Download 19FMEReview_Solutions

Demonstrations of Force, Motion and Energy for Review
In this activity you will visit each station in the classroom (starting with the setup at
your own table), perform the instructed activity, and answer the appropriate question(s).
Following the completion of these activities, we will briefly discuss the observations
made at each lab station. Instead of handing in a yellow answer sheet today, each group
will, in turn, summarize the results of the activity at their own starting table in a
concluding class discussion. To keep the activities moving along, you will spend 5
minutes maximum at any given station (your instructor will tell you when to change) you will have a chance during the end-of-class discussion to catch up on any questions
you were unable to answer in this allotted time.
Station #1: Can gravity roll an object uphill?
Materials: V-shaped “ramp” device, conical object made of 2 funnels
Investigate the “ramp” device - which end of the device has a higher elevation, the
narrow or wide end?
The wide end has the higher elevation.
Place the 2-cone object at various starting points on the ramp so it can “roll”. Describe
the observed motion.
The 2-cone object rolls from the lower end to the higher end.
Try to explain why this device behaves in the observed manner, in terms of its motion and
energy.
While the object may have been rolling from the lower end to the higher end, the
actual elevation of the object decreased as it rolled to the higher end, due to the
specific design of its shape. Therefore, it started at a position of higher potential
energy, and ended at a position of lower potential energy.
Please remove the conical object from the ramp before moving on to the next station.
Station #2: Exploding carts
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Materials: 2 Pasco carts, several 200 g masses
Practice pressing the “recoil rod” so that you can rig the two carts “explode” with the
press of the button, as shown in the diagram.
Make the
carts
“explode” and observe the process. Repeat the “explosion” , but with several 200 g
masses piled on cart A. Then, move the masses to cart B and repeat the process again.
For all 3 cases, fill in the charts below:
AFTER THE EXPLOSION:
Experiment
How do the final
speeds of carts A and
B compare?
mass of A = mass of B
opposite
equal
mass of A > mass of B
opposite
speed A < speed B
mass of A < mass of B
opposite
speed B > speed B
DURING THE EXPLOSION:
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How do the final
directions of carts A
and B compare?
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Experiment
How do the forces on
carts A and B
compare?
How do the
accelerations of carts
A and B compare?
mass of A = mass of B
equal and opposite
equal
mass of A > mass of B
equal and opposite
accel. A < accel. B
mass of A < mass of B
equal and opposite
accel. A > accel. B
Remove the masses from the carts before moving on to the next station.
Station #3: Colliding carts
Materials: 2 Pasco carts, several 200 g masses
Collide two carts of equal mass, as shown in the diagram above, so they “bounce” apart
upon collision. (Cart A should start in motion, Cart B should start at rest.) Repeat the
collision with several 200 g masses piled on Cart A. Then, move the masses from Cart A
to Cart B and repeat it again. For all 3 cases, fill in the charts below:
AFTER THE COLLISION:
Experiment
How do the final
speeds of carts A and
B compare?
mass of A = mass of B
A was almost still; B
moved along the same
direction
speed of B > speed of A
(A was practically still)
mass of A > mass of B
same direction
speed of B > speed of A
mass of A < mass of B
opposite directions
speed of B < speed of A
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How do the final
directions of carts A
and B compare?
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DURING THE COLLISION:
Experiment
How do the forces on
carts A and B
compare?
How do the
accelerations of carts
A and B compare?
mass of A = mass of B
equal and opposite
equal
mass of A > mass of B
equal and opposite
accel. A < accel. B
mass of A < mass of B
equal and opposite
accel. A > accel. B
Remove the masses from the carts before moving on to the next station.
Station #4: Hammering a nail
Materials: Hammer, nails, wood blocks
There is a lot of physics involved in simply hammering a nail into a wood block. Try
hammering a nail into a wood block. Observe the forces involved, where the forces are
applied, and how energy transfers in this situation.
Hitting a nail with a hammer causes a large force on the nail. How can we explain this
using the Law of Force and Acceleration and the Law of Interaction?
You exert a force to accelerate the hammer, which in turn impacts the nail, exerting
a force on the nail; the nail exerts an equal and opposite force back on the head of
the hammer. The accelerating nail in turn exerts force on the wood, which exerts an
equal and opposite force back on the nail.
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This situation can also be discussed using energy concepts.
How does the hammer acquire energy, and how does the acquired energy relate to how
the hammer is held?
The hammer acquires energy from the person wielding it - holding the hammer
lower on the handle gives the head of the hammer more kinetic energy when it is
swung.
How does the nail remove the energy of the hammer?
The hammer acquires a very large kinetic energy - this kinetic energy is transferred
to the nail. The acquired energy = force times distance; the distance the nail moves
is very small, so the force must be very large - large enough to drive it through the
wood.
Station #5: Sailing with an electric fan
Materials: Pasco cart with a “sail”, small electric fan
Position the fan, resting on the table so that it blows air into the “sail” when switched on,
causing the cart to roll forward. Describe the forces acting on the cart. (Draw a quick
diagram if you prefer.)
The force of air blows on the sail, accelerating the cart forward. The forces on the
cart are unbalanced in this situation.
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Now, snap the fan into place on the cart so that it blows air into the “sail”. Record your
observations when the fan is switched on.
The cart did not accelerate or move (at least it didn’t if it was on a level part of the
table!)
Describe the forces acting on the cart when the fan is mounted on it. (Draw a quick
diagram if you prefer.)
When the fan is sitting on the cart, the air blowing on the sail is countered by an
equal and opposite force pushing the fan backward - this causes the forces on the
cart to balance, and there is no acceleration.
(Before, when the fan was on the table, the reaction force was still THERE, but since
the fan wasn’t “part of” the boat, the reaction force pushing on the fan did not act
on the boat.)
If you are stuck on a sailboat with no wind, and have a huge a electric fan, is there a way
you can direct the fan to make the boat move? Try reorienting the fan so that this
happens.
Yes - you can aim the fan backward, so that the reaction force of the fan pushing on
the air propels the boat forward.
Turn off the fan and remove it from the cart before moving on to the next station.
Station #6: Newton’s cradle
Materials: “Newton’s cradle” device made of several suspended steel spheres
Try using this familiar-looking “toy”. Observe what happens when you lift and release
one sphere on one end. Also, try lifting and releasing two spheres together on one end,
and observe. Try this with three and four spheres and observe. Record your observations.
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What does the Law of Inertia (Newton’s 1st Law) say about this behavior?
A metal sphere will move with a constant speed and direction until acted upon by
another force - in this case, either contact with another sphere, or gravity and and
tension in the string when it swings higher.
What does the Law of Interaction (Newton’s 3rd Law) say about this behavior?
Metal spheres in contact will exert equal and opposite forces on one another - a
forward force accelerates a sphere on the far end forward, and an opposite force
accelerates the moving sphere to a stop.
What does the Law of Force and Acceleration (Newton’s 2nd Law) say about this
behavior?
The acceleration of the spheres is directly related to the force of impact and the
mass - all the masses are equal, so the motion appears symmetrical on both sides.
Because the masses are equal, the force of the weight is also equal on each sphere, as
well.
Station #7: Ring the chime
Materials: Box with a string attached to a hammer and metal “chime”
By pulling on the string, we pull on the spring which pulls on the hammer. One pull
doesn't do much, but by pulling many times we can make the hammer hit the chime.
Practice using this device until you can reliably get the hammer to strike the chime.
Describe what action was necessary to get the chime to ring.
The string had to be pulled once per swing in rhythm with the swinging hammer,
when it was at the highest point back in its “swing”.
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What is the implication of the relationship
Energy transferred = force × distance
to this device? (In other words, what conditions were necessary to transfer enough
energy to the hammer so it swings high enough to strike the chime?)
Only a small amount of string was given (a small distance), so the energy
transferred to the hammer by any given pull was very small - therefore energy had
to be added to the swinging hammer in small increments. Timing was also essential,
as a poorly timed tug could as easily remove a small amount of energy from the
hammer as it could add it.
Station #8: Falling and sinking objects
Materials: book, sheet of paper, graduated cylinder, 100 g mass, water
If a piece of paper and a book are dropped side-by-side from a height of 50 cm, will they
fall at the same rate?
Obviously not - the paper falls more slowly.
Predict what will happen if you drop the book with a piece
of paper flat on TOP of it. Will the paper and book fall at the
same rate? Test your prediction.
This time, they both fell at the same rate. Since the book
was underneath, it pushed any air out of the way as it fell, so that the paper did not
have to. Without air resistance, all falling objects have the same rate of acceleration.
Fill the graduated cylinder about 3/4 full with water. Drop a 100 g
mass in the cylinder and observe it “sink” to the bottom. As the
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100 g mass “sinks” through the water, is it accelerating, or is its speed fairly constant?
The speed of the weight is fairly constant as it sinks.
What can you tell about the forces on the 100 g mass as it “sinks”?
Because there was no acceleration, there was no net force on the 100 g mass as it
sank - the force of gravity was exactly balanced by the upward buoyancy of the
water underneath it.
How is a mass sinking in water similar to a falling object experiencing air resistance?
Both experience an upward force that counters the force of gravity.
(The highest speed an object reaches during its fall due to air resistance is called its
“terminal velocity”, a term you hear used in skydiving, for example.)
Empty the graduated cylinder and remove the 100 g mass before moving on to the next
station.
Station #9: The floating ping pong ball
Materials: ping pong ball, wad of paper, balance, hair dryer
Using the balance, find how the mass of a ping pong ball compares to the mass of a paper
wad.
The masses are approximately equal.
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Aim a hair dryer upward and turn it on. Try to balance a ping pong ball in
the stream of air, so that it hovers in place. Describe the forces acting on
the ping pong ball while it is in this position.
Because the ping pong ball is held (more or less) stationary in the air,
all the forces acting on the ball, up and down, left and right, forward
and backward, must be balanced, in every single direction. In other
words, the sum of the weight of the ball and all the air pressure against
the ball must cancel in every direction.
Try to repeat the same procedure, except this time, use a wad of paper instead of a ping
pong ball. Record your observations, and think about what differs between the paper and
the ping pong ball.
This didn’t work with the paper wad - this means the forces were not balanced.
There must be something about the shape and/or smoothness of the ping pong ball
that causes the forces to balance (it’s not the mass - these were the same!)
Place materials back into their initial positions before moving on to the next station.
Station #10: The truck on the ramp
Materials: A truck with a heavy mass on a ramp countered by a pulley with a hanging
smaller mass
Pull the truck to the bottom of the ramp, and release it. The truck (with the weight in it)
weighs about 6 Newtons, while the hanging 2 Newton weight is just able to pull the truck
up the incline.
How is this related to an activity that you did?
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In the Force and Energy lab, you measured the amount of force required to pull a
cart up an incline with varying steepness. This force lessened as the distance
increased.
How is this related to the concepts of potential energy, kinetic energy, and the
conservation of energy?
The energy in the system was constant - the 2-N weight started with potential
energy, which, as it fell, transferred to the 6-N truck rolling uphill in the form of
kinetic energy and added potential energy to the truck. Force times distance lost
cannot be greater than the force times distance gained by the truck - so although the
heavier weight is lifted, the distance upward it is lifted is much smaller than the
distance the 2-N weight falls.
Station #11: Riding on a roller coaster
Materials: Metal ramp with “hills” and “valleys”, 2 steel balls of different size
Gently release the small steel ball from one end of the ramp. Describe your observations.
The ball rolls back and forth between the 2 ends (which are at equal height),
gradually slowing down to a complete stop in one of the two “valleys”.
Explain what happens to the energy of the ball as it moves on the track after it is released.
The ball starts with potential energy when lifted to either end - when it is released,
potential energy is converted into kinetic energy as it speeds up, and vice versa when
it slows down going uphill. Eventually, friction causes energy to leave the system,
and the ball slows to a stop.
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Try releasing the ball from various points in the track, and observe the behavior. Using
your observations and the principle of energy, explain why you know the ball will always
remain on the ramp after it is released.
Conservation of energy says no new energy can enter the ball when it is released - so
it cannot end up at any location higher than the end of the ramp, as this would
require it have more potential energy than it started with at rest.
Using the principle of energy, predict if the large steel ball will behave any differently
than the small steel ball. Test your prediction.
The large steel ball behaves similarly - even though the steel ball has more potential
energy when it is lifted to the top, it also acquires more kinetic energy when it
speeds up. Both potential energy and kinetic energy are proportional to the mass of
the object, so both the large and small steel balls behave similarly.
When you have completed all 11 stations, return to your original lab table, and
spend a few minutes discussing how you will briefly explain the results for the
station at your table to the class. As much as possible, incorporate physics terms
like energy, force, acceleration, velocity, interaction, inertia, etc. in their proper
contexts into your explanation.
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