Newton

1
Newtons First and Second Laws of Motion,
Inertia, Mass, Volume, Weight, centripetal force
and rotational inertia.

• Stations 1-7

2
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.

3
Station 1

• 1. The penny at rest stays at rest due to its
  inertia. This illustrates static equilibrium.
• 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).
• 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).

4
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.

5
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.
• 3. How do Newtons first and second laws of
  motion apply to the air track and sled?
• 4. As more mass is added to the sled, what
  happens to the sleds inertia?
• 5. If the air track was one mile long, describe
  the sleds motion after receiving a push. Would
  it require a force to keep moving? What type of
  equilibrium does this demostrate?

6
Station 2

• 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 sleds
  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.

7
36. To pull a wagon across a lawn at a constant
velocity, you have to exert a steady force. Does
this contradict Newtons 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.

8
Station 3 Penny with Hanger

• What type of motion is illustrated when the penny
  is rotated in a circle using the hanger?
• What is keeping the penny on the hanger?How does
  this activity illustrate Newtons first law of
  motion?
• What would the penny do if the hanger was
  removed? Use the first law of motion to explain.

9
Station 3

• Acceleration is illustrated by the rotating
  penny.
• 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.

10
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.

11
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.

12
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.

13
Station 4

1. Most mass bowling ball
2. Greatest volume bowling ball
3. Greatest weight bowling ball
4. Most dense bowling ball
5. 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.

14
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)

15
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)

16
38. If an elephant was chasing you, its enormous
mass would be very threatening. But if you
zig-zagged, the elephants mass would be to your
advantage. Why?

• Because the elephants 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.

17
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.

18
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.

19
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.

20
Station 5

• What happened to the cart and clay figure during
  the test?
• Why did the clay figure continue to move after
  the cart hit the wood block? Use Newtons First
  and Second laws of Motion to explain.

21
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).

22
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.

23
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.

24
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.

25
Station 6

• Weight Calculation Practice (show work, use sig
  figs)
• Measure the mass of the block
• Convert mass to kilograms
• Calculate the weight of the block in pounds.
• Convert the weight from pounds to newtons.
• Multiply the mass (in kg) by acceleration rate
  due to gravity (on Earth) to find Fgrav (the
  weight of the object in newtons).

26
Station 6

• 1. Mass of wood block 217.2 g
• 2. Kilograms-- 0.2172 kg
• 3. 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

27
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 Earths surface or
  to another place (such as the moon).

28
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.
• 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 its
  the passenger that moved.

29
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 linemoving of its own inertia.