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Z Physics > 1. Attracted To The Earth > Flashcards

1. Attracted To The Earth Flashcards

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1
Q

What is the gravitational field strength, g? What is the unit?

A

The force per unit mass acting on a body

Measured in Nkg^-1

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2
Q

What is the equation for weight?

What happened equation is similar to this equation and why?

A

Weight = mass x gravitational field strength
w=mg

It is similar to f=ma and g can be viewed as the acceleration so that Nkg^-1= ms^-1

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3
Q

What is ms^-1 equivalent to?

A

Nkg^-1

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4
Q

What is the unit for weight?

A

Newtons

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5
Q

What is responsible for the sensation of weight when sitting on a seat?

A

The normal reaction force from the chair, not the gravity

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6
Q

Why does a person jumping out of a plane not experience their weight? Why is there a feeling of weightlessness?

A

There is no reaction force (there is, however, drag/ air resistance which they would feel). There is a sensation of weightlessness because although the gravitational attraction to the earth is present, there is NO REACTION FORCE.

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7
Q

What are two possible reasons why an object should be weightless?

A
  • there is no mass

- there is no net gravitational field

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8
Q

Why can true weightlessness almost never be achieved?

A

Everywhere in the universe is subject to some gravitational field so true weightlessness can only be achieved if two identical gravitational forces act on an object and cancel each other out however there will still be gravitational effects from the rest of the universe

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9
Q

What is the point at which gravitational fields are cancelled out called?

A

The neutral point

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10
Q

Describe the forces experienced by a person going up from rest to the the tenth floor in a lift

A

At rest, the forces of weight due to gravity and the normal reaction force experienced are in equilibrium. There is no resultant force as the lift is stationary. As the lift accelerates upwards, the normal reaction force increases then levels out as the lift reaches a constant speed so the forces are in equilibrium. As the lift decelerates as it reaches the tenth floor, the normal reaction force decreases due to f being equal to mg (f=ma) so the person feels lighter.

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11
Q

What factors do human tolerance to acceleration depend on? What is the max g force that a person can tolerate compared to a pilot?

A

Depends on:
- acceleration
-how fit the person is
-duration of acceleration
-position of those when accelerated
A person can withstand about 5g (vertical acceleration) whereas a pilot can withstand 9g for a longer period of time
Downward acceleration drives blood to the head so people can withstand less- about 2/3g

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12
Q

What is viscosity?

A

A fluid’s resistance to flow or its “stickiness”

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13
Q

What is the SI unit of the coefficient of viscosity?

A

Pa s (pascal seconds)

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14
Q

What happens when a fluid flows over an object?

A

The layers in the fluid exert resistive forces on each other, producing stress across the fluid

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15
Q

What causes viscosity in a gas?

A

Viscosity in gases arises from the molecular diffusion which carries momentum between the different layers in a moving gas- this is INDEPENDENT OF PRESSURE but IS affected by increasing TEMPERATURES

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16
Q

What is streamline flow?

A

When molecules of the fluid remain in layers and do not cross over into other layers

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17
Q

What is turbulent flow?

A

When molecules move from layer to layer in a chaotic manner eg. If an aircraft wing is pitched at too large an angle, the turbulence produced can cause the aircraft to rise and fall in an unpredictable way

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18
Q

What limits the maximum speed a rocket accelerating upwards can achieve?

A

-gravity
-drag
They oppose the motion of the rocket

19
Q

Why does the tip of the rocket heat up when coming back to earth?

A

Air resistance opposes the motion of the rocket. air in front of it is compressed which heats up to high temperatures

20
Q

What is the process called in which the material coating a space capsule melts then evaporates?

A

ABLATION

This removes heat from the surface of the capsule because vaporising the material on the surface requires LATENT HEAT.

21
Q

What are the stages in making a safe landing in s space shuttle?

A
  • shuttle inverts to gain protection from solar radiation
  • rear thrusters fire to move shuttle into a tail first position which helps to slow it down.
  • shuttle returns to a nose first position once it has slowed down
  • leftover fuel burnt to prevent it igniting
  • shuttle slows down further by making wide S shaped turns-at this time, the air resistance raises the temperature so astronauts loose communication for about 12 minutes
  • 600m above land, the shuttle is brought down at a very steep angle and landing gear is deployed
22
Q

What did Newton propose about a falling Apple, a cannon ball fired from a cannon and the moon?

A

An apple falls from a tree no matter how tall the tree is so there must be a force causing it to fall. A cannonball will eventually fall to the ground when fired but choosing taller and taller towers the distance the cannonball travels increases and the curvature of the earth will mean it eventually falls below the horizon. Eventually the curvature of the earth will match the cannonballs trajectory so the ball will be in a fixed orbit around the earth. This is the same with the moon- it is held in orbit by gravity. If this happens to the moon, it can happen with other objects- all objects attract all other objects.

23
Q

Using Kepler’s experimental laws of planetary motion, what did Newton conclude?

A

He concluded that gravity was an inverse square law which takes the form:
F= G x m1m2/r^2

24
Q

What does Newton’s universal law of gravitation show?

A

That every particle in the universe attracts every other particle and is directly proportional to their masses (m1m2) and inversely proportional to the square of the distance (r)

The distance between two objects is measured from the centre of mass

25
Q

What does G stand for?

A

The universal gravitational constant and has a value of 6.67x10^-11Nm^2kg^-2

It is such a small constant that to be significant, the masses of the objects must be very large

26
Q

What do field lines represent? What does the density of the lines indicate?

A
  • they represent the direction an object would travel if placed on the line
  • the density shows the strength of the fields
27
Q

What is centripetal force?

A

A force acting on an object moving in a circle wi a constant speed which is directed towards the centre of the circle

28
Q

what provides centripetal force?

A
  • weight
  • tension
  • reaction
  • gravitation
29
Q

What is the equation for centripetal force?

A

F=mv^2/r

30
Q

Describe the movement of a ball being swung around a point at a constant velocity

A

Speed is constant but velocity is changing due to the direction continually changing so the object is continually accelerating and therefore requires centripetal force to do this. At any instant, the direction of the objects velocity is at tangential to the circle

31
Q

What is the angle traveled by an object completing a circle?

A

2pi radians

32
Q

A object is travelling in a circle (circular motion). What is the object’s steady angular speed (represented by omega(w))?

A

T=2pi/omega (w)

33
Q

What is weight

A

Weight is the force of gravity acting on a body

34
Q

How would you find the mass of the sun?

A

Equate the centripetal force to the gravitational attraction

35
Q

(1 mark)

Explain why a geostationary satellite is neccesary for communication eg TV broadcasting

A

Avoids dish having to track/ stationary footprint

36
Q

(3 marks)

Explain the meanings of g and G

A
  • G is the gravitational constant and g is the gravitational field strength
  • g is the force on 1kg on or close to the earth’s surface
  • G is the universal constant relating attraction of any two masses with their separation/ constant in Newton’s laws of gravitation
37
Q

(4 marks)
In a fairground there is a ride that spins round and the floor is dropped whilst the rider says pinned to the wall. Explain there is a minimum rotational speed required to ensure the rider remains pinned to the walls.

A
  • Mention of centripetal force
  • Reaction of wall provides centripetal force
  • Frictional force (+ reaction of floor) equals the weight
  • As the ride accelerates, reaction of wall increases
  • This increases the frictional force
38
Q

(3 marks)
Whilst in motion on a spinning ride called a rotor, a rider drops a glove. Describe and explain the subsequent motion of the glove

A
  • Glove falls under gravity
  • Also travels along tangentially to circle (as its velocity is tangential to the circle)
  • Until it hits the wall which can provide the necessary reaction to give required centripetal force (wall)/ friction (floor)
39
Q

(3 mark)

Explain why a rocket would be able to escape Mars with an initial speed much less than the calculated escape velocity.

A
  • Differentiates from escape speed- unpowered etc
  • Fuel/ chemical energy
  • Transferred into gravitational potential energy
  • This can be done at any speed (given enough fuel)
40
Q

(2 marks)
The earth and Mars rotate around the sun in the same directional sense. Why does the distance between the sun and mars vary?

A
  • Gravitational attraction of earth
  • Varies with relative positions
  • Orbital speeds of Mars and Earth different
41
Q

(2 marks)
State how the gravitational potential energy of a rocket travelling from Mars to the Earth changes
-Relative to the Earth
-Relative to Mars

A
  • Relative to Earth- decreases (more negative)

- Relative to Mars- increases (less negative)

42
Q

(2 marks)
The gravitational potential energies of the sun and the earth affect a rocket journey from mars to the earth. Suggest why a space agency might wish to consider the relative positions of the planets when planning such a journey.

A

Any two from:

  • Gravitational attraction is maximised by proximity of the planets
  • Reduce journey time
  • Shortest distance to travel
  • Shortest distance to neutral point
  • Less work done by rocket
  • Less fuel needed
  • Wrongly aligned planets can attract shuttle on wrong course
43
Q

(2 marks)

state two reasons why stokes law cannot be applied to the motion of a skydiver.

A

any two from

  • not spherical
  • not small
  • air not viscous enough
  • moving too fast
  • non-laminar flow
44
Q

(1 mark)

State the factors that affect gravitational field strength at the surface of a planet.

A
  • Mass of the planet

- Radius of the planet

Z Physics (13 decks)

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