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Unit 8.4.2 Study Guide: Gravity—Force and Energy
I Can
Statements
“I Can Statements” are the learning targets for each unit. By the time you take the test for this
unit, you should be able to confidently say:
I can explain the difference between mass and weight.
I can describe how Earth’s gravitational force on an object depends upon the mass of the object.
I can describe how the Earth’s gravitational force on an object depends on the distance of the object
from the Earth.
I can design and build structures to support a load.
I can describe how energy changes back and forth from potential energy to kinetic energy (e.g. a
bouncing ball or pendulum).
Teach the
Concepts
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One great way to study is to explain concepts to others. Teach the following concepts to a
parent, grandparent, aunt, uncle, or even an older sibling.
Explain the difference between mass and weight.
Describe gravity on different planets and give examples.
Explain the two factors that impact gravity’s force.
Describe the difference between kinetic and gravitational potential energy.
Give examples of kinetic energy changing to potential energy, and vice versa.
Explain how humans are impacted by gravity in how we build structures.
Signature _______________________________
Read/Summarize
Gravity: is the attractive force between all objects in the universe. It is the force that causes
Earth to orbit around the sun. It is the force that pulls objects to Earth. It’s not only planets and
stars that have gravity, thought: all objects that have mass also have a gravitational force.
Summarize the paragraph(s) above:
The strength of gravity (the “gravitational force”) depends on two things: the masses of the
objects and the distance between the objects. The larger the mass of the objects the
stronger the gravitational pull the will be. This is why the Earth is capable of pulling objects
(including you) towards it: the Earth has a very large mass! Even a tiny dust particle has
gravity—however, it is not a very strong force because the mass of the dust particle is so small.
Summarize the paragraph(s) above:
Do you have a gravitational force? Of course you do - but compared to Earth, your gravity does not affect many things around you.
Distance also has an effect on gravity. For instance, if you were an astronaut, and you went farther into space, why would you feel
weightless? Well, as your distance from Earth increases, the force of gravity between you and Earth decreases. So when you're in space,
gravity has less effect on your mass. The closer you are to a large mass, like the earth, the more effect its gravity has on your mass.
Summarize the paragraph(s) above:
Gravity is constantly pulling on all the things on and around the earth, helping the natural processes that wear down mountains and
shape the surface of the earth. Humans have had to consider the effects of gravity as they designed and built structures on Earth. The
buildings, dams and bridges we build must not only support our use but also withstand the force of gravity.
Summarize the paragraph(s) above:
Weight and Mass: Sometimes people use the words weight and mass to mean the same thing. This, however, is not accurate.
Scientists’ measure weight using a unit called the Newton (N). Usually we refer to weight in pounds and ounces. Weight is a measure of
the gravitational force on an object. So, weight is really just a force. You could imagine that you are pressing down on the Earth with the
force of your weight!
Mass, on the other hand, is the amount of matter in an object. You will have the same amount of mass no matter where you go,
even if you are on another planet or the moon. Scientist’s measure mass using a unit called the gram (g). If an object is moved to a
location of greater gravitational force, such as Jupiter, nothing physically changes about the object. Only its weight will increase, but its
mass remains the same. The only way to change the mass of an object is to take away part of the object or add to it.
Summarize the paragraph(s) above:
Weight on Other Planets: An object with a mass of one kilogram (kg) on Earth has the same mass on another planet, like Jupiter.
However, because Jupiter has more mass, the weight of the object would be different. In other words, the two planets would exert a
different gravitational force on the same object. Similarly, if you traveled to another planet, your mass would not change. However, if the
planet had a different mass than Earth, you would notice a change in the gravitational force pulling on you. On Jupiter you would
experience a much stronger pull of gravity than you do on Earth. In other words, your weight would increase, even though your
body would be the same!
What if you travelled to the moon? The moon has much less mass than Earth. Would your mass be different on the moon than it is on
Earth? No, it would remain the same! However, your weight, or gravitational force, would be much lower than on Earth. The moon has
only 1/6 the mass of Earth. This means that your weight on the moon would be 1/6 of your weight on Earth. For example, imagine that
you weigh 120 lbs here on Earth. If you stood on a bathroom scale on the moon you would find that you weighed only 20 lbs! So, next
time you hear someone talking about losing weight, you might want to suggest they take a trip to the moon.
Summarize the paragraph(s) above:
Gravitational Potential Energy is stored energy. The "stored" energy is possessed by an object by virtue of its position or state.
When you lift a heavy object you exert energy, which later will become kinetic energy when the object is dropped. A lift motor from a roller
coaster exerts kinetic energy when lifting the train to the top of the hill. The higher the train is lifted by the motor the more potential
energy is produced; thus, forming a greater amount if kinetic energy when the train is dropped. At the top of the hills the train has a
huge amount of potential energy, but it has very little kinetic energy. Potential energy cannot be transferred to other objects.
Kinetic Energy: The word "kinetic" is derived from the Greek word meaning to move, and the word "energy" is the ability to move.
Thus, "kinetic energy" is the energy of motion – an objects ability to do work. The faster the body moves the more kinetic energy is
produced. The greater the mass and speed of an object the more kinetic energy there will be. As the train accelerates down the hill the
potential energy is converted into kinetic energy. There is very little potential energy at the bottom of the hill, but there is a great amount
of kinetic energy. Kinetic energy can be transferred from one moving object to another, say, in collisions.
Summarize the paragraph(s) above:
Check Yourself
What two things determine an object's kinetic energy?
What determines an object’s gravitational potential energy?
Give 3 Examples of Kinetic Energy:
Give 3 examples of Gravitational Potential Energy:
Example of Kinetic energy transforming into Gravitational
Potential energy:
Example of gravitational Potential energy transforming into Kinetic
energy:
To the right is a diagram of a Pendulum Bob swinging. Label the
diagram as follows:
1. Label the place(s) where Bob has the greatest gravitational
potential energy.
2. Label the place(s) where Bob has the greatest kinetic
energy.
3. Label the place(s) where Bob has a mix between
gravitational potential energy and kinetic energy.
The diagram below shows snapshots of a ball bouncing. Use the diagram to answer the following questions.
Color each ball with green representing gravitational potential energy and yellow representing kinetic energy. (For example,
if at point 2 the ball has mostly kinetic energy and just a little gravitational potential energy, color pretty hard with yellow and
shade lightly over it with green.)
1
2
3
10
4
9
5
6
11
12
8
7
At which number does the ball have 100% gravitational potential energy and 0% kinetic energy? Explain.
At which number does the ball have some gravitational potential energy AND some kinetic energy? Explain.
At which number does the ball have 100% kinetic energy? Explain.
Where is the ball moving the fastest? What kind of energy is greater when an object is moving faster?
Where is the ball not moving? What does this tell you about kinetic energy?
Where is the ball the highest? What kind of energy is related to high an object is off of the ground?
If object A and B have the same mass, which object would be experiencing more gravitational pull from the planet? Why?
A
B
Planet
If you found yourself on a planet with twice the mass of Earth, how would your mass and weight be different than your mass and weight
on Earth?
If every object has its own gravitational pull, why doesn’t your body act like a magnet to the things around you?
Why do you not notice the gravitational pull of the Sun or the Moon?
Why do we say that things are weightless in space?
Mass is a measure of ___________________.
Weight is a measure of ___________________.
Explain why you would weigh less on the moon than on the Earth:
List some ways that person could change their MASS.
List some ways that a person could change their WEIGHT, without changing their mass.