P4: Energy and Hydrostatics Flashcards
What is energy and what is its unit?
the ability to do work. Unit is joules J
True or false: thermal energy is always a product and by product of energy transformation
true
what is work and what is its unit?
work is the energy spent in moving an object through a particular distance.
W = F x d
The definition of Force. what is its unit?
a push or pull that will change the size, shape, or motion of an object. its unit is the Newton(N)
What is power and what is its unit?
power is the rate at which work is done. Unit is watt(W)
equation: P = Energy used up/time= work done/ time = E/t (Js-1 or W)
1W is the same as
1Js-1 = 1 joule per second
What are the different types of forces?
- Gravitational force(Weight)
- Frictional force(friction)
- Spring force(elastics and springs)
- Tensional Force(strings)
- Electrical Force(electric fields)
- Magnetic force(magnetic fields)
- Applied force(push or pull)
- Normal force (reaction force eg book on table)
- Nuclear force(radioactivity)
- Air resistance force(drag)
Whenever a force causes a change in an object (1)_________ is done and therefore (2)_______________ is converted from one form to another, and therefore (3)____________ can be determined
- work
- energy
- power
What is weight? How is it calculated?
The gravitational pull on an object It is a force
W = gavitational constant x mass
What is the moment of a force and what is its formula
its the turning effect of an applied force
T = F x distance form the pivot or fulcrum
principles of moments
sum of clockwise moments about a pivovt = sum of anticlockwise moments about the same pivot
using Newtons second law to calculate force - relationship between force and momemtum
F = ma
momentum(p) = mv
but a = (change in velocity)/time
therefore F = m(v-u))/t
F = (mv-mu)/t
but mv and mu = final and initial momentum
Law of conservation of linear momentum
total momentum of objects before interraction = total momentum of objects after interraction
what is the centre of gravity of an object?
a point in a body from which its weight is considered to act.
What is Hook’s law
The extension of a spring is directly proportional to the stretching force acting on it, providing the force does not extend the spring beyond its elastic limit
Equation to use when calculating spring forces
e = extension
- k = spring constant*
- F = extending force*
What is pressure
Pressure is the force acting over a unit area
Unit Pascal(Pa) = Nm-2
Formula for Pressure in a liquid
Important facts to remember about pressure in a liquid
- it is not affected by the shape of the container
- the pressure is the same at all points on the same horizontal level
- pressure acts equally in all directions at the same depth in the liquid.
- the pressure in a liquid is directly peoportional to the depth below the surface
- pressure is directly proportional to the density of the liquid.
What is Archimides Principle?
The upthrust force on an object wholly or partially immersed in a fluid is equal and opposite to weight of the fluid displaced by the object
When working problems regarding Archimedes principle(flotation) we need to know the following _________
we need to use derived equations for the initial
Upthrust, U = weight W of object = weight of displaced fluid
but weight of object = mg
and weight of a liquid = ρvg
- but volume of object = Ah*
- therefore U = Ah*ρg
Also V = volume of object = volume of fuid displaced.
Pascal’s Law
Pressure applied to an enclosed fluid is transmitted to every part of the fluid, as well as to the walls of the container.
Practical applications of Pascals Law
- Hydraulic lifts
- Hydraulic brakes
- Hydraulic presses
Newtons Laws
- If there is no resultant force acting on a body it will remain at rest or at constant velocit.
- The acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass
- If a body A exerts a force on body B, body B will exert an equal and opposite force on body A
Applications of Newtons 1st Law
- rocket moving is space with engines of moves with constant velocity
- An austronaut moving in a ship at constant velocity will move at the same velocity of he ship if he step out, unless some other resultant force acts on him
Applications of Newtons Second Law
F = ma
How to choose among Newton’s laws
- why the earth does not float away from the sun or get pulled into it - 3rd Law
- A marble on a table stays there unless pushed - 1st law
- A rocket is about to take off when the fuel ignites - Newtons 1st Law
- A rocket begins to move from rest - Newtons 1st law
- A Hummingbird hovers near a flower to get nectar - 1st Law
- An aircraft accelerates on a runway. the Passengers accelerate with it - 2nd law
- The seats prevent the passengers from sliding to the back of the plane - 3rd Law
- A car is moving at contant velocity - 1st law
How to determine the centre of gravity of an irregular object?
- An object balances at its centre of gravity.
- Hang the object for 3 different positions and mark the vertical lines representing the direction of gravity in each case.
- the centre of gravity is where the lines intersect
3 different types of equilibrium and examples
- Stable eg a square lock on a table
- unstable eg a pencil standing on its tip
- Neutral eg a ball on a table
Factors affecting equilibrium of an object
- Height of its centre of gravity
- width of its base
- its weight
what is the behavior of the centre of gravity in each of the objects A, B and C when it is gently displaced?
A - it rises
B - it falls
C- does not change
which type of equilibrium will the ball experience i each of the positions A, B and C
A - Stable
B - Neutral
C - Unstable
How to determine the behavior of the centre of gravity(CG) of stable and unstable objects.
- Draw a horizontal line through the centre of gravity.
- With the fulcrum acting as the centre. use a compass to draw a curve through the CG in the direction of the displacing force.
- If the curve goes above the line it means the CG will go up and so we are in stable equilibrium
- If the curve goes below the horizontal line it means it goes down and we have unstable equilibrium.
How does centre of gravity behave to maintain equilibrium of an object?
Since the line of action of gravity is always vertical, the CG will only go up, down, or remain stationary along this line.
