ACTIVITY #7: GRAVITY: THE INVISIBLE FORCE

Gravity is a force from which we can not escape its effects. Right now you are being pulled downward by the Earth’s gravity; it’s the force that we commonly call our weight. But it may surprise you that you are also being pulled by Mar’s gravity, Pluto’s gravity, and by many other planets and stars outside of our solar system.

When we look at pictures of astronauts in space, they appear to be floating around as if there were no gravity at all. The Apollo space missions fascinated people around the world as they watched the astronauts bounce and jump around the surface of the Moon. With past and present missions in space, gravity is a force that must be understood and considered to allow for a safe and healthy space mission. These observations of space raise a large number of questions.

•What is gravity?•Do astronauts in space really have no weight?•What effect does space travel have on the human body?

•How does gravity affect your life? ACTIVITY OVERVIEW

The first part of the activity is the viewing of a video about gravity, which serves as an overview for the entire activity. This leads into several other investigations and demonstrations, each targeting a specific variable that affects gravity.

 Masses of objects and the distance between them affects the attractive force of gravity between them.  Gravity is always present and can act without touching, hence the reason that it is often called the “invisible force.”gravity alone keeps the planets, and other objects, in our solar system in orbit around the Sun and that is the Sun’s primarily responsible in our solar system.  Mass and weight represent two different quantities.•The human body has adapted over time to life with gravity and prolonged space travel affects the human body.

•Since each planet in our solar system has a different mass, each has its own unique gravitational force on other objects. The Sun is the gravitationally dominant object in our solar system•Gravity has an influence on observable phenomena such as tides, planetary orbits and our own ability to move and function on Earth

MAIN IDEAS: In this lab activity, you will learn…………

What is Gravity? Gravity is one of the four fundamental forces in our universe. It is an attractive (pulling) force between objects, and the objects can exert a gravity force on one another without even touching. It is for this reason that gravity is sometimes called the invisible force. The amount of force between the objects depends on the masses of the objects and the distance between them. If the mass increases, so does the force of gravity. If the distance between the objects increases, the force of gravity decreases; but the force of gravity never goes to zero.

All objects exert their force of gravity (we could even calculate your own personal “gravity force” if we wished), but in most cases, objects are too small to really concern ourselves with their affects. The force involves two objects and will change size based upon the characteristics of the second object. This is not the case with large objectssuch as planets, stars and asteroids.

What Newton discovered is that the force of gravity was universal - it behaves the same way everywhere in the universe. The Sun and the planets have a gravitational pull on each other, but because the Sun is so much more massive, its gravity dominates the solar system and is the reason why the planets orbit the Sun, not the other way around. The Earth’s gravity is responsible for keeping our Moon in orbit around our planet.

Other forces:

Strong force-holds nucleus together

Electromagnetic-attraction,repel

Weak-beta(radioactive)decay

Gravity and Planets – Why are most space objects spherical?

On Earth, we typically say that gravity pulls things straight downward, but the truth is that gravity actually pulls toward the center of the Earth. If you look at a globe, vertically downward in Delaware is not the same direction as vertically downward in Hawaii. So, when talking about the force of gravity on a global or astronomical scale, it would be more accurate to say that gravity acts inward, towards the Earth’s center. When our solar system was forming, there were many small pieces of gas and dust, each having their own very small amount of gravity. As these small pieces started to stick together, the combined ( now larger and more massive) object increased its gravity and pulled more and more pieces together from all directions. This caused the formation of objects in a spherical shape.

So How Do Astronauts Float Around in Space?

We have all seen the images of astronauts floating around in space and it is referred to as a “weightless” or “microgravity”. But this contradicts our previous discussion that gravity never goes to zero. The terms weightlessness, zero gravity, and microgravity refer to a sensation of being weightless. By all appearances, gravity has disappeared, but this is not true. The space shuttle, the space station, and satellites are actually falling towards the Earth because the Earth’s gravity is pulling them downward just like it would do to any object that get tossed up into the air. The spacecraft and astronauts are moving forward while they are also falling downward. Since the Earth is a sphere, the astronauts and the spacecraft actually fall around the Earth.

You would feel the same sensation, for a short amount of time if you were to ride the Free Fall ride at Six Flags Great Adventure in New Jersey or if you rode in the Vomit Comet, which is an aircraft that is used to train astronauts for this free fall condition Due to this free fall situation, it appears as if all objects have no weight, when actually they have not traveled have not traveled far enough away from the center of the Earth to experience any significant weight loss. What is really happening is that all of the objects are falling at the same rate around the Earth. Since things in this free fall environment behave like there is no gravity at all, scientists use the terms weightlessness, zero gravity, and microgravity to describe it even though the gravity is not zero or “micro”.

How Do Humans React to Space?Humans may live in space for longer periods of time in the future. Past missions in space travel have revealed that this free fall (or microgravity) environment can have negative effects on the human body. Our bodies have evolved to handle the Earth’s always-present downward force of gravity, but when put into a situation, such as in orbit, where the body feels as if the force of gravity was turned off, the human body adapts to the new environment. This is the reason that astronauts must exercise regularly in space. As scientists study ways to combat gravity related problems in space, such as bone degeneration, they have also made advances in similar problems already existing on our planet.

Part A: Video, Gravity: The Invisible Force After viewing videoT/F

1.Our body structure would be the same if we grew up in zero gravity. 

  F

2. The rings of Saturn are moons that were ripped apart by gravity. 

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3. Gravity helps in allowing life to exist on planet Earth. 

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4. It takes 2 days to reach outer space in the space shuttle. 

  F

5. We have muscles to pick things up because of gravity. 

  T

6. The height of mountains is affected by gravity. 

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7. Human life is very different in the absence of gravity. 

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8. Humans get weak in space, but feel strong. 

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9. Astronauts get stronger muscles when they go into space.   F

10. Bones become stronger in space. 

  F

 

Part 2: Video Discussion Questions1.      Give one example of how the human body has evolved because of gravity. Bones, muscles and blood pressure are all designed to defeat gravity. We can keep our feet on the ground because of gravity. 2. What effect did gravity have on Saturn? The enormous gravitational pull ripped apart nearby moons, therefore forming the rings. The gravity of Saturn was so strong that it prevented moons from forming, and so we have rings. 3. How does gravity contribute to life on Earth? Gravity contributes to the success of living organisms because of its ability to hold the atmosphere and water in place on our planet. 4. Give one example of how humans defy gravity. We can hold our heads up, by using our neck muscles as we walk. Our heart pumps blood upwards and our muscles support our body. Our vestibular organ allows us to balance ourselves and stop from falling over. 

 

5. Give an example of something gravity allows you to do? We can shape ourselves with exercise using weights. Roller coasters move downward at a very fast rate and we feel a brief “weightless” moment (exactly the same experience that astronauts feel in orbit). Any other examples.

 6. How does an astronaut’s perception of the space around him change in the free fall/microgravity environment of space? It is very hard to tell up from down. Astronauts are able to lift very heavy objects. It is very difficult to walk (we float).

 7. What happens to an astronaut’s organs when they are in space? The heart, diaphragm and liver move as ligaments relax. The face becomes puffy as fluids move into the head and tissue.

 

8. What happens to an astronaut’s bones when they are in space? Bones demineralize and become brittle in space which could lead to bone fractures.

 9. What happens to an astronaut’s muscles when they are in space over a period of time? Muscles atrophy and get weaker in space. (It could be easily related a student who had a leg or arm in a cast for several weeks and then feels that they muscles are weaker by their non-use during that period)

 

PART B: How much would you weigh at different locations in our Solar System?

 

How much you weigh depends on the force of gravity at your location. The table shows what the force of gravity would be at different locations in our solar system based on a value of 1 on the Earth’s surface. For example, if you weighed 100 pounds on Earth, you would weigh 17 pounds on the Moon. 100 pounds (your Earth weight) multiplied by 0.17 (the gravity factor for the moon) = 17 pounds. This is about 1/6 the gravity on Earth. Remember, your mass does not change at different locations. Your mass remains the same; it is your weight that changes due to the force of gravity on the different planets.

Question: Put your list of locations in order from where you weigh the most to where you weigh the least. After looking at this data, what do you think causes the variation in your weight?

PART C: How does the Mass of a Planet affect Jump height?

 Problem:

How does the Mass of a Planet affect how high you can jump?

Independent Variable:Dependent Variable: Hypothesis:

 Materials: Meter Stick Calculator Procedure:•Working in groups of three, have one member of the group hold a meter stick vertical to the ground with the zero end touching the ground.The second group member will observe and record the jump height of group member three. Group member three makes a standing jump next to the meter stick. This height is recorded for three jumps. Average the three jumps together and record the average jump height on Earth. •Reverse roles two more times so that each member of the group is able to be the jumper and get their average jump height on Earth. Calculate average jump height on Earth

JUMP TRIAL 1 TRIAL 2 TRIAL 3 AVERAGE JUMP

HEIGHT 41 42 42 42

1. Using Table 2, calculate your average jump height at other locations in the solar system. Example to follow.

2. Complete Table 3 by listing the planets, Sun and dwarf planet Pluto and the height of your jump from the planet with the least mass to the planet with the greatest mass. Example to follow.

3. Create a graph comparing the planets in increasing order of mass to the jump height. Example to follow.

Table 2: How Jump height is determined by the Mass of the Solar System Object Location Mass of the

Solar System object 1023 kg

Average Height of Jump on Earth (cm)

Conversion for the Height of Jump

Jump Height on the location

(cm)

Mercury 3.3 42 (cm) X 2.65 111.3 (cm)

Venus 48.7 42 X 1.10 46.2

Earth 59.8 42 X 1 42

Mars 6.42 42 X 2.64 110.9

Jupiter 19000 42 X .039 16.38

Saturn 5690 42 X 0.94 39.5

Uranus 868 42 X 1.10 46.2

Neptune 1020 42 X 0.88 36.9

Dwarf Planet Pluto

0.129 42 X 13.2 554.4

Sun 19,900,000 42 X 0.04 1.68

Solar System Object Height of Jump

1. Dwarf Planet Pluto 554.4 cm

2 .Mercury 111.3

3. Mars 110.9

4. Venus 46.2

5. Earth 42

6. Uranus 46.2

7. Neptune 36.9

8. Saturn 39.5

9. Jupiter 16.38

10. Sun 1.68

Jump height on planets from planets least to most massive

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Using the data from the above tables, create a bar graph comparing the mass of the planets and dwarf planet to the height of your jump. Use the list of planets and dwarf planet Pluto to organize the planets on the x axis from least to greatest mass. 

Answer the following questions using your data  1. What relationship do you see with the mass of the object and Jump Height?As the mass of the planet increases, the jump height decreases. 2. What does the mass of the planet have to do with gravity? Why does this influence the Jump Height? The greater the mass of the planet, the stronger the gravitational pull. The greater the mass of the planet, the lower the jump height will be because the gravitational pull in greater on more massive planets.

Part D: Modeling Gravity’s Effects on Planets

Demonstration 1 QUESTIONS: What keeps the Earth from traveling outside of our Solar System?What makes it revolve around the Sun and not just move further into outer

space? 1.      Get to in pairs and hold hands. You should spin around together in a circle. You should feel the force on your arms that is between you your partner, causing both of you to move in a circular pattern. 

2. What do you think would happen if you were to let go. This is a topic we addressed in 6th grade Force and Motion,

where it is emphasized that a force is needed to cause an object to move in a circle. It is not natural for any object to move in a circle-a force must be present for this to happen. In this demonstration, the pulling force of your arms represents gravity (Gravity is a pulling force ) and that one person would represent a planet while the other person represents the Sun. This model also serves the purpose of showing that a gravity force is exerted by both objects on each other.

Demonstration 2

   Use the string and ball supplied and swing the ball around in an arc over your head. To

demonstrate that there is a force on the string, let go. We have selected a ball which will not injure anyone whom it might hit. Questions: What force caused the ball to move

in a circle?

 What happened when the force was no longer present?

What would happen if the Sun’s gravity

were to suddenly turn off?

For this demonstration, we will use a lightweight ball tied to the end of a string to model the situation again, but with this demonstration, students can actually test out what would happen if they let go of the string.

Distance and its Influence on Gravity

Table 1: Average Orbital Speed of the Planets in Our Solar System

Planet Average Orbital Speed (km/sec)Mercury 48Venus 35Earth 30Mars 24Jupiter 13Saturn 9.7Uranus 6.8Neptune 5.4Dwarf Planet Pluto 4.7

  Table 2: Planet Distance from the Sun

Planet Distance from Sun in AU

Mercury 0.39

Venus 0.72

Earth 1.0

Mars 1.52

Jupiter 5.20

Saturn 9.58

Uranus 19.20

Neptune 30.05

Dwarf Planet Pluto 39.24

Average Orbital Speed (km/sec)

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Distance from Sun in AU

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Distance from Sun inAU

Graphs: The Influence of Distance on Gravity     

  The astronomical unit (AU or au or a.u.) is a unit of length. It is approximately equal to the mean distance between Earth and Sun. The currently accepted value of the AU is about 150 million kilometers or 93 million miles.

                            

 Discussion Questions:1.      Using the graphs above, do you see any correlation between the distance of the planet from the Sun and the influence of gravity on its orbital speed? The orbital speed is the average speed the planet is moving as it revolves around the Sun one time. Explain how gravity and distance would influence this speed.   2.      Name and describe two factors that influence gravitational pull of a planet.

Investigating Further…Investigating Further… 

 

 When planning future long term missions in space, many health factors must be addressed. Due to the effects of microgravity on the human body, many health problems have occurred that must be addressed in future long term space missions. Microgravity means that there is a very small amount of gravity present in space. After extended periods of time in space, bone disorders, cardiac problems, sleep disorders, radiation effects, immune disorders, muscle changes, inner ear and balance issues and psychological factors have surfaced. The National Space and Biomedical Research Institute is actively researching the effects of microgravity on the human body and looking for ways to counteract these harmful effects. Search the internet for a more complete description of harmful effects of long duration space flights. The NSBRI web site is a great site to start. The site is as follows: www.nsbri.org

Copy and Answer the following questions. Use your data to support your responses.

 What is gravity? How does microgravity affect the human body? What does gravity allow us to do on Earth? What keeps the Earth and Moon in orbit? What is the difference between your weight and mass? What two factors influence gravity? Why is gravity called the invisible force?

 

Applying what you have learned … 1. In future long term missions to Mars, what gravitational influences (related to travel and health), will mankind need to address?  2.  If a human was born in space, why would they have difficulty if they returned to Earth?   3.  Why do astronauts feel like “superman” when they are in space?  4. How do these activities explain how gravity keeps the solar system

held together?   5. Use what you have learned to explain how games such as football, basketball, gymnastics, hurdles or other sporting events would be very different on other planetary bodies.