance (in a vacuum), free falling objects should hit the ground at the same time. The sum of the
forces on the object is:
FT=Fg
To solve for the acceleration on the object, you substitute maT for the FT (Newton's second law), and get
maT=mg
aT=g
The force that gravity exerts on a given object is related to its mass. (Weight is a measure of the force of gravity--it
changes depending on the whether the object is on the moon or earth or another place.)
In the case of the parachutes, the force of gravity is counteracted by the force of air resistance. Air resistance is what
keeps the two different weight objects from hitting the ground at the same time.
A heavier object with similar air resistance (the same sized parachute with more washers) will have a higher terminal
velocity. This fact seems to go against the principle that mass doesn't matter in the acceleration due to gravity.
However, the force of gravity in the two cases is different. The air resistance increases as the velocity increases. The
sum of the forces on the parachute is
F = mg - Fair
where m is the mass of the parachute, g is the acceleration of gravity, and Fair is the force of the air resistance, which
is a function of the velocity. If you solve for acceleration, first you substitute F with ma then,
ma = mg - Fair
a = g - Fair/m
From this one can show that the higher the mass, the less the air resistance decreases the acceleration. At terminal
velocity, when a = 0, then mg = Fair . Therefore, the larger the mass, the large the force of the air resistance must be.
As air resistance is a function of velocity, then the larger the mass, the higher the velocity when the acceleration
equals 0.
Helicopter Script
What we will talk about today--forces and motion.
[This activity, like the previous one, deals with forces, motion, and gravity. Try to tie in lessons from the previous
activity as much as possible.]
Review motion vocabulary with students. Words that might come into the discussion will be distance, velocity,
acceleration, force, gravity.
Take two pieces of paper. Wad one up into a ball. Ask students to predict which will hit the ground first. Why do they
think the wadded one hit the ground first? [Less air resistance.]
What does air resistance mean? It is a force that counteracts the force of gravity.
Show students the helicopter with one paper clip. What do they predict will happen when you let go of the helicopter?
Demonstrate it.
What happened? Try to elicit motion words such as accelerated, same velocity, etc. Why do they think the helicopter
spun? Draw an approximate force diagram showing where the forces are acting.
What factors/variables might influence the terminal velocity of the helicopters? What can we measure to determine
the effect of these variables?
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Size of the helicopter shaft
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Weight of the helicopter
Material used to make the helicopters
Height from which the helicopters were dropped
How the helicopters were dropped
Where the helicopter was held as it was dropped
Where we added the paper clips to the helicopter
Once the students have come up with several possible variable, have the groups decide which variable they want to
test. Pass out paper helicopters from pre-printed (and probably precut) templates (Template1, Template2,
Template3) depending on the variables they choose. (Cut on solid lines, fold on dotted lines.)
Remind students to do multiple trials and record their data.
After students have had some time to test their variable, have them report their results to the class. Encourage them
to use scientific terms where possible. If time allows, let students try a second variable.
Challenge questions for students: What would happen to the helicopters in a vacuum? [They would all fall at the
same speed, regardless of size or weight.] What would happen on the moon? [The moon has no atmosphere but a
lower gravity than Earth, so they would fall slower than in a vacuum on Earth, but wouldn't spin. Mass wouldn't
matter.] What would happen to them in the space station? [Nothing--no gravity. Of course, if there were a fan blowing,
they might move.]
Forces on the helicopter
The two forces acting on the helicopter, as with the parachute, are gravity and air resistance. In this case, however,
the air resistance is not resulting in only an upward force on the helicopter. The wings are only attached on one side,
so, in the absence of gravity (if it was being blown by a fan), the wings would line back up with the body of the
helicopter. That tendency indicates that there is both a component of the force upward and a component inward.
Since the inward component on one wing is in the opposite direction and on the opposite side of the center of the
helicopter from the inward component on the other wing, the helicopter starts to spin.
One way to think about it is, as the air hits the wing, it can be viewed as being deflected to the side. As Newton's laws
state, for every action there is an equal and opposite reaction. Therefore, when the air moves one direction, the
helicopter moves the other.
The remainder of the background is much like that of the parachute activity.