Our Dynamic Universe

Question 1

Quantities

Answer 1

Scalar quantities: have magnitude only (e.g. speed, distance, frequency, energy)

Vector quantities: have both magnitude and direction (e.g. velocity, displacement, acceleration, force)

Question 2

Terminal velocity

Answer 2

As the velocity of an object increases the size of the frictional forces acting on it also increases. This effect means as the falling object accelerates the unbalanced force on it decreases. Eventually the object falls at a velocity where the forces acting on it are balanced - this is the object’s terminal velocity.

Question 3

Acceleration

Answer 3

Acceleration is the rate of change of velocity

Units: ms-2 (meters per second per second)

Question 4

Law of conservation of energy

Answer 4

Energy cannot be created or destroyed, but can only be converted from one form into another
(e.g. Ek lost = Ep gained)

Question 5

(in/) elastic collisions

Answer 5

In an elastic collision both momentum and total kinetic energy is conserved
(total Ek before = Ek after)

In an inelastic collision momentum is conserved, but total kinetic energy is not conserved
(total Ek before ≠ Ek after)

Question 6

Law of conservation of momentum

Answer 6

The total momentum before a collision is equal to the total momentum after, provided there are no external forces acting on the object

total p before = total p after
m1u1 + m2u2 = m1v1 + m2v2

Question 7

Newton’s laws of motion

Answer 7

First law:
when forces acting on an object are balanced the object remains at rest or continues to move at a constant speed in a straight line

Second law:
When forces acting on an object are unbalances the object accelerates (F=ma)

Third law:
For every action there is an equal and opposite reaction

Question 8

Gravitation

Answer 8

The universal constant of gravitation (G) = 6.67x10^-11

Units: m^3 kg^-1 s^-2

Gravitation is the mutual force of attraction between objects. When an object with mass is placed in a gravitational field it experiences a force. Every object attracts every other object.

Question 9

Weight

Answer 9

Weight is the downwards force due to gravity

W=mg, weight = mass x gravitational field strength

Weight acting on a slope:

• component of weight down a slope = mgsinθ
• component of weight at right angles to the slope = mgcosθ

Question 10

Projectiles

Answer 10

A projectile is an object that is in free fall, i.e. the only force acting on it is gravity

A projectile follows a curves path, the motion of which can be split into:

• horizontal motion: is a constant velocity (s=vt)
• vertical motion: is a constant downwards acceleration (on earth a= 9.8ms-2) (v=u+at) (suvat)
• when at maximum height the vertical velocity = 0

Question 11

Special relativity laws

Answer 11

• when two observer are moving at constant speeds relative to one another, they will observe the same laws of physics
• the speed of light (in a vacuum) is the same for all observers
• however, in order to agree on the speed of light being constant observers in different frames of reference have to disagree about their measurements of time and distance

Question 12

Doppler effect

Answer 12

The doppler effect causes shifts in wavelengths as a source moves relative to an observer

As the source moves towards an observer the wavelength appears to be shorter and the frequency higher (the distance between the wavefronts becomes compressed)

As the source moves away from an observer, the wavelength appears to be longer and the frequency lower (the distance between the wavefronts become elongated)

fo = fs (v/ v +- vs)
+ when source is moving away, - when source is moving towards

Question 13

Length contraction

Answer 13

Length contraction is the apparent decrease in length (in the direction of travel) of an object moving relative to an observer

l’ = l √ 1 - (v/c)^2

l' = length measured by an observer, which the object is moving relative to (in a different frame of reference) 
l = length measured in same frame of reference as the moving object

Question 14

Time dilation

Answer 14

Time dilation is the apparent increase in time of events on an object moving relative to an observer

t’ = t/ √ 1 - (v/c)^2

t' = time measured by an observer which the object is moving relative to (in a different frame of reference)
t = time measured in the same frame of reference as the moving object

Question 15

Redshift

Answer 15

Redshift (z) is when light emitted from objects moving away from an observer appears shifted to longer wavelengths (closer to the red end of the spectrum).

Blueshift/ negative redshift is when light emitted from objects moving towards an observer appears shifted to shorter wavelengths (closer to the blue end of the spectrum)

Question 16

Temperature of stellar objects

Answer 16

The colour of a star indicates the surface temperature of that star (red, cooler –> blue, hotter)

The emitted radiation (energy) of a stellar object is distributed over a wide range of wavelengths. A graph of this emitted radiation has a characteristic shape known as a black-body radiation curve.

The peak wavelength gives the star’s temperature. The peak wavelength is shorter for hotter objects than for cooler objects.

At all wavelengths the radiation emitted per unit surface area per unit time for hotter objects is greater than that for cooler objects.

Question 17

Big bang

Answer 17

The big bang theory is that the universe began as a single point with an extremely large amount of energy. The early universe was very small and incredibly hot. As time went on the universe expanded and cooled, causing the energy to condense into matter which gradually formed atoms and eventually elements.

Evidence that supports the big bang includes:

• redshifts of galaxies
• cosmic microwave background radiation
• obler’s paradox
• measurements from Hubble’s law

Question 18

Cosmic background microwave radiation (CMB)

Answer 18

Cosmic microwave background radiation is radiation left over from the big bang

Originally, the radiation had a much shorter wavelength (so higher frequency) but as the universe expanded and cooled over time the emitted radiation became a lower temperature, so a lower energy and therefore a lower frequency (so longer wavelength).

CMB comes from all directions in space and is thought to have been emitted from a singular point (the big bang).

Question 19

Obler’s paradox

Answer 19

If the universe is static, infinite, infinitely old and contains an infinite number of stars, then theoretically a star should be visible in all lines of sight, causing the night sky to appear white.

This is not the case as the universe is expanding at an increasing rate so stars and galaxies are accelerating away from each other, from a single point, so their light will never catch up to the earth.

(Therefore evidence for the big bang)

Question 20

Dark energy

Answer 20

Our universe is expanding at an increasing rate

However, the force of gravity acting on the universe (determined by the universe’s mass) should eventually slow down and overcome the universe’s expansion, causing all matter to accelerate back towards a central point and collapse in on itself - this is the fate of a closed universe.

Since this is not happening to our own universe it suggest there is a force acting against gravity, pushing matter apart - this force is known as dark energy.

Universes that continue to expand indefinitely are ‘open universes’. This type of universe will undergo a ‘heat death’. This means eventually all energy will become heat energy and as time goes on matter will be so far apart that the heat energy of the universe would be spread too far apart to allow any further production of stars/ galaxies.

Question 21

Dark matter

Answer 21

Galaxies rotate about their cores due to gravity

Stars on the outer arms of the galaxy should travel slower than those towards the galactic core as they are further from the central mass and therefore experience a smaller gravitational force

If an object has a great enough velocity it can escape the gravitational field it is in, therefore escaping from the orbit

Observations of the velocity of stars in galaxies shows stars with a greater orbital radii are moving much faster than predicted for the mass visible to us, and yet remain in orbit. This therefore suggests there must be a significant amount of mass unaccounted for - known as dark matter.