Download 6.26.2(es)

Inquiry 6.2
Testing Balanced and Unbalanced Forces
Directions: Follow the procedures below. Write down the title, purpose, and background information.
Purpose: How is an unbalanced force different from a balanced force?
Background Info:
Inertia: Objects will not change their motion until an unbalanced force acts on them.
Balanced forces: Forces that are the same but pull or push in opposite directions. Balanced forces do
not change the motion of an object.
Unbalanced forces: Forces that cause a change in the motion of an object.
Newton (N): A unit force in physics.
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Procedure:
1. Place the white paper in the bottom of the plastic box. Put the metal ring on
top of the paper as if you were putting the metal ring onto a jar. Trace the
outside of the ring onto the paper. Remove the metal ring from the paper.
Mark 4 points at an equal distance around the circle. Number the places 1 to 4
going clockwise, like in the image on the right.
2. Place the metal ring on the circle as you did before. Now place the marble
inside the metal ring. Without moving the metal ring, describe the motion of the marble.
a. How is the marble moving right now? (Don’t over think your answer)
3. Now make the marble move in a clockwise circle (inside the ring). Keep the ring on the paper at all times.
Discuss your observations:
b. What is happening to the marble as you move the metal ring?
c. How does the metal ring produce an unbalanced force that influences the marble’s motion?
d. Why is the marble not moving to the sides of the plastic container?
4. Discuss as a group what would happen if you lifted the ring while the marble was moving inside. (This
would be removing the unbalanced force.) Move the marble in clockwise circle again by lifting the ring.
e. What happened to the marble?
Table A Number the marble
5. While having the marble move in a clockwise circle, try to pick up the ring
left the circle at:
when the marble at mark #2. Record in Table A which mark # the marble
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actually left the hand drawn circle. Then draw the path of the marble (on
the sheet of paper in the box) as it left the circle and hit the wall of the
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container. Do this 3 times and record your data. Discuss what you noticed
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about the path of the marble.
g. Did the marble leave the circle exactly at number 2?
6. Discuss what you noticed about the path of the marble as it left the drawn circle.
7. Just like the marble, the planets move forward due to inertia and inward due to an unbalanced force.
Together, these forces cause the planets’ paths to curve.
h. What is the unbalanced force that keeps the planets in orbit?
i. What would happen to the planets without this unbalanced force?
j. What evidence do you have to support your answer to question h above?
Orbital Motion
How does gravity affect orbital motion in the solar system? The Sun’s gravity continuously pulls the
planets toward the Sun. But the planets also have a natural tendency to move forward in their orbits at a
constant speed and in a straight line until acted upon by an outside force. These two motions – forward motion
and motion toward the Sun under the influence of gravity – keep the planets traveling in curved paths in
elliptical (oval shaped) orbits around the Sun. If the gravitational force pulling on a planet is bigger, in addition
to the planet’s tendency to continue moving forward, then its orbital velocity will be bigger too. Without the
Sun’s gravity, all objects in the solar system would move a straight line into outer space.
All planets orbit the Sun in a counterclockwise direction (as seen form Earth’s North Pole) with those
planets closest to the Sun traveling the shortest distance. Because the planets and the Sun originally were
formed from the same rotating disk of dust and gas, it is logical that the counterclockwise direction of planetary
revolution coincides with the Sun’s counterclockwise rotation. Most of the planets rotate in a counterclockwise
direction as well. Venus is an exception because it is tilted nearly 180˚ onto its axis. This means that from the
Earth’s North Pole, Venus appears to rotate in a clockwise, or retrograde, direction.
Unbalanced Forces Defined
Unbalanced forces are forces that cause a change in the motion of an object.
Any push or pull is a force. A force can do three things to an object: cause an object
to start moving, stop moving or change direction. To describe a force, you must
know two things. You must know the size of the force and the direction of the force.
Suppose two teams are playing tug of war. Each team is pulling with equal force, but
in opposite directions. Neither team can make the other team move. Forces that are equal in size but opposite in
direction are called balanced forces.
Balanced forces do not cause a change in motion. When balanced forces act on an object at rest, the
object will not move. If you push against a wall, the wall pushes back with an equal but opposite force. Neither
you nor the wall will move. Forces that cause a change in the motion of an object are unbalanced forces.
Unbalanced forces are not equal and opposite. Suppose that one of the teams in tug of war pulls harder than the
other team. The forces would no longer be equal. One team would be able to pull the other team in the direction
of the larger force.
Force and Motion
More than one force can act on an object at the same time. If you hold a paper clip near a magnet, you,
the magnet and gravity all exert forces on the paper clip. The combination of all the forces acting on an object is
the net force. When more than one force is acting on an object, the net force determines the motion of the
object. In this example, the paper clip is not moving, so the net force is zero.
How do forces combine to form the “net force”? If the forces are in the same direction, they add up
together to form the net force. Suppose you and a friend are asked to move a piano for the music teacher. To do
this, you pull on one end of the piano, and your friend pushes on the other end. Together, your forces add up to
enough force to move the piano. This is because your forces are in the same direction. Because the forces are in
the same direction, they can be added together to determine the net force.
If two forces are in opposite directions, then the net force is the difference between the two forces, and it
is in the direction of the larger force. Consider two dogs playing tug of war with a short piece of rope. Each is
exerting a force, but in opposite directions. Notice that the dog on the left is pulling with a force of 10 N, and
the dog on the right is pulling with a force of 12 N. Because the forces are in opposite directions, the net force
is determined by subtracting the smaller force from the larger one. In this case, the net force is 2 N in the
direction of the dog on the right. Give that dog a treat!
Unbalanced Forces in Action
Unbalanced forces can change the motion of an object in two ways. When unbalanced forces act on an
object at rest, the object will move. Unbalanced forces are necessary to cause a non-moving object to start
moving. Second, when unbalanced forces act on a moving object, the velocity of the object will change.
Remember that a change in velocity means a change in speed, direction, or both speed and direction.
For example, consider a soccer game. The soccer ball is already moving when it
is passed from one player to another. When the ball reaches the second player, the player
exerts an unbalanced force - a kick - on the ball. After the kick, the ball moves in a new
direction and with a new speed.
Reading Questions: Answer in complete sentences in your spiral -
1) Describe a “balanced force” and give an example of one.
2) Describe an “unbalanced force” and give an example of one.
3) What is the definition of “force”?
4) List 3 things that a force can do to an object?
Interactive 6.2
Testing Balanced and Unbalanced Forces
Procedures:
1) Cut and paste this link: https://phet.colorado.edu/sims/html/forces-and-motion-basics/latest/forcesand-motion-basics_en.html
2) Click on “net forces”
3) In the upper right hand corner of the screen, put a check in the boxes for both “sum of forces” and
“values” before having each tug-of-war challenge. Make sure to record your answers in the table.
Left Force (blue emos)
Right force (red emos)
1 large blue
1 large red
1 large blue + 1 small blue
2 small reds
1 medium blue
2 small reds
1large blue + 1 med. blue
2 small reds + 1 med. red
Sum of the
forces
Type of force:
Balanced or unbalanced?
Questions:
A. If Mr. Kusibab pushes on a chair with a force of 10 N and Mr. Ragaller pushes the chair in the opposite
direction with a force of 3 N, then which direction will the chair move? __________________________
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B. If Mr. Glennon and Mr. Ragaller push a chair in the same direction, each with force of 6 N, and Mr.
Kusibab pushes the chair in the opposite direction with a force of 10 N, which way will the chair move?
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C. Write a couple of sentences answering the purpose question for Inquiry 6.2.
(Hint: What are the differences between a balanced and an unbalanced force?) ____________________________________________________________________________________
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