4.2 Out Into Space Flashcards
What is a vector quantity?
Something that has magnitude and direction.
Give the equation for momentum.
p = m v
What the the units of momentum?
kg m s^-1 (or Ns)
Is momentum always conserved?
Yes.
What is Newton’s 1st law of motion?
The velocity of an object will not change unless a resultant for acts upon it.
What is the resultant force on an object moving at a constant speed?
Zero
What is Newton’s 2nd law of motion?
The rate of change of momentum of an object is directly proportional to the results force which acts upon the object.
Give the equation for Newton’s 2nd law.
F = ∆mv / ∆t
Give the equation for Newton’s 2nd law for when mass does not change.
F = ma
What is the resultant force?
The vector sum of all forces acting on an object.
What are the units for force?
Newtons, N.
What are the units for mass?
Kilograms, kg.
If which direction will acceleration occur if there is a resultant force?
In the direction of the resultant force.
What is Newton’s 3rd law?
If object A exerts a force on object B, then object B exerts an equal but opposite force on object A.
Gravity pulls down on a book whilst a table pushes upward on the book. Why is this not a case of Newton’s 3rd law?
Because the forces are not of the same type. Gravity pulling the Earth up would be an example of Newton’s 3rd law though.
Work is done when…
Energy is transferred.
Give the equation for work done.
Foce x Distance
What are the units for work done?
Joules, J.
How could you find work done from a graph?
Find the area under a F vs s graph.
W = F s cosθ where θ is…
The angle between the force and the direction of motion.
Energy cannot be created or destroyed but it can be…
Transferred from one form to another.
Give the equation for efficiency.
Efficiency = useful power output / total power output
Name three types of potential energy.
Gravitational, elastic and electrical.
Give the equation for gravitational potential energy of objects moving close to the Earth’s surface.
E = m g ∆h
Define radians.
The arc-length divided by the radius of the circle.
How many radians are in a complete circle?
2π.
Convert 45º into radians.
π/4 rad.
What is the definition of angular speed?
The angle an object rotates though per second.
Give the equation for angular speed, ω.
ω = θ / t
Give the equation for linear speed in terms on angular speed.
v = r ω
What is the equation for frequency in terms of time period?
f = 1 / T
Give the angular speed of an object in terms of frequency (in revolutions per second).
ω = 2 π f
A car is travelling at a constant speed in a circular motion. Why is it accelerating?
It’s velocity is changing since it’s direction is changing and acceleration is rate of change of velocity.
Give an equation for radial acceleration in terms of linear speed.
a = v^2 / r
Give an equation for centripetal acceleration in terms of angular speed.
a = ω^2 r
Give an equation for radial acceleration in terms of angular speed and linear speed.
a = v ω
Give two equations for centripetal force.
F = mv^2 / r F = m ω^2 r
Any mass in a gravitational field will experience an…
Attractive force.
Force experienced in a gravitational field is always…
Attractive, never repulsive.
Gravitational force experience depends on what?
The masses of the two objects and the distance between them.
What is a point mass?
Objects that behave as if all their mass is acting at a single point.
What is Newton’s law of gravitation?
F = -(G M m) / r^2
What does r represent in Newton’s law of gravitation?
The distance between the centre of the two masses.
What is G in Newton’s law of gravitation, and what is it’s value?
The gravitational constant. 6.67 x 10^-11
What are gravitational lines of force (field lines)?
Lines with arrows that show the direction of the force that masses would feel in a gravitational field.
If you double the distance between two objects, the gravitational force experienced by the objects will…
Be one quarter of the original value.
Descrive a radial field.
A field where the field lines meet at the centre of a mass..
What is a uniform field and where might you find one?
A field where the field lines are parallel. At the Earth’s surface.
What is field strength, g, defined as?
Force per unit mass.
What are the units of field strength?
N kg^-1
g is a vector quantity. Where does the vector point?
To the centre of the mass of the field being described.
Why do you assume g is constant around the surface of the Earth?
Because the field at the surface of the Earth is almost uniform.
What is the value of g (acceleration due to gravity) at the Earth’s surface?
-9.81 m s^-2 (N kg^-1)
What is the gravitational force experienced by an object in a uniform field?
mg
The force experienced by a mass in a uniform field is?
Constant.
In a uniform gravitational field, potential energy increases with…
Height.
What is the equation for change in gravitational potential energy in a uniform gravitational field?
∆PE = m g ∆h
In a radial field, g is inversely proportional to…
r^2
Give the equation for field strength in a radial field.
g = -(G M) / r^2
In a radial field, you gain gravitational potential energy if you move away from…
The mass causing the field.
Give the equation for gravitational potential energy in a radial field.
E grav = - (G M m) / r
Where is potential energy defined as zero?
At an infinite distance away from the mass.
Is gravitational potential energy at the Earth’s surface positive or negative?
Negative.
What is gravitational potential defined as?
The gravitational potential energy per unit mass.
What are the units of gravitational potential?
J Kg^-1
What is the symbol for gravitational potential?
V
What is the equation for gravitational potential in a radial field?
V = -(G M) / r
Where is gravitational potential defined as zero?
At an infinite distance away from the mass.
What does the gradient of a E grav against radius graph represent?
The value of the gravitational force at that point.
What does the gradient of a V against radius graph represent?
The gravitational field strength at that point.
What is a satellite?
A small mass that orbits a far greater mass.
How could you calculate the speed of an orbiting satellite?
Equate Newton’s law of gravitation with F = (m v^2) / r and rearrange to make v the subject.
