Physics quiz 3
moon period, speed, and distance
27.3 days, slowest, 60 radius of earth
When a satellite in circular orbit slows, perhaps due to the firing of a "retro rocket," it ends up gaining more speed than it had initially. Why?
Upon slowing it spirals toward Earth and in so doing has a component of gravitational force in its direction of motion that causes it to gain speed. When it slows and spirals toward Earth there is a component of gravitational force that does work to increase the KE of the satellite.
black holes
VERY dense
torque
a force that causes rotation
Why it is easier to remove an old bolt if we add an extension to the wrench handle?
a higher torque is applied
centripetal force
any force that causes a body to move in a circular path
The center of gravity of a basketball is located _______.
at its geometrical center
At what point do satellites moving with an elliptical path reach a maximum speed?
at the closest point to the planet
a space shuttle orbits about 200 km above earth's surface so as to be above earth's
atmosphere
center of mass
average position of mass
center of gravity
average position of weight
what is the centripetal force on the water when you spin a bucket
bucket
The kinetic energy of a satellite remains constant when the satellite follows a __________.
circular path
why does the moon mainly cause tides?
difference (6.7% difference from earth)
if the mass is from the axis of rotation is the rotational inertia greater or less
greater
When an object moves away from the surface of Earth, the gravitational force decreases because __________.
the distance between Earth and the object increases
centrifugal gravity
the force you feel pulling you outward when going in a circle (caused by inertia)
geosynchronous orbit period, speed, and distance
1 day, slower, 5 radius of earth
escape energy
11.2 km/s
for maximum range of a projectile (neglecting air resistance), at what angle should the projectile be launched
45 degrees
maximum range
45 degrees without air, 39degrees to 42degrees with air
a projectile is launched at 20 degrees. at what other angle can the projectile be launched to land in the same place
70 degrees
earth period, speed, and distance
90min, 8km/s, radius of earth
the centers of gravity of three trucks parked on a hill are shown by the dots. which truck will tip over?
A, the one farthest down the hill
You sit at the middle of a large turntable at an amusement park as it is set spinning and then allowed to spin freely. When you crawl toward the edge of the turntable, does the rate of the rotation increase, decrease, or remain unchanged? What physics principle supports your answer?
As you crawl outward, the rotational inertia of the system increases . In accord with the conservation of angular momentum, crawling toward the outer rim increases the rotational inertia of the spinning system and decreases the angular speed.
The Mediterranean Sea has very little sediment churned up and suspended in its waters, mainly because of the absence of any substantial ocean tides. Why do you suppose the Mediterranean Sea has practically no tides? Similarly, are there tides in the Black Sea? In the Great Salt Lake? Your county reservoir? A glass of water? Explain.
Because of its relatively small size, different parts of the Mediterranean Sea and other relatively small bodies of water are essentially equidistant from the Moon (or from the Sun). So one part is not pulled with any appreciably different force than any other part. This results in extremely tiny tides. Tides are caused by appreciable differences in pulls.
Two golfers each hit a ball at the same speed, but one at 60° with the horizontal and the other at 30°. Which ball goes farther? Which hits the ground first? (Ignore air resistance.)
Both balls have the same range. The ball with the initial projection angle of however, is in the air for a shorter time and hits the ground first.
gravitation fields
Einstein's theory of gravitation that warping of space by mass
newton's law of universal gravitation
F=Gm1m2/r^2
A circularly moving object requires a centripetal force. What supplies the centripetal force for satellites that orbit Earth?
Gravitation supplies the centripetal force on satellites.
If you drove your car off the edge of a cliff, why would you be momentarily weightless? Would gravity still be acting on you?
In a car that drives off a cliff you "float" because the car no longer provides a support force. Both you and the car are free fall. But gravity is still acting on you, as evidenced by your acceleration toward the ground. So, by definition,
With respect to a tidal calendar, when are the tides lowest and best for digging clams?
Lowest tides occur along with highest tides, spring tides. So the tide cycle consists of higher-than-average high tides followed by lower-than-average low tides (best for digging clams!).
if air drag can be neglected, at what point in its trajectory does an upwardly thrown baseball have its minimum speed? How does this compare with the horizontal component of its velocity at other points?
Minimum speed occurs at the top, which is the same as the horizontal component of velocity anywhere along the path.
If all of Earth's inhabitants moved to the equator, how would this affect Earth's rotational inertia? How would it affect the length of a day?
Rotational inertia would increase. By angular momentum conservation, the rotation of Earth would slow (just as a skater spins slower with arms outstretched), tending to a longer day.
Dan and Sue cycle at the same speed. The tires on Dan's bike are larger in diameter than those on Sue's bike. Which wheels, if either, have the greater rotational speed?
Sue's tires have a greater rotational speed for they have to turn more times to cover the same distance.
Which will have the greater acceleration rolling down an incline—a bowling ball or a volleyball? Defend your answer.
The bowling ball will have greater acceleration. Rotational inertia is less for the solid ball so it is easier to accelerate. The object with the least rotational
A heavy crate accidentally falls from a high-flying airplane just as it flies directly above Mike's shiny red Corvette in a parking lot. Relative to the Corvette, where does the crate crash?
The crate will not hit the Corvette, but will crash a distance beyond it determined by the height and speed of the plane.
What happens to the force of attraction between two planets when the distance between them is doubled?
The force decreases to one quarter.
Most people know that the ocean tides are caused principally by the gravitational influence of the Moon, and therefore they think that the gravitational pull of the Moon on Earth is greater than the gravitational pull of the Sun on Earth. What do you think?
The gravitational pull of the Sun on Earth is greater than the gravitational pull of the Moon. The tides, however, are caused by the differences in gravitational forces by the Moon on opposite sides of Earth that are greater than the corresponding difference in forces by the stronger pull of the more distant Sun.
Why do the passengers in high-altitude jet planes feel the sensation of weight, while passengers in an orbiting space vehicle, such as a space shuttle, do not?
The high-flying jet plane is not in free fall so a passenger experiences no net force. The upward support force of the seat equals the downward pull of gravity, providing the sensation of weight. The orbiting space vehicle, on the other hand, is in a state of free fall. No support force is provided by the seat. With no support force, the force of gravity on the orbiting passenger is not sensed as weight.
Fragments of fireworks beautifully illuminate the night sky. (a) What specific path is ideally traced by each fragment? (b) What paths would the fragments trace in a gravity-free region?
The paths are parabolas. The paths would be straight lines.
if a satellite circled Earth at a distance equal to the Earth-Moon distance, how long would it take for it to make a complete orbit? In other words, what would be its period?
The period of any satellite at the same distance from Earth as the Moon would be the same as the Moon's, 27.3 day
Without anyone holding your feet steady, which is more difficult—doing sit-ups with your knees bent, or with your legs straight out? Defend your answer.
With your legs straight out, your CG is farther away and you exert more torque sitting up. So sit-ups are more difficult with legs straight out, a longer lever arm.
Does Earth's gravity pull harder on some people than on others?
Yes, gravity pulls harder on more massive people.
Compared to slowly rolling a ball off a tabletop, will a faster-moving ball hit the floor with a higher speed? Defend your answer.
Yes, it will hit with a higher speed in the same time because the horizontal (not the vertical) component of motion is greater.
Why must you bend forward when carrying a heavy load on your back?
You bend forward when carrying a heavy load on your back to shift your CG and your load above the area bounded by your feet—otherwise you topple backward.
a zoo-keeper devises a rubber-band gun to shoot food to a monkey who is too shy to come down from the trees, of the monkey does not move should the zoo keeper aim above, at, or below the monkey?
above
What is required to change the angular momentum of a system?
external torque
The torque exerted by a crowbar on an object increases with increased _______.
force and leverage distance
what is the centripetal force when you spin a bucket around on the bucket
hand
fast moving projectiles (satellites)
high tangential velocity (8 Km/sec)
conservation of angular momentum
if there is no external torque then the angular momentum of a system stays the same
If the polar ice caps on Earth's solid surface were to melt, the oceans would be deeper. Strictly speaking, what effect would this have on Earth's rotation?
in accord with the conservation of angular momentum, if mass moves farther from the axis of rotation, as occurs with ice caps melting, rotational speed decreases. So Earth would slow in its daily rotation. Earth's daily rotation slows down.
What would be the path of the Moon if somehow all gravitational forces on it vanished to zero?
in accord with the law of inertia, the Moon would move in a straight-line path instead of circling both the Sun and Earth.
Rotational inertia of an object rotating about an axis becomes greater with _______.
increased mass and increased distance to mass concentration
when you whirl a tin can in a horizontal circle overhead, the force that holds the can in in the circular path acts in a direction
inward
how is the force between two planets altered when they are moved twice as far apart
it is 1/4 as much
how does the melting of the polar ice caps effect the rotation of the earth
it slows down the rotation of the earth
which planet in our solar system is moving the fastest?
mercury
rotational speed
number of rotations in a given time interval (angular)
rotational intertia
resistance of an object to change its rotational state of motion
angular momentum
rotational inertia x rotational velocity
consider the various position of the satellite as it orbits the planet as shown, with respect to the planet, in which position does the satellite have the maximum total mechanical energy?
same at all positions
as a bug walks in halfway from the edge of a spinning turntable what happens to its rotational speed? what happens to its tangential speed?
stays the same, decrease
projectile motion
the horizontal velocity is constant, the vertical acceleration is constant, the horizontal and vertical direction are independent
tangential speed
the speed of something moving along a circular path
if your roll a hoop, a solid disk, and a solid, sphere down ramp, which one will get to the bottom of the ramp first
the sphere
what pulls harder the earth, the sun, or the moon?
the sun
circular satellite orbits
velocity and force (speed is constant) always creates a right angle
a rocket fired vertically at a speed of 11.2 km/s will escape the earth. if it is instead launched horizontally at the same speed and it doesn't hit mountains how other obstructions, and air resistance can be neglected, will it still escape the earth?
yes
