PHYS006: Violent Universe

Speed

rate at which an object moves

• the distance an object moves in a given amount of time

Velocity

a change in an object’s speed in a certain direction

• 10 m/s moving east

Acceleration

• change in an object’s velocity

• Change in either the speed or direction of an object

Forces

forces can change the motion of an object

force: anything that can change the momentum of an object

That means force can change (mass x velocity), but since we usually do not see a change in mass, it just changes velocity

When forces are applied on an object = acceleration

Momentum

a measure of an object’s motion

(mass (matter/stuff) x velocity)

Gravity

is a force that acts between any two objects

• 2 factors influence gravitational attraction between objects: mass & distance

• the force of gravity acts on all objects

• If the mass increases, gravitational forces increase

• If the distance increases, gravitational forces EXPONENTIALLY decrease

  • so they decrease rlly fast

    • If distance btwn objects doubled, their gravitational forces would decrease by a factor of 4

Mass v. Weight

Mass: the amount of matter in an object

Weight: a measure of the forces acting on an object

• i.e. how much gravity is pulling on you

examples:

• Scales → springs → pulled down by gravity = weight (# on scale)

• You weigh less on the moon, but your mass is the same as it is on Earth

Acceleration of Gravity

• remember, forces cause masses to accelerate

• As an object falls it accelerates because of the force of gravity

• The higher you drop an object, the more time it has to fall which increases its velocity since it has more time to increase its speed

Newton’s Laws of Motion

1. An object’s momentum does not change if left alone

   1. It only changes if a force is exerted on it

   2. We don’t really see an application of this law because there’s forces all around us

2. A force can change the object’s momentum

3. For any force, there is an equal and opposite reaction force; momentum is always conserved

Angular Momentum

momentum involved in spinning / circling (mass x velocity x radius)

Conservation of Angular Momentum

• Angular momentum depends on mass, velocity, and radius

• If there is no torque, the total angular momentum (m x v x r) of a system remains constant

• That means that if one quantity decreases, another quantity has to increase so that the total angular momentum remains the same & vice versa

  • In-class demo, spinning with a wide radius = slower turn, spinning with a small radius = faster turn

    • Radius V velocity ^

Tides

• Since gravitational force decreases with (distance)^2, the moon’s pull on Earth is strongest on the side facing the Moon, and weakest on the opposite side

  • Water will move toward the moon

• Earth gets stretched along the Earth-Moon line

• THe oceans rise relative to land at these points

• Tidal bulge

  • Earth and moon are equally attracted to each other

  • The gravitational pull of the moon causes a tidal bulge on one side of the earth as it rotates

  • The other side of the bulge lags behind

    • The bigger bulge is closest to the moon

  • The furthest side from the moon is attracted to the moon,but since its further the force is not as strong

  • As the earth rotates, it goes through these two bulges causing a shift in tides (two high, two low)

    • The moon as it orbits the earth causes the bulges to shift (high tides happen 50 minutes later)

• Every place on earth passes through these points called high tides, twice per day as Earth rotates

• High tides occur every 12 hours 25 minutes

  • Remember the moon moves! So the high tide comes a little later every day

• Sun’s tidal effect on Earth is not as strong

  • The Sun is so far, that the difference between the force  of gravity on one side of Earth and the other is not as great

• But you do get the greatest tides when the Sun and Moon are on the same side

Light year

• Light takes time to travel, though it travels very fast

• A light-year is the distance that light can travel in one year

• One light year is equal to 5.88 trillion miles (don’t memorize)

• Light-year is a unit of distance, not of time