Mechanics Flashcards
Motion
Speed= Distance / Time
Measures in meters per second (m/s) or if units are in miles and hours it will be miles per hour (mph).
Speed and Velocity
There is a difference.
Velocity has a direction associated with it, and is called a vector quantity.
Displacement
The length and direction together of a line (or object movement) is known as displacement and is also a vector quantity.
Displacement and Velocity Example
Car going from london to bristol (200km west). Therefore it has a displacement of 200km.
But the distance travelled will be more than this as roads are not straight and sometimes the car will be travelling north to south as the roads go.
At those times, the displacement is not increasing, so although it has speed, it has no velocity.
Speed, Velocity and Acceleration Definition
Velocity and displacement have associated direction and are known as vector quantities.
Speed is the rate of change of distance.
Velocity is the rate of change of displacement.
Acceleration is the rate of change of velocity.
Momentum
Is the product of mass and velocity (mass x velocity).
e.g. 2kg object travelling at 10m/s has a momentum of 20kgm/s.
Force
Product of mass and acceleration
F=MA (in kgm/s^2 aka newtons).
Is 2kg object is at rest, and then a force is applied to make it accelerate at 10m/s^2, the force is 10x2=20N.
Force of Gravity
Provides an acceleration that acts on everything.
At earth surface anything dropped will accelerate at 9.81m/s^2. Referred to as the symbol ‘g’.
This also determines the forces that are responsible for the movement of hot gases in a fire environment.
Weight of a body
A measure of how strong the force due to gravity is on an object.
Weight and mass have different meanings.
Weight
Is a force due to gravity which acts on an object.
e.g. a mass of 2kg experiences a force due to gravity, therefore 2x9.81= 19.62 Newtons.
Work
If a body moves because a force acts on it, we say that work is being done on that object.
Work is only being done if the object moves, if it does not then no work is done.
Work Equation
W=Fs
W is work done in Joules.
F is the constant force applied in Joules.
s is the distance moved in the direction of the force in Metres.
Energy
Energy is the ability to do work, cannot be created or destroyed.
If something is capable of doing work then it possesses energy.
Mechanical energy can be either potential or kinetic.
Potential Energy
Amount of energy that an object possesses because of its position, or the arrangement of its components.
E.g. a body held above the ground has potential energy as it could do work whilst it is falling.
If a 2kg object is raised 2m then the potential energy is equal to the work required to raise it to that height (mass x acceleration x height) which has the unit of joules. Its a consequence of the position of the mass in the earths gravitational field.
Kinetic Energy
The energy that a body possesses by virtue of the fact that it is moving.
The kinetic energy that a body possesses is equal to the amount of work that must’ve been done to it to increase its velocity from zero to whatever it now is.
Equation= 1/2mv^2 (m= mass and v=velocity) in kg.m^2/s^2 or joules.
Example of kinetic and potential energy
Hydro-electric dam:
water is held in a reserviour and has potential energy, but as it is released and flows by gravity down to the turbine, the potential energy is converted into kinetic energy and propels the generator which converts the energy into electrical energy.
Power
The rate at which energy is used or transformed. (Rate of energy consumption).
The word energy has no connection with time whereas power does. Power is a time based quantity and depends on how fast a job is done.
Two identical jobs can be done at two different rates. The work done is the same in both cases but the power is different.
Power Equation
Power (Watts (W))= energy (J) / Time (s)
Friction
Whenever an object is moved over a surface, both the object and the surface experience a frictional force along the common surface, each in a direction that opposes the relative motion of the surfaces.
Even occurs on ‘perfect’ surfaces due to tiny imperfections. These imperfections may interlock and i the case of metal, can weld together under very high local pressures.
Friction and Heat Energy.
The locking and bonding will inhibit motion and energy is required to overcome these.
When energy is expended to overcome friction, it appears as heat.
This is why brake pads get hot and how matches work.
Simple Machines
Lever is simplest of all- enables mechanical energy involving a small force to be converted into mechanical energy involving a large force.
Class 1 Lever
Has the fulcrum placed between the effort and the load e.g. pair of pliers, scissors or oars on a boat.
The movement of the load is in the opposite direction of the movement of the effort.
Effort arm can be greater, less than or equal to load arm.
Class 2 Lever
Has the load between the effort and the fulcrum e.g. wheelbarrow, crowbar or wrecking bar.
The length of the effort arm goes all the way to the fulcrum.
Effort arm is greater than the length of the load arm.
Class 3 Lever
Has the effort in between the load and the fulcrum.
Both the effort and the load are in the same direction.
E.g. Spades and shovels, brooms, tweezers.
Load arm is greater than the effort arm.
Mechanical Advantage
Mechanical Advantage= load / effort.
Load and effort are measures in Newtons.
Therefore, a load to be moved is 500N, and the effort applied is 100N, the mechanical advantage is 5N.
