Transcript First Law of Motion - Fort Thomas Independent Schools

Newton’s First and Second
Laws of Motion, Inertia, Mass,
Volume, Weight, centripetal
force and rotational inertia.
Stations 1-7
Station 1: Cup and pennies with card
 1.
How does the law of inertia apply to this activity? What
type of equilibrium does this illustrate?
 2.
Use the 1st and 2nd laws of motion to explain why the penny
did not move away with the card when the card was flicked
away?
 3.
Predict what would happen if the index card was replaced
with sandpaper? How could the 1st and 2nd laws of motion to
explain.
Station 1
1. The penny at rest stays at rest due to its inertia. This
illustrates static equilibrium.
2. The penny resists any change in velocity unless it
receives an unbalanced force (1st law). Acceleration
of the penny did not occur because the net force
acting horizontally on the penny was essentially
equal to zero (2nd law).
3. Sandpaper would increase the friction force on the
penny, which could provide an unbalanced force
greater than zero (2nd law) that could cause the
penny to be accelerated and overcome inertia (1st
law).
34.
Tim practices a demonstration before doing it for
Sunday dinner. What concept is he illustrating, and why is he
careful not to pull the tablecloth slightly upward?
 He is demonstrating the law of inertia (objects
at rest stay at rest unless receiving an
unbalanced force). Pulling upward would lift
the plates and cause them to be unbalanced.
Station 2: Air Track Questions
 1.
What force is removed from the track when the positive air supply
is turned on?
 2.
Describe the motion of the sled in one direction when it is pushed
with the air supply turned on.
How do Newton’s first and second laws of motion apply to the air
track and sled?
 3.
As more mass is added to the sled, what happens to the sled’s
inertia?
 4.
If the air track was one mile long, describe the sled’s motion after
receiving a push. Would it require a force to keep moving? What type
of equilibrium does this demostrate?
 5.
Station 2
1.
2.
3.
4.
5.
Friction is removed.
The sled on the air track demonstrates that an object in
motion will continue moving with constant velocity until an
unbalanced force changes the velocity.
Objects in motion stay at a constant velocity unless acted
upon by an unbalanced force (1st law). The sled does not
receive an unbalanced force until making impact with the
bumper (2nd law).
As more mass is added to the sled, the sled’s inertia
increases.
The sled would continue to move at a constant velocity and
requires no force to keep it moving, due to its inertia. This is
dynamic equilibrium.
36. To pull a wagon across a lawn at a constant velocity, you
have to exert a steady force. Does this contradict Newton’s
first law?
 No. The key is net force. Your steady force
is balancing the rolling friction, which means
that the net force = 0, which means dynamic
equilibrium, which means constant velocity,
which means no acceleration. Removing
friction would allow the wagon to continue
moving at the same velocity without any more
pull force.
Station 3: Penny with Hanger
1.
What type of motion is illustrated when the penny is
rotated in a circle using the hanger?
2.
What is keeping the penny on the hanger?
How does this activity illustrate Newton’s first law of
motion?
3.
What would the penny do if the hanger was
removed? Use the first law of motion to explain.
Station 3
1. Acceleration is illustrated by the rotating
penny.
2. It takes an unbalanced force to cause the
penny to turn in a circle or to keep it on the
hanger. This unbalanced force is called
centripetal force, and keeps the penny
turning.
3. If the hanger was removed, the penny would
take a straight path due to its inertia.
8.
When you whirl a can at the end of a string in a
circular path, what is the direction of the force that acts on
the can?
 The force is directed inward towards the axis
of rotation. This inward-directed force is
called centripetal force.
42.
Can an object move along a curved path if no force
acts on it?
 No. An object moving along a curved path is
accelerating and requires an unbalanced
force to cause it to turn. This type of force is
referred to as centripetal force.
Station 4
1.
Which ball has the most mass?
2.
Which ball has the greatest volume?
3.
Which ball has the greatest weight?
4.
Which ball is the most dense?
5.
Rank the balls in order from greatest to least inertia.
6.
Predict whether a solid disk or a hollow disk with roll down a
ramp faster. Test to verify and explain why.
Station 4
Most mass= bowling ball
Greatest volume= bowling ball
Greatest weight = bowling ball
Most dense = bowling ball
Most Inertia to least= bowling ball, tennis ball,
styrofoam ball
6. The solid disk rolls faster down the ramp because it
has less rotational inertia. The greater the rotational
inertia, the greater the resistance to rotation.
Rotational inertia increases as the mass of the
object is distributed further from the center of
rotation.
1.
2.
3.
4.
5.
32. In an orbiting spacecraft, you are handed two identical closed
boxes, one filled with sand and the other filled with feathers. How can
you tell which is which without opening the boxes?
 The one that is easier to shake back and forth
is the one with less mass (less inertia, less
resistance to changes in motion)
37.
When a junked car is crushed into a compact cube,
does its mass change? Its volume? Its weight?
 Mass remains unchanged.
 Volume is reduced.
 Weight remains unchanged (no change in
position within the gravitational field so no
change in gravitational force)
38. If an elephant was chasing you, its enormous mass would
be very threatening. But if you zig-zagged, the elephant’s
mass would be to your advantage. Why?
 Because the elephant’s mass is greater than
yours, so is its inertia. Therefore, the
elephant would have more difficulty switching
directions than you because its inertia is
greater.
40.
Which has more mass, a 2-kg fluffy pillow or a 3-kg
small piece of iron? More volume? Why are your answers
different?
 The iron has more mass (greater) and the
pillow has more volume (takes up more
space. The answers differ because mass
and volume are completely different
concepts.
20.
Beginning from rest, a solid disk, a solid ball and a
hollow disk race down an incline. What happens?
 The solid ball moves fastest, followed by the
solid ring and the hollow disk. The solid ball
has the least amount of rotational inertia
because its mass is distributed closest to the
center or axis of rotation.
31.
Consider two rotating bicycle wheels, one filled with
air and the other with water. Which would be more difficult
to stop rotating? Explain.
 The bicycle wheel filled with water would be
the more difficult wheel to stop rotating
because it has the greater amount of
rotational inertia or resistance to change its
rotational motion.
Station 5
1. What happened to the cart and clay figure
during the test?
2. Why did the clay figure continue to move
after the cart hit the wood block? Use
Newton’s First and Second laws of Motion to
explain.
Station 5
1.
Both the cart and the clay figure move at the
same speed in the same direction until the cart slams
into the wood block. Then the clay figure moved
forward after the cart stopped.
2.
The cart receives an unbalanced force, which
causes the cart to decelerate rapidly (2nd law), but the
clay figure does not, so it continues to move at the
same speed and in the same direction due to its
inertia until it receives an unbalanced force (1st law).
33.
Many auto passengers suffer neck injuries when
struck by cars from behind. How does NL of I apply? Why
headrests?
 The body is accelerated forward with the
seat, but the head remains (behind) at its
current velocity until an unbalanced force
pulls it forward (which is the neck, which can
cause whiplash if forceful enough).
 Headrests provide the unbalanced force
needed to accelerate the head with the body.
35.
Suppose you place a ball in the middle of a wagon that is at
rest and then abruptly pull the wagon forward. Describe the motion of
the ball relative to the ground.
 Except for some change in motion due to
friction between the wagon and ball, the will
be no motion of the ball relative to the
ground; but relative to the wagon, the ball will
appear to move toward the back.
43.
The head of a hammer is loose and you wish to tighten it by
banging it against the top of a workbench. Why is it better to hold the
hammer with the handle down as shown rather than with the head
down?
 The handle stops when it hits the bench, but
the relatively massive head tends to keep
moving towards the handle and tightens.
Station 6
Weight Calculation Practice (show work, use sig figs)
1.
Measure the mass of the block
2.
Convert mass to kilograms
3.
Calculate the weight of the block in pounds.
4.
Convert the weight from pounds to newtons.
5.
Multiply the mass (in kg) by acceleration rate due to gravity (on Earth) to find
Fgrav (the weight of the object in newtons).
Station 6
1.
Mass of wood block: 217.2 g
2.
3.
Kilograms-- 0.2172 kg
Weight of wood block: convert using 1 kg = 2.2 lb
0.2172 kg x 2.2 lb = 0.48 lb
Convert pounds to newtons 1 lb = 4.45 N
0.48 lb x 4.45 N = 2.1 N
Calculate weight using w = mg
w = 0.2172 kg x 9.8 m/s2 = 2.1 N
4.
5.
41.
Is it more accurate to say that a dieting person loses
mass or loses weight?
 It is more accurate to say that a dieting
person is losing mass (the amount of matter
that composes the person). A reduction in
mass leads to a reduction in weight.
 It is possible to change weight (only) if the
person moves further from the Earth’s surface
or to another place (such as the moon).
Station 7
The baseball, although encountering a small
amount of air resistance across the diamond,
continues to move without a force, due to its inertia.
Only an unbalanced force with change its
horizontal velocity.
2. Tendency for an object to resist acceleration is
inertia.
In the plane= 0 km/h, outside observer = 925 km/h
 The coin keeps the same velocity in every situation.
It is the passenger that changes speed and
direction. This makes it appear to the passenger
that the coin moved, but the coin maintains its
velocity due to its inertia; it’s the passenger that
moved.
1.
31. A space probe can be carried by a rocket into outer space. Your
friend asks what kind of force keeps the probe moving after it is
released from the rocket and on its own. What is your answer?
 Nothing keeps the probe moving. With no
propelling force it continues moving in a
straight line—moving of its own inertia.