Forces and the Universe Unit Review:
Weight [N] = 4.44 x Weight [lbs]
mass [kg] = weight [lbs] / 2.2
F=ma
Fg = m ag
Answer Each Question on a separate sheet on paper.
1. What is a force, and what does it cause objects to do? A force is a push or pull. When unbalanced forces cause
objects to acceleration. If balanced, a force will cancel out leaving the object to stay at rest or continue to move
at a constant velocity.
2. What are mass and weight? How are they different? Mass is the amount of matter in something and is
measured in [kg]. Weight is the force of gravity acting on something and is measured in [N].
3. List three ways to accelerate. Speed up, slow down, and change direction.
4. List two ways not to accelerate. Stay at rest, and move at a constant velocity.
5. What are the symbol and unit for mass, acceleration, force and weight? Mass: m [kg], acceleration: a [m/s2],
force: F [N], and weight: Fg [N]
6. What does it mean when using the term “net force?” The addition of all forces in the same direction, and
subtraction of all forces in opposite directions. So if you have 10 [N] up, 4 [N] up and 6 [N] down, the net force is
8[N] up.
7. What are the possible motions when the net force equals 0? At rest, or constant velocity.
8. What are the possible motions when the net force is equal to something greater than 0? Speeding up, slowing
down, or changing direction.
9. What happens when the motion and the force are in the same direction? An object speeds up.
10. What happens when the motion and the force are in opposite directions? An object slows down.
11. Find the net force for each case: a) 5 [N] left and 8 [N] left, b) 12 [N] up and 18 [N] down. A: 13 [N] left, B: 6 [N]
down.
12. Convert the following weights from pounds to Newtons: 10 [lbs], 50 [lbs], and 100 [lbs]. Weight in [N] = weight
[lbs] x 4.44. Using that equation 10 [lbs] = 44.4 [N], 50 [lbs] = 222 [N] and 100 [lbs] = 444 [N].
13. Convert the following weights in [lbs] to mass in [kg]: 10 [lb], 50 [lb], and 100 [lb].
Mass [kg] = Weight [lbs] / 2.2. Using that equation 10 [lb] = 4.5 [kg], 50 [lb] = 22.7 [kg], 100 [kg] = 45.5 [kg].
14. How much force is needed to move a mass of 10 [kg] with an acceleration of 15 [m/s2]? F = m a, F = 10 x 15 =
250 [N].
15. A force of 50 [N] is applied to a mass of 12.5 [kg]. What is the acceleration? F = m a, so a = F / m. a = 50/12.5 =
4 m/s2
16. 150 [N] of force causes an acceleration of 3 [m/s2]. Find the mass. F = m a, so m = F / a. m = 50 [kg]
17. What two factors control the force of gravity? The mass of the two objects, and the distance between the two
objects. So a person’s weight is determined by the mass of the person, the mass of Earth, and the distance
between center of the Earth and the person.
18. What happens to the force of gravity as the objects gain more mass? The force of gravity increases.
19. What happens to the force of gravity as the objects get farther apart? The force of gravity decreases.
20. Why does the moon have less gravity than the Earth? The moon has less mass than the Earth.
21. What is the weight [N] and mass [kg] of a 20 [kg] rock at each location: Earth (ag = 9.8 [m/s2]), Mercury (ag = 3.7
[m/s2]), and Jupiter (ag = 24.8 [m/s2]. The equation for weight is Fg = m ag. Mass is constant. For Earth Fg = 196.2
[N] and m = 20 [kg], for Mecury Fg = 74 [N] and m = 20 [kg], for Jupiter Fg = 496 [N] and m = 20 [kg].
22. Why does friction exist? At the microscopic level no surface is perfectly smooth. So friction is the resistance
that exists when the imperfections grind against each other.
23. Give an example of friction slowing an object down. Give an example of friction causing an object to move
forward? When a driver steps on the breaks really hard, the tires skid across the ground. Friction points in the
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opposite direction of the motion and slows the car down. When a person walks, her foot pushes backwards on
the ground, friction then pushes the foot forward allowing the person move forward.
What is a normal force? It is the force of a surface pushing back in response to gravity. Think about it, if your
weight is 400 [N] down and you are standing still, then there must be another force pushing back up to balance
you out. That upward force by the ground is called normal force. Without this force you would fall through the
ground.
If a book is sitting on a table and has a weight of 100 N, what is the value of the normal force? FN = 100 [N]
If you were to kick a cinder block wall very hard, explain why it hurts using one of Newton’s laws. Newton’s 3rd
law tells us that for every force there is a separate, equal and opposite reaction force. So if you kick a wall with
a force of 200 [N], then the wall kicks your foot back with the same 200 [N] force.
If you kick a soccer ball explain why it speeds up when in contact with your foot using one of Newton’s laws.
Newton’s 2nd law tells us that if you apply a force to an object then the object will accelerate. So if you apply a
force to the soccer ball with your foot, the soccer ball will speed up and move (accelerate).
If you put a book on shelf, explain why the book is still on the shelf a few days later using one of Newton’s laws.
According to Newton’s 1st law an object at rest will continue to stay at rest so long as nothing applies a net force
on it. So if the book is left on the shelf, and nobody picks it up, it will remain at rest on the shelf.
How are meteoroid, meteor, and meteorite different? A meteoroid is a rock in space, a meteor is rock that
burns up in the atmosphere before hitting the ground, and a meteorite is a rock from space that makes it
through the atmosphere and hits the ground.
How does an asteroid differ from a comet? An asteroid is very large rock in space (like the size of a state), and a
comet is ball of dust, ice and gases. Comets also have a tail from the trailing gasses it leaves behind.
What percentage of the surface is covered in water and land? 71% water, 29% land.
What percentage of the water on Earth is drinkable? What percentage is salt water? 0.2% of the water on Earth
is drinkable (clean and easy to get). 97% of the water on Earth is salt water.
What three factors on Earth are considered necessary for life? A protective atmosphere, liquid water, and
consistent temperatures.
List the three main services provided by the Earth’s atmosphere. Protects us from cosmic radiation, moves
resources (oxygen, water, and carbon dioxide) with wind around the planet, and it smoothes out the
temperatures on the planet with wind.
List the three main services provided by the Earth’s atmosphere. Wind movers the heat around to smooth out
the temperatures. Wind moves resources around Earth like oxygen, water, and plant seeds. The atmosphere
and magnetic field of the Earth work together to create a shield against harmful radiation from the sun.
Why does the temperature on the moon get a lot more extreme than the temperatures on Earth, even though
both places are nearly the same distance from the sun? The moon does not have an atmosphere. Therefore the
moon does not have an atmosphere to smooth out the temperatures.
How long does it take the earth to rotate once, and what is this called? It takes the Earth 24 hours to rotate
once. One rotation of the Earth is called a day.
How long does it take the earth to revolve once around the sun, and what is this called? It takes the Earth
365.25 days to revolve once around the sun. One revolution of Earth around the sun is called a year.
How long does it take the moon to revolve once around the Earth? It takes the moon 29 days to revolve once
around the Earth.
Explain why the Earth has seasons? The Earth has seasons because the it is tilted. When the northern
hemisphere is tilted towards the sun it is summer in the northern hemisphere and winter in the southern
hemisphere. When the northern hemisphere is tilted away from the sun it is winter in the northern hemisphere
and summer in the southern hemisphere.
41. What causes tides on Earth? When causes high tide to occur? The tides are caused by the moon’s force of
gravity pulling up on the Earth’s water. High tide occurs when the moon is directly overhead, and on the
opposite side of the Earth. Low tide occurs half way in between the times for two high tides. So if high tide is
expected at 12:00 PM and 8:00 PM, then low tide will happen at 4:00 PM.
42. How is it possible to see the moon, even though it is not a star? Sun light reflects off the surface of the moon
allowing us to see it. So it is right to say that the moon is an illuminated object (lit up).
43. In terms of Earth’s away, how far away is the moon from Earth? The moon is 30 Earths away from Earth. (Clone
Earth 30 times, and line them up for the distance to the moon.)
44. What happens in a lunar eclipse? The Earth casts a shadow on the moon. The sun, Earth, and moon need to be
a straight line for this to happen.
45. What happens in a solar eclipse? The moon casts a shadow on the Earth. The sun, moon, and Earth need to be
in a straight line for this to happen.
46. Describe the moon in each case: a) getting brighter and less than 50% lit, b) getting dimmer and more than 50%
lit, c) getting brighter and more than 50% lit, d) getting dimmer and less than 50% lit, e) 100 % lit, and f) 0% lit.
A) Waxing crescent, B) waning gibbous, C) waxing gibbous, waning crescent, full moon, and new moon.
47. What is the sun, what is it made out of, how far is it from earth, how old is it, and how long will it continue to
burn? The sun is a star which is a burning ball of gas. The sun is made of hydrogen and helium. The sun is
11,724 Earth’s away from Earth (line up 11,724 Earths for the distance to the sun). The sun is about 4 billion
years old. It is estimated to have about 4 billion years worth of fuel left to burn. So it will be around for quite a
while.
48. What is a light year? What does it mean to say something is 10 light years away? A light year is a unit of
distance. It the amount distance that light travels in one year. Something that is 10 light years away takes light
10 years to travel to the observer on Earth. Therefore the observer sees the object as it was 10 years ago, since
the light is outdated by 10 years. This is the way astronomers look into the past. The simply need to farther
away.
49. How can you use the terms star, solar system, constellation, galaxy and universe to describe the organization
system of the universe? A star is burning ball of gasses. A solar system is one star and the planets that orbit it.
A constellation is a collection of solar systems. A galaxy is a collection of constellations. The universe is a
collection of all the galaxies.
50. Name a special fact about each of these four stars: Sun, Polaris, Proxima Centauri and Sirius. The sun is only star
close enough to Earth that creates day time light. Proxima Centauri is the second closet star to Earth after the
Sun. Sirius is the brightest star in the night sky. Polaris is the North Star and is the only star in the sky to stay in
the same position in the sky. This is true because Polaris is located above the North Pole on Earth.
51. Describe the life cycle of the Sun. It begins as a nebula which is a cloud of gas in space. The nebula contracts
due to gravity, heats up, and births a star. A star like the sun then burns mainly hydrogen for few billion years.
Then the star runs out of hydrogen and starts burning helium. Helium has more energy, so it causes the star to
grow in size into a Red Giant. The Red Giant is so big that some the star’s mass floats away into space. The
remaining mass contracts into a much smaller object called a White Dwarf. The White Dwarf is very bright and
hot. Over time this object cools down into a Black Dwarf which is very cold and dim.
52. What is the general trend between the brightness and temperature of stars on a H-R diagram? The H-R diagram
shows us that for main sequence stars (the majority of stars) the brighter they are, the higher temperature they
have.
53. What color are low temperature stars? What color are high temperature stars? (Look at a H-R diagram). Low
temperature stars are red in color, and high temperature stars are blue in color.
54. What is luminosity on an H-R diagram? Using the H-R diagram roughly determine the temperature of the sun.
What color is star with a temperature of 10,000 K? Luminosity is a sun of brightness. On the H-R diagram the
luminosity is showing the brightness of different temperature stars. The Sun is yellowish-orange star. Using the
H-R diagram this puts the temperature around 5,700 K. A star with a temperature of 10,000 K is greenish blue in
color.