Download NEWTON`S LAWS

NEWTON’S LAWS
Sir Isaac Newton (1642-1727) is probably one of the most remarkable men in the history of science. He
graduated from Cambridge University in England at the age of 23. Records indicate that although he
was a good student, he was not outstanding. He had intended to stay on after graduation but the
University closed down for two years due to the plague. Newton returned home and during these two
years he worked out the law of universal gravitation, explained the motion of the solar system, developed
the laws of motion, invented the calculus, invented the reflecting telescope, and proposed a theory of light
and color. When the University reopened he was made a professor at the age of 26. He lived to the ripe
old age of 85, but never again approached the productivity achieved during the two years Cambridge
closed down.
Here is a list of concepts that you will need to include in your observations and explanations:
MASS, WEIGHT, INERTIA, ACCELERATION, FORCE, DIRECTION, AND FRICTION.
STATION #1: MASS VS WEIGHT
The mass of an object is the amount of matter in the object and can be measured by comparing the mass
of an unknown object with the mass of a known object. This is exactly the principle used when measuring
the mass of an object on a triple-beam balance. The standard unit of mass is the gram. The weight of an
object is a downward force. Standing on a bathroom scale measures your downward force exerted on the
scale and is dependent upon your mass (the quantity of you) and the downward pull of the Earth’s
gravity on your mass. The standard unit of force is the Newton, thus your weight (or downward force on
the scale) is measured in Newtons.
•
Stand on the bathroom scale and record your mass in kilograms. 90.0 kg
The average acceleration of all objects toward the earth is the same, approximately 9.80 m/s/s. Using
Newton’s Second Law of Motion, Force (N) = mass (kg) x acceleration (9.80 m/s/s), calculate your
downward force on the scale.
•
Your weight: 90.0 kg x 9.8 m/s2 = 882 N
STATION #2: AIR FRICTION
An object falling through air encounters an upward force due to the air pushing in the opposite direction
of the moving object. This is air friction. Friction always acts in the direction opposite in which the object
is moving. The greater the mass of the falling object, the greater the force it exerts in a downward
direction. If the downward force is greater than the upward force of the air, the object will accelerate
(speed up) on its way down. If the object's downward force is equal to the upward force of the air,
then the object will fall with a constant velocity (terminal velocity) and an acceleration of zero.
Drop the piece of paper and the metal disk side-by-side to see which one hits the floor first. Repeat, but
place the piece of paper on top of the metal disk so it rides "piggy-back" on the disk. Make sure that the
disk is horizontal. How do you account for your results?
When the paper disk and the metal disk are dropped side-by-side, the metal disk falls at a
greater velocity. Since they are approximately the same size, they encounter about the same air
resistance (upward force) on the way down. The metal disk, since it has a greater weight, or
downward force, accelerates, or has a net force in the downward direction. The paper disk, which
has a smaller weight (less of a downward force), encounters an equal but opposite upward force as
it falls. Therefore, there is no net force on the paper disk, and the paper disk reaches terminal
velocity (no acceleration).
Newton’s Laws
1
When the two are placed "piggy-back", the metal disk pushes the air around the disk, removing
any air resistance, which the paper disk would encounter. This indicates that all objects fall at the
same rate if air resistance is removed.
With the paper provided construct a paper helicopter as you did in the laboratory on Speed &
Acceleration. Take an identical piece of paper and roll it into a tight ball. Drop them simultaneously
side-by-side. What are you observations?
Since they are identical in mass, which has the greater downward force? The helicopter, the paper ball, or
are their downward forces equal?
Provide an explanation why there is a difference in their rates of fall?
The paper ball falls faster than the paper helicopter. This is because the air resistance is less on the
paper ball. Both the helicopter and the ball have the same mass, thus the same weight. Weight is
a force that acts in the downward direction. The reason why the paper helicopter falls slower is
because the air resistance, which acts in the upward direction is greater. When an object falls at
terminal speed, the downward force and the upward force are equal. With no NET FORCE, there
is no acceleration and the object falls at a constant speed.
STATION #3: ACTION / REACTION
A force acting in one direction encounters a force equal and opposite in direction. Adjust the postal scales
to read zero. Hold the scales in front of you and press the right hand scale against the left hand scale.
Read the weight (force) on each scale.
Repeat, but this time press the left hand scale against the one on the right. Note the force on each. Try
increasing the force on the right hand scale. How did the left hand scale react. Describe your results and
provide an explanation.
Newton’s Third Law of Motion states that for every action, there is an equal but opposite reaction.
When one scale is pushed against the other, both scales record the same force. Notice that the
scales are being pushed in opposite directions.
STATION #4: IT'S HAMMER TIME
Common sense tells you that it is not a good idea to pound on the palm of your hand with a hammer.
Newton says its OK, if you place a massive object (2 kg) between your palm and the hammer. Repeat
the same experiment with the less massive object (0.5 kg mass).
Prepare a statement that describes the mass in your hand and the force applied by the hammer required
to accelerate it.
When the hammer applies a force on the 2 kg mass, you do not feel the acceleration of the mass as
much as when the hammer exerts a force on the 0.5 kg mass. The greater the mass, the more
inertia, and the greater the resistance to accelerate (Newton's First Law). The sensation you feel on
your hand when the masses are hit with the hammer is the acceleration of the mass (Newton's
Second Law). You will also notice that the hammer bounced back when striking the larger mass.
This is the reaction force or the force of the mass accelerating the hammer in the opposite direction
(Newton's Third Law).
STATION #5: CELEBRITY BOWLING WITH ISAAC NEWTON
Inertia is a measure of mass. Inertia is the resistance of an object to change its motion (increase or
decrease its speed, or change its direction.) Place one of the bowling balls in front of the mallet. Pull the
mallet back and allow it to strike the ball when you release it. Observe the motion of the ball. Repeat the
procedure, but now use the other bowling ball. How do their accelerations compare?
___________________________________________________________________________
Prepare a statement that describes the relationship between the masses of the bowling balls and their
accelerations when a constant force is applied to them by the mallet.
Newton’s Laws
2
The two bowling balls have different masses. When a constant force is applied to them by the
mallet they have different accelerations. The more massive ball has a lower acceleration than the
plastic ball. The real bowling ball has a greater inertia than the plastic ball. Both Newton’s First
and Second Laws can clearly be applied to this station. When you tap the balls with the mallet
you will feel the reaction force of the balls on the mallet. If the mallet hits the ball, then the ball
hits the mallet (Newton's Third Law).
STATION #6: SPRING INTO ACTION / REACTION
Compress the spring in one of the carts and place it in contact with the other cart. Release the spring and
observe the accelerations of the two carts. Compare the acceleration of each of the carts? Repeat, except
this time double the mass of one of the carts by placing a brick on it (the mass of the brick is equal to the
mass of the cart.) Compare their accelerations.
Although the force of the spring was the same in both cases, how do you account for the resulting
acceleration of the carts in each case? Explain in terms of Newton's Laws.
When the piston of one of the carts is compressed and released, both carts have a force exerted on
them which are equal and opposite in direction (Newton’s Third Law). When a brick is placed on
one of the carts and the experiment is repeated, the cart with the greater mass has a lower
acceleration than the less massive cart (Newton’s Second Law). The greater the mass the greater
the resistance to accelerate (Newton's First Law). Notice that for all situations when an object is
moving or not, all three laws apply.
STATION #7: SPUDS IN MOTION
Push the knife about an inch into the potato. Holding the potato upright with the knife, bring the handle
of the knife down sharply onto the table. Explain your observations.
Your reasoning for behavior of the potato on the knife is exactly why you should wear seat belts in your
car. Why?
This is a clear example of Newton’s First Law of Motion. When the potato is set in motion it has a
tendency to stay in motion. Then, when the knife suddenly stops, the potato keeps moving. Inertia
is a tendency for an object to keep on doing whatever its doing. When a mass is at rest, it resists
anything trying to change its position (that is positively or negatively accelerate it). When a mass
is set in motion it has a tendency to keep moving and resist anything that will change its motion.
Think about this the next time you decide not to wear a seat belt.
STATION #8: A PENNY FOR YOUR THOUGHTS
Place the card on top of the glass and place the penny on top of the card directly over the glass. Give the
card a quick flick with your finger. Did the penny drop into the glass? Provide an explanation for your
observations.
Replace the card with a piece of sandpaper so that the rough side is in contact with the penny. Try to flick
the sandpaper out from under the penny. Why are the results different this time?
You may have witnessed (or tried) to pull a tablecloth out from under a table of dinnerware. Would you
have better luck completing this demonstration with heavy china or with paper plates and cups? Why?
When the coin is placed on the card, and the card is "flicked" out from under the coin, the coin falls
into the cup. Since the coin has inertia, it remains stationary. When the card is replaced by
sandpaper, the increased frictional force between the coin and the sandpaper would accelerate
the coin and it would not fall into the cup. Even though the coin has the same amount of inertia
(mass) the friction of the sandpaper is enough to accelerate the coin. Remember Newton's Second
Law: When a force is applied to a mass it accelerates.
Newton’s Laws
3
STATION #9: PHYSICS IN THE TOY STORE
A. Wind up the surfer-dude and place him in the water. What is the action/reaction pair of
forces responsible for making him move through the water?
The propeller pushes the water backward (action force). The water pushes back on the
propeller pushing the dude forward (reaction force).
B. Place the shaft of the helicopter between your palms and quickly slide your hands apart to
rotate the helicopter. Identify the action/reaction forces that cause it to fly.
The propeller pushes the air downward (action force). The air pushes back on the propeller
pushing the helicopter upward (reaction force).
POST LAB QUESTIONS:
1. Using the concepts from this laboratory, how would you describe how the
motor moves the boat forward through the water?
The propeller of the motor pushes the water backwards from the boat.
The water, in turn, pushes the propeller in the opposite (forward)
direction. The force from the propeller on the water is equal in
magnitude and opposite in direction to the force of the water on the
propeller. This is similar to a jet airplane (air is pushed backwards by the engine) and how a
sprinter in the “blocks” pushes backwards at the starting line.
2. Describe the forces (upward and downward) and the motion of the parachutist after
jumping out of the airplane. Then, describe the forces (upward and downward) and the
motion of the parachutist after the parachute has been deployed.
The downward force (weight) of the parachutist, before after the parachute opens,
are identical. Initially, when the parachutist jumps out of the airplane, the
downward force is greater than the upward air resistance, thus there is a net force in
the downward direction and the parachutist accelerates. Opening the parachute
increases the upward force (air resistance). When the upward and downward forces
are equal, there is no net force on the parachutist and the parachutist stops
accelerating.
3. Tug-of-War was an Olympic event until 1920 and is still a popular sport in Europe. Two equally
matched teams are depicted below. What can you say about the forces acting on the rope? What about
two mismatched teams?
Equal teams have equal forces in opposite directions, thus there is no net force on the rope. When
one team is stronger (a greater force) there is a net force acting in the direction of the stronger
team. The rope (and the other team) accelerates in the direction of the greater force.
4. One shopping cart is full of groceries and the other is empty.
•
If you push both carts with the same force, which has the greater acceleration?
The cart with the greater mass has the lower acceleration.
• Which cart has the greater inertia?
The cart with the greater mass has the greater resistance to changing its motion.
• If both carts were rolling down the hill in Stop & Shop’s parking lot, which would
require a greater force to stop it?
The cart with the greater mass has the greater resistance to stopping (negative
acceleration).
• If you were to push each cart with the same acceleration, which cart would require
a greater force?
The cart with the greater mass would require the greater force.
5. Any object with mass has gravity. Because the mass of the Sun is so large, it’s gravity
is strong enough to keep all of the planets in the solar system orbiting around it. While
there may be no atmosphere on the Moon, it does have gravity. The mass of the Moon
is 1/6 that of the Earth, therefore the acceleration due to the Moon’s gravity is 1/6 that of
the Earth’s gravity. Calculate your weight on the surface of the Moon.
Since the mass of the Moon is 1/6 that of earth, then the acceleration due to the
Moon’s gravity on the surface of the Moon is 1/6 that of earth. 1/6 of 9.80 m/s/s is
1.63 m/s/s. My weight on the surface of the Moon is: 100 kg x 1.63 m/s/s = 163 N.
Newton’s Laws
6