Lec 6

Question 1

speed

Answer 1

The speed of the car tells us how far it will go in a certain amount of time

For example, “100 kilometers per hr” (about 60 miles per hour) is a speed, and it tells us that the car will cover a distance of 100 kilometers if it is driven at this speed for an hour

Question 2

velocity

Answer 2

The velocity of the car tells us both its speed and its direction

For example, “100 kilometers per hour going
due north” describes a velocity

Question 3

acceleration

Answer 3

The car has an acceleration if its velocity is changing in any way, whether in speed or direction or both

Question 4

acceleration of gravity

Answer 4

acceleration of a falling object (g) in 9.8 m/s

-gravity doesn’t affect horizontal velocity

Question 5

momentum

Answer 5

the product of its mass and velocity; that is, momentum

=mass x velocity

Question 6

net force

Answer 6

(or overall force) acting on
an object represents the combined effect of all the individual forces put together

There is NO net force on your
car when you are driving at constant velocity, because the force generated by the engine to turn the wheels precisely offsets the forces of air resistance and road friction.

A change in momentum occurs only when the net force is not zero

Question 8

what does it mean to change an object’s momentum?

Answer 8

means changing its velocity, as long as its mass remains constant

A net force that is NOT zero therefore causes an object to accelerate
–whenever an object accelerates, a net
force must be causing the acceleration
—that is why you feel forces (pushing you forward, backward, or to the side) when you accelerate in your car

We can use the same ideas to understand many astronomical processes
–for example, planets are always accelerating as they orbit the Sun, because their direction of travel constantly CHANGES as they go around their orbits
–we can conclude that some force must be causing this acceleration –> gravity

Question 9

angular momentum

Answer 9

-total momentum from each part of his body as he spins

Any object that is either spinning or moving along a curved path has angular momentum, which makes angular momentum very important in astronomy

Question 10

torque

Answer 10

The type of force that can change an object’s angular momentum is called a torque, which you can think of as a “twisting force.”

Changing a tire offers an example of
torque
–turning the bolts on a tire means making them rotate, which requires giving them some angular momentum
–a longer wrench allows you to push from farther out than you can with a short wrench, so you can turn the bolts with less force

Question 11

mass vs weight

Answer 11

-mass is the amount of matter in your body

-weight is the FORCE that a scale measures when you stand on it
–depends on BOTH mass and on the forces (gravity) acting on your mass

Question 12

free-fall

Answer 12

falling without any resistance to slow you down

The floor drops away at the same rate that you fall, allowing you to “float” freely above it, and the scale reads zero because you are no longer held to it
–in other words, your free-fall has
made you weightless

Question 13

Aristotle

Answer 13

made many claims about the physics of
motion, using his ideas to support his belief in an Earth-centered cosmos

also maintained that the heavens were totally distinct from Earth, so physical laws on Earth did not apply to heavenly motion

Question 14

Newton’s laws of motion

Answer 14

1) An object moves at constant velocity if there is no net force acting upon it

2) force= mass x acceleration

3) for any force, there is always an equal and opposite rxn force

Question 15

newtons first law

Answer 15

An object moves at constant velocity if there is no net force acting upon it

In other words, objects at rest (velocity = 0) tend to remain at rest, and objects in motion tend to remain in motion with no change in either their speed or their direction
–Newton’s first law says that the
car should keep going at the same speed forever unless a force acts to slow it down

Question 16

newtons 2nd law

Answer 16

tells us what happens to an object when a net force is present

a net force will change an object’s momentum, accelerating it in the direction of the force

Question 17

newtons 3rd law

Answer 17

This law is very important in astronomy, because it tells us that objects always attract each other through gravity
–for example, your body always exerts a gravitational force on Earth identical to the force that Earth exerts on you, except
that it acts in the opposite direction

Of course, the same force means a much greater acceleration for you than for
Earth (because your mass is so much smaller than Earth’s), which’s why you fall toward Earth when you jump off a chair, rather than Earth falling toward you

Newton’s third law also explains how a rocket works:
A rocket engine generates a force that drives hot gas out the back, which creates an equal and opposite force that propels the rocket forward.

Question 18

conservation of angular momentum

Answer 18

rotating or orbiting objects have angular momentum because they are moving in circles or going around curves,
and that angular momentum can be changed only by a “twisting force,” or torque. The law of conservation of angular momentum states that as long as there is no external torque, the total angular momentum of a set of interacting objects cannot change

An individual object can change its angular momentum only by transferring some angular
momentum to or from another object

Question 19

angular momentum and earths orbit

Answer 19

1. Earth needs no fuel or push of any kind to keep orbiting the Sun—it will keep orbiting as long as nothing comes along to take angular momentum away
2. Because Earth’s angular momentum at any point in its orbit depends on the product of its speed and orbital radius (distance from the Sun), Earth’s orbital speed must be faster when it is nearer to the Sun (and the radius is smaller) and slower when it is farther from the
   Sun (and the radius is larger).

Question 20

orbital energy

Answer 20

A planet orbiting the Sun has both kinetic
energy (because it is moving around the Sun) and gravitational potential energy (because it would fall toward the Sun if it stopped orbiting)

The amount of kinetic energy depends on orbital speed, and the amount of gravitational
potential energy depends on orbital distance.

Because the planet’s distance and speed both vary as it orbits the Sun, its gravitational potential energy and kinetic energy also vary

However, the planet’s total orbital energy—the sum of its kinetic and gravitational potential energies, stays the same

Question 21

gravitational encounters

Answer 21

Although orbits cannot change spontaneously, they can change through exchanges of
energy

One way that two objects can exchange orbital energy is through a gravitational encounter, in which they pass near enough that each can feel the effects of the other’s gravity

-for example, in the rare cases in which a comet happens to pass near a planet, the comet’s orbit can change
dramatically

Question 22

atmospheric drag

Answer 22

-friction can cause objects to lose orbital energy

a satellite experience drag from earths atmosphere, and the drag causes the satellite to lose orbital energy until it plummets
–The satellite’s lost orbital energy is converted to thermal energy in the atmosphere, which is why a falling satellite usually burns up

Question 23

escape velocity

Answer 23

An object that gains orbital energy moves
into an orbit with a higher average altitude

For example, if we want to boost the orbital altitude of a spacecraft, we can give it more orbital energy by firing a rocket
–the chemical potential energy released by the rocket fuel is converted to orbital energy for the spacecraft

the minimum speed an object must have to break free from a planet’s or moon’s gravitational pull without needing more energy or propulsion

Question 24

how does gravity cause tides?

Answer 24

the moon’s tidal force
-gravity attracts Earth and Moon toward each other (with the Moon staying in orbit as it “falls around” Earth), but it affects different parts of Earth slightly differently
–because the strength of gravity declines
with distance, the gravitational attraction of each part of Earth to the Moon becomes weaker as we go from the side of Earth facing the Moon to the side facing away from the
Moon

This difference in attraction creates a “stretching force,” or tidal force , that stretches the entire Earth to create two tidal bulges, one facing the Moon and one opposite the moon

Question 25

the origin of tides

Answer 25

Earth must be stretching from its center in both directions (toward and away from the Moon) This stretching force, or tidal force, arises from the difference in the force of gravity attracting different parts of Earth to the Moon Many moons are stretched into slightly oblong shapes by tidal forces caused by their parent planets, and mutual tidal forces stretch close binary stars into teardrop shapes

Question 26

high vs low tides

Answer 26

Earth’s rotation carries any location through each of the two bulges each day, creating two high tides Low tides occur when the location is at the points halfway between the two tidal bulges

Question 28

neap tides

Answer 28

When the tidal forces of the Sun and the Moon counteract each other, as is the case at first- and third-quarter moons, we get the relatively small tides known as neap tides

Question 29

spring tides

Answer 29

When the tidal forces of the Sun and the Moon work together, as is the case at both new moon and full moon, we get the especially pronounced spring tides (named because the water tends to “spring up” from Earth)

Question 31

what effects does the slight misalignment of the tidal bulges with the earth-moon line cause?

Answer 31

1) the Moon’s gravity always pulls back on the bulges, slowing Earth’s rotation 2) the gravity of the bulges pulls the Moon slightly ahead in its orbit, adding orbital energy that causes the Moon to move farther from Earth these effects are barely measurable on human time scales

Question 32

synchronous rotation

Answer 32

when the Moon always shows (nearly) the same face to Earth -natural consequence of tidal friction

Question 33

why does earths tidal force have a greater effect on the moon than the moon's tidal force has on earth?

Answer 33

This tidal force gives the Moon two tidal bulges along the Earth–Moon line, much like the two tidal bulges that the Moon creates on Earth. (The Moon’s tidal bulges are not visible but can be measured in terms of excess mass along the Earth–Moon line.) If the Moon rotated relative to its tidal bulges in the same way as Earth, the resulting tidal friction would cause the Moon’s rotation to slow down

Question 34

tidal effects on other worlds

Answer 34

Tidal forces and tidal friction affect many worlds Synchronous rotation is especially common. For example, Jupiter’s four large moons keep nearly the same face toward Jupiter at all times, as do many other moons Pluto and its moon Charon both rotate synchronously: always keep the same face toward each other

Question 35

how do tidal forces play other roles in the cosmos?

Answer 35

They can alter the shapes of objects by stretching them along the line of tidal bulges

Question 36

why do all objects fall at the same rate?

Answer 36

If you drop a rock, the force acting on the rock is the force of gravity --the two masses involved are the mass of Earth and the mass of the rock Newton’s equations show that the acceleration of gravity is independent of the mass of a falling object, so all objects fall at the same rate

Question 37

escape velocity

Answer 37

If an object gains enough orbital energy, it may achieve escape velocity and leave the gravitational influence of the object it was orbiting

Question 38

how does gravity cause tides?

Answer 38

The Moon’s gravity creates a tidal force that stretches Earth along the Earth–Moon line, causing Earth to bulge both toward and away from the Moon Earth’s rotation carries us through the two bulges each day, giving us two daily high tides and two daily low tides. Tidal forces also lead to tidal friction, which is gradually slowing Earth’s rotation and explains the synchronous rotation of the Moon

Question 39

newton's law of gravitation

Answer 39

There is a force b/w any objects in the universe The force is proportional to the product of the masses of each object The force is inversely proportional to the square of the distance

Question 40

newtonian physics

Answer 40

Acceleration: rate of change of velocity (change in speed OR direction) Speed: rate of change of position

Question 41

angular momentum

Answer 41

Angular momentum (m x v x r) If you add up the angular momentum of everything in a closed system you will find that no matter what happens, this momentum is always conserved More mass=more momentum More velocity=more momentum Collision transfers momentum from the first ball to the second ball Conserved as Earth orbits the sun -greater distance=smaller velocity

Question 42

newton's universal law of gravitation

Answer 42

Tells us the strength of the gravitational attraction b/w the 2 objects There is a force b/w any 2 objects in the universe The force is proportional to the product of the masses of each object Double the mass, double the force Force is inversely proportional to the square of the distances Double the distance, the force decreases by 2 x 2 =4 Half the distance, the force increases by 2 x 2 =4x

Question 43

acceleration by gravity

Answer 43

When you drop something, it speeds up as it falls Near the earth, ignoring air resistance, all objects speed up by around 10 m/s every second If there is no air resistance, then it doesn’t matter what the object is It will speed up at the same rate

Question 44

escape velocity

Answer 44

The path of objects (elliptical orbits) is described by Newton’s laws An object can strike the earth, orbit the earth, or escape from the earth, depending on how fast it is going. The velocity required to escape the earth is called the escape velocity It does not depend on the object’s mass. Only on the Earth’s mass.

Question 45

What is the conservation of energy and momentum and how does this affect celestial bodies?

Answer 45

Momentum is mass x velocity, and its conservation explains the orbits of planets around the Sun

Question 46

What is the escape speed and how does it relate to the orbit of solar system objects?

Answer 46

Speed required to escape the gravity of an object Depends on the mass of the object you are trying to escape from

Question 47

Why do all objects fall at the same rate?

Answer 47

Acceleration due to gravity is mass independent

Question 48

speed

Answer 48

Definition: Speed is the distance an object travels in a certain amount of time. Example: A car moving at 100 km/h means it covers 100 kilometers every hour. Key Point: Speed is a scalar quantity—it has magnitude but no direction

Question 49

velocity

Answer 49

Definition: Velocity tells us both the speed and direction of an object’s motion. Example: “100 km/h due north” is a velocity. Key Point: Velocity is a vector—it includes direction as well as magnitude

Question 50

acceleration

Answer 50

Definition: Acceleration is the rate of change of velocity—it occurs whenever speed or direction (or both) changes. Example: Speeding up, slowing down, or turning are all forms of acceleration. Key Point: Even turning at constant speed counts as acceleration

Question 51

force

Answer 51

Definition: A force is any interaction that can change an object’s momentum; in other words, it causes acceleration. Equation: F=m×a (Newton’s Second Law) Unit: Newton (N) = kg·m/s² Key Point: A net force is required to change motion; if there's no net force, motion stays constant

Question 52

momentum

Answer 52

Definition: Momentum is the product of mass × velocity. It measures how much motion an object has. Equation: Momentum=m×v Key Point: A moving truck has more momentum than a moving bicycle, even at the same speed, because of its greater mass

Question 53

angular momentum

Answer 53

Definition: Angular momentum is a special form of momentum for spinning or orbiting objects. Equation: Angular Momentum=m×v×r, where 𝑟 is the radius or distance from the center. Key Point: Objects conserve angular momentum unless acted upon by a torque (twisting force). This is why Earth keeps rotating and orbiting the Sun

Question 54

newtons 1st law

Answer 54

Newton’s First Law (Law of Inertia) Definition: An object moves at constant velocity unless a net force acts to change its speed or direction Application to Gravity: A planet in space would move in a straight line at constant speed if no force acted on it. However, the gravitational force from the Sun continuously changes the planet’s direction, pulling it into a curved (elliptical) orbit. Without gravity, the planet would not orbit—it would fly off in a straight line.

Question 55

Newton’s Second Law

Answer 55

(Law of Acceleration) Definition: Force = mass x acceleration or force= rate of change of momentum Application to Gravity: The gravitational force from the Sun causes planets to accelerate continuously—changing their velocity’s direction as they orbit. The closer a planet is to the Sun, the stronger the gravitational force, and the greater the acceleration. That’s why Mercury orbits faster than Neptune. The second law quantifies this relationship.

Question 56

Newton’s Third Law

Answer 56

(Action-Reaction Law) Definition: For every action, there is an equal and opposite reaction Application to Gravity: The Earth pulls on the Moon with a gravitational force, and the Moon pulls back on the Earth with an equal and opposite force. Even though Earth’s mass is much greater, the forces are equal. This explains tidal effects and why both bodies actually orbit around a common center of mass.

Question 57

Summary of Application to Gravity:

Answer 57

Gravity is the net force that constantly accelerates planets and moons in curved paths, rather than allowing them to travel in straight lines. According to Newton’s second law, this curved motion is a result of the gravitational force acting on the mass of the object. Thanks to the third law, we know that gravitational interactions are mutual—the Sun pulls on Earth just as much as Earth pulls on the Sun, even though the Sun barely budges due to its massive size. Newton’s insight showed that the same physical laws apply on Earth and in the heavens, uniting celestial and terrestrial motion in a single framework.

Question 58

Describe how the force of gravity between two objects depends on their masses and the distance between them

Answer 58

The force of gravity between two objects depends on both their masses and the distance between them, as described by: Newton’s Universal Law of Gravitation: Newton formulated the gravitational force as: Where: Fg= gravitational force between the 2 objects M1 and M2= masses of the 2 objects d= the distance between the centers of the 2 masses G= the gravitational constant Key principles Every mass attracts every other mass through gravity. The gravitational force is directly proportional to the product of the two masses. Doubling either mass doubles the gravitational force. The gravitational force is inversely proportional to the square of the distance between the objects. Doubling the distance reduces the gravitational force to one-fourth. This is known as the inverse-square law.

Question 59

real world example of the force of gravity

Answer 59

If you increase the mass of Earth or the Moon, the gravitational pull between them increases. If you move the Moon twice as far away, the gravitational pull becomes ¼ as strong. This equation explains why: Massive objects like planets and stars have strong gravitational pulls. Gravity weakens rapidly with distance. It underpins all orbital motion, such as Earth orbiting the Sun or moons orbiting planets.

Question 60

define free-fall

Answer 60

Free-fall refers to the motion of an object when no force other than gravity is acting upon it In this state, the object experiences weightlessness because there is nothing supporting it against gravity—not even air resistance. The object is accelerating downward at the acceleration due to gravity, which on Earth is approximately: g=9.8m/s^2 Key Characteristics of Free-Fall (as described in the PDF): Acceleration: All objects in free-fall accelerate at the same rate, regardless of their mass, when air resistance is negligible. Weightlessness: In free-fall, a person or object experiences a sensation of weightlessness, as there is no contact force pushing back against gravity. Examples: An astronaut orbiting Earth is in continuous free-fall around the planet. A person who jumps from a height before hitting the ground is in brief free-fall. A broken elevator in free-fall would make you feel as if you’re floating. Common Misconception Clarified: Many believe that astronauts in space experience weightlessness because there is "no gravity." In reality, gravity is still present; astronauts feel weightless because they are in a constant state of free-fall as they orbit Earth, essentially falling around the planet rather than into it

Question 61

Understand why astronauts appear ‘weightless’ in space

Answer 61

Astronauts appear weightless in space not because there is no gravity, but because they are in a state of continuous free-fall around Earth Continuous Free-Fall Astronauts in orbit are constantly falling toward Earth, but because of their high horizontal velocity, they keep missing Earth This creates a situation in which they are in free-fall around the planet, just like the spacecraft they are in. Gravity pulls them down. Their sideways speed keeps them moving forward. The combination makes them fall around Earth in a curved path (i.e., an orbit), rather than crashing into it. This is just like Newton’s thought experiment: if you could run fast enough off a tall tower, you'd fall around Earth instead of into it.

Question 62

when in free fall

Answer 62

Free-Fall = Weightlessness When in free-fall: There is no force pushing up from a floor or support (unlike standing on Earth). A scale would read zero, because nothing is resisting gravity. You and your spacecraft fall at the same rate, so you feel no contact or resistance. This results in the sensation of floating—what we call weightlessness