Mechanics Flashcards
What is scalar quantity ?
Has only magnitude and specified by a number and unit.
Examples are speed, distance, time, mass, volume, temperature and frequency.
What is a vector quantity ?
Has both magnitude and direction.
Examples are force, velocity, acceleration, momentum and displacement.
What is resolving a vector?
When a vector is replaced by two or more others, creating components of the initial vector.
The components are normally always perpendicular to each other.
What are coplanar forces ?
Forces whose line of action act in the same plane.
What is translational equilibrium?
Where two forces are equal in magnitude and opposite in direction.
What is a couple ?
When two equal but opposite forces are present, whose lines of action are not coincident they will create a rotation termed a couple.
What is the moment of a couple?
Equal to the magnitude of the force, multiplied by the distance between them.
What is rotational equilibrium?
if the two forces equal in magnitude but opposite in direction act along a common line of action.
what are concurrent forces ?
Where three forces in translational equilibrium pass through a common point so that the object is also in rotatinal equlibrium.
What is a moment?
The turning effect of a force. It can be either clockwise (CWM) or anti-clockwise (ACWM)
How is a moment measured?
Moment (Nm) = Force x Perpendicular Distance from the line of action of the force to the point.
If a body is at equilibrium, what conditions must apply ?
The components of the forces in both directions must balance.
The sum of the CWM of any point equals the sum of the ACWM about the same point.
What is weight (N) ?
The gravitational force by which the object is attracted to the earth’s centre. The product of an object’s mass and its acceleration due to gravity.
Weight equation?
Weight = mass x gravity
N = kg x 9.8
What is the centre of gravity (CofG) ?
A unique point in an extended rigid body at which the total gravitational force (weight) appears to act.
Why is deteremining the Centre of Gravity a complicated procedure?
The mass and weight may not be uniformly distrubuted throughout an object.
If the mass is uniformly distrubted the problem is greatly simplified.
How do components fail ?
From being over-stressed, not overloaded.
What is stress defined as?
The force (N) acting on a unit cross-sectional area.
stress equation ?
Stress (Pa) = force / area
What are the 5 types of stress in a mechanical body?
Tension, Compression, Torsion, Bending and Shear.
what is tensile stress?
Describes the effect of a force that tends to pull an object apart.
An example is a flexible steel cable.
what is compressional stress?
The resistance to an external force that tries to force an object together.
An example is a rivet.
What is torsional stress?
A combination of both tension and compressional stress.
Tensional stresses occur diagonally across the material and the compressional stresses at right angles to the tensional stresses.
An example is an engine crankshaft.
What is bending stress?
Where a long object is anchored at one end with a load appplied at the other end, the object will bend in the direction of the applied load.
An example is an aircraft wing on the ground and in the air.
What stresses occur to an aircraft wing when on ground ?
The top of the wing os subject to tensile stress and the underside is under compressional stress.
The wing bends (droops) downwards.
What stresses occur to an aircraft wing when in flight?
The top pf the wing is subject to compressional forces and the underside is under tensile stress.
The wings are pulled upwards.
What is shear stress?
Stress that attempts to slice an objec apart.
An example is a rivet under tensile stress, or a clevis pin.
What is the definition of strain?
The relative change of size or shape due to an applied stress.
It is a ratio and can be expressed as a percentage.
What is the equation for finding the intensity of strain?
intensity of strain = change in length / original length.
What is the definition of elasticity?
The property of certain materials that enables them to return to their original dimensions after an applied stress has been removed.
What is the elastic limit?
Up to this point, an object will return to its orginal state after the stress is removed.
Beyond this point, there is permanent deformation when the stress is removed. The material has ceased to be elastic.
When is an elastic material said to have become ‘plastic’ ?
When it has become deformed from stress applied beyond the material’s elastic limit.
What is Hooke’s Law?
States that strain proportionately increases with stress until the limit of proportionality is reached.
What is the yield point of the elastic material?
After the elastic limit, where there is a sudden increase in strain with further increases in stress.
At what point does an elastic material break?
The fracture point.
What is Young’s Modulus?
Stress / Strain = constant (E)
Which is the same as,
E = F/A / change in length / original length.
What are examples of a simple machine?
Levers, winches, pulleys, inclined planes and screws.
Allow a smaller force to be applied to achieve a result.
What is the definition of a lever?
Described as a rigid beam supported at a point or fulcrum that is fixed, and about which the beam can turn.
What is a first class lever?
Fulcrum is between the load (R) and effort (E).
An example would be a see-saw, or opening and can of paint with a screwdriver.
What is a second class lever?
The load (R) is between the effort (E) and the fulcrum. Less effort is required if the load is close to the fulcrum.
An example would be a wheelbarrow.
What is a third class lever?
The effort (E) is between the fulcrum and the load (R). The effort is applied close to the fulcrum whilst the load is at the end of the lever .
The effort expended is greater than the load in order to gain distance.
An example is landing gear.
What is the definition of actual mechanical advantage ?
The ratio between the output force (load) it exerts and the input force (effort) that is applied to it.
Actual Mechanical Advantage equation?
MA = Load / Effort.
Velocity Ratio equation ?
VR = input dispalcement of effort / output displacement of load.
VR = dE / dI
What is mechanical efficiency ?
Measures the perofrmance of a machine as a ratio of the energy it delivers to the energy fed to it.
Mechanical efficiency equation?
ME = (Output / Input) x 100
OR
MA / VR
A perfect machine operating at 100% mechanical efficiency, can be shown by what equation?
MA = VR
What is the definition of friction ?
The force that resists the motion of one surface relative to another with which it is in contact with.
What is static friction ?
A force that prevents an object from moving.
e.g Tilting an object on an incline with it staying stationary.
What does static friction depend on?
The materials, the smootheness of the surfaces in contact and the component of the force (N) pressing the two surfaces together.
What is the co-efficient of static friction?
Fs max = us x N
where (us) is a constant for two materials of specified smoothness.
What is dynamic friction?
If an object is sliding over a surface at a constant velocity, and there will be some conversion of kinetic energy of the object into heat.
What does the force of Dynamic friction depend on?
The material of the two surfaces, their smoothness and the component of the force (N) that presses the two surfaces together.
What is the co-efficient of dynamic friction?
Fd = ud x N
The dynamic friction that occurs is usually less than the starting friction, so less force is needed to keep the object moving than to start it.
