# Physics – Chapters 15 & 16

(There is no way you can tell whether you are being pulled by gravity or being accelerated)

Who created the “general theory of relativity”?

Albert Einstein

What does the “general theory of relativity” show? It shows how ______ is related to _____ and ______

gravity, space, time

Einstein reasoned that space and time are two parts of one whole called…

space-time

What does Einstein’s “special theory of relativity” describe? It describes how ____ is affected by _______ in space at a _______ velocity, and how ____ and ______ are related.

time, motion, constant, mass, energy

From the viewpoint of special relativity, you travel through a combination of _____ and _____.

space, time

If you stand still, are you traveling?

Yes, you are moving through time

If you move a little bit, how are you traveling?

some is through space and most of it is through time

If you move at the speed of light, how are you traveling?

all travel is through space and none through time

Light travels through space only and is …

timeless

A photon traveling from one part of the universe to another, takes how much time?

none (it is traveling at the speed of light)

How can you describe a person’s travel from the viewpoint of special relativity?

space and time

What is a postulate?

assumption

What is time dilation?

the stretching of time

According to special relativity, mass and energy are …

equivalent

According to general relativity, gravity causes …

space to become curved and time to undergo changes

What is the speed limit in the universe for an object with mass?

speed of light

What does “p=mv” stand for?

momentum equals mass times velocity

What is p=(mv)/sqr(1-(v^2/c^2)), where v = speed of an object and c = speed of light

realistic momentum

As an object approaches the speed of light, its momentum…

increases dramatically

When is Newton’s definition of momentum valid?

at low speeds

“Rest mass” of an object, represented by m in the relativity equation, is a true constant, a property of the object no matter what …

speed it has

How does an object’s momentum change as it approaches the speed of light?

It increases dramatically.

For a particle with a large momentum, the trajectory is …

stiffer

A piece of matter, even if at rest and even if not interacting with anything else, has an “energy of being” called its…

rest energy

What does E = mc^2 stand for?

Energy = Mass x (speed of light)^2

In E = mc^2, the speed of light, “c”, is a large quantity and its square is even larger. This means that a small amount of mass stores…

a large amount of energy

In E = mc^2, the quantity c^2 is a “conversion factor.” It converts the measurement of mass to the measurement of …

equivalent energy

For all chemical reactions that give off energy, there is a corresponding…

decrease in mass

What is the relationship between mass and energy?

Mass and energy are the same.

The correspondence principle states…

that new theory and old must overlap and agree in the region where the results of the old theory have been fully verified.

According to the correspondence principle, if the equations of special relativity (or any other new theory) are to be valid, they must correspond to those of Newtonian mechanics when speeds are…

much less than the speed of light.

The equations of special relativity hold for all speeds, although they are significant only for speeds…

near the speed of light

The general theory of relativity was a new theory of gravitation, in which gravity caused space to become _______ and time to ____ _____.

curved, slow down

The principle of equivalence states that local observations made in an accelerated frame of reference cannot be distinguished from…

observations made in a Newtonian gravitational field

(There is no way you can tell whether you are being pulled by gravity or being accelerated)

(There is no way you can tell whether you are being pulled by gravity or being accelerated)

The presence of mass produces a curvature or warping of …

space-time

A curvature of space-time reveals the presence of

mass

The ripples that travel outward from the gravitational sources at the speed of light are…

gravitational waves

What is the relationship between the presence of mass and the curvature of space-time?

mass produces the curvature of space-time

What is an illustration of how mass produces the curvature of space-time?

a ball resting on a waterbed

The first person to understand the relationship between space and time was…

Albert Einstein

Light travels through _____ only and is _______.

space timeless

What does the first postulate of special relativity state?

It states that all laws of nature are the same in all uniformly moving frames of reference.

Einstein’s first postulate of special relativity assumes…

our inability to detect a state of uniform motion.

Einstein’s second postulate of special relativity assumes…

that the speed of light is constant.

The second postulate of special relativity states…

that the speed of light in empty space will always have the same value regardless of the motion of the source or the motion of the observer.

Time dilation occurs ever so slightly for ______ speeds, but significantly for speeds approaching __________

everyday, the speed of light

Does time dilation mean that time really passes more slowly in moving systems or that it only seems to pass more slowly?

Time really does pass more slowly in a moving system compared with one at relative rest.

If you were moving in a spaceship at a high speed relative to Earth, would you notice a difference in your pulse rate? In the pulse rate of the people back on Earth?

No relative speed between you and your pulse. There would be a relativistic effect between you and people back on Earth. You would find their pulse rate slower and vice versa.

Distance/Time =

C or the speed of light

How does time dilation at everyday speeds compare with time dilation at light speed?

It is smaller at everyday speeds.

Will observers A and B agree on measurements of time if A moves at half the speed of light relative to B?

When A and B have different motions relative to each other, each will observe a slowing of time in the frame of reference of each other. So, they will not agree on the measurements of time.

If A and B move together at 0.5c relative to Earth, will they agree on measurements of time?

Yes, they share the same frame of reference and will agree on measurements of time.

Time, as we know it, travels only one way –

forward

Why does space travel at relativistic speeds seem impossible?

The amounts of energy required to propel spaceships to relativistic speeds are billions of times the energy used to put the space shuttles into orbit.

The observable shortening of objects moving at speeds approaching the speed of light is…

length contraction

The amount of contraction is related to the amount of _____________.

time dilation

For ______ speeds, the amount of contraction is much too small to be measured.

everyday

As relative speed gets closer and closer to the speed of light, the measured lengths of objects contract closer and closer to…

zero.

A meterstick traveling at 87% the speed of light relative to an observer would be measured as only ____ as long as normal.

half

Space contracts in only one direction, the direction of _____.

motion

A spacewoman travels by a spherical planet so fast that it appears to her to be an ellipsoid (egg shaped). If she sees the short diameter as half the long diameter, what is her speed relative to the planet?

87% of the speed of light

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