Power, Work and Load Flashcards
What is the definition of ‘work’?
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The product of force and distance moved
Work = Force x Distance Moved
Work = Torque x Angle moved (in Radians)
What are the units of ‘work’?
Joules (the standard unit of energy)
What is the definition of ‘power’?
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The rate at which energy is transferred
Power = Force x Velocity
Power = Torque x Angular Velocity
What are the units of ‘power’?
Watts
What is the principle of ‘The Conservation of Energy’?
And
What does this mean in terms of mechanisms?
Energy cannot be created or destroyed, only transferred from one place or form to another
A mechanism cannot introduce new energy into the system, but it can shift the balance between force and movement
What are ‘Mechanical Advantage’ and ‘Speed Ratio’ defined as?
What is the important that they tell us?
They are two important ways of quantifying the behaviour of mechanisms, they determine the relationships between the input force and motion and the output force and motion.
They tell us whether a mechanism is sacrificing movement to increase force or sacrificing force to increase movement
What the equations associated with Mechanical Advantage and Speed Ratio?
Ideal Mechanical Advantage = Speed Ratio
(Rarely happens in practice)
Ideal Mechanical Advantage = Applied Force/Effort Force
Ideal Mechanical Advantage = Input Velocity/Output Velocity
Speed Ratio = Input Velocity/Output Velocity
= (Input distance/time) / (output distance/time)
= Input distance/output distance
= Input angular velocity/output angular velocity
= (Input angle/time) / (output angle/time)
What happens to the force and movement when the mechanical advantage is:
(A) >1
(B) <1
(C) =1
(A) Force increases, Movement decreases
(B) Force decreases, Movement increases
(C) Force stays the same, Movement stays the same
Why aren’t mechanical systems 100% efficient?
There will always be some friction between moving parts which needs to be overcome. So some work (and therefore power) is lost as heat and part of the effort force goes into overcoming this friction.
The loss always comes from the force, never the motion, so the speed ratio is unaffected.
Define Efficiency…
(Greek letter: weird n)
The ratio of the actual mechanical advantage to the ideal mechanical advantage defined by the speed ratio.
What are the equations for efficiency?
Actual mechanical advantage = Applied force/effort force = n x speed ratio
Output work = n x input work
What does it mean if n = 1 and what is the real life value that n will be?
If a mechanism is ‘ideal’ then it is 100% efficient (n=1)
For all real mechanisms, n<1 and some losses occur
What is mechanical load?
Load in this case describes the force or torque that tries to resist the movement of the mechanical system
The difference between the applied force and the load force determines the rate at which the object will be accelerated
What are the equations involving load force?
Resultant Force = Applied Force - Load Force
Applied force - load force = mass x acceleration
How can load affect torque?
Load can be given as a torque that resists the applied torque, trying to prevent rotation.
It is the resultant torque that causes motion and this can be calculated as:
Resultant torque = applied torque - load
What are the common sources of load forces?
Two most common: Gravity and friction
But could be a direct force against the mechanism such as air resistance or springs
How do you calculate the force due to gravity?
Force due to gravity = mass x acceleration due to gravity
What is the effect of gravity?
The gravitational pull of the earth causes every object to have a downward force equal to its weight in Newtons
What is the gravitational constant?
Where force is in Newtons, mass in Kilograms, acceleration due to gravity (g) = 9.81 m/s
What is friction? How does it affect efficiency?
Friction describes the force that resists movement between two surfaces
Efficiency represents the losses that occur due to friction inside a mechanism
What is kinetic friction?
Friction between two moving objects
What is static friction (stiction)?
Friction between two static objects, it represents the force that must be overcome before the objects will move relative to one another and its effect is generally greater than that of kinetic friction
How do you calculate the Load due to Friction?
F = uN
(F) - Load due to friction
(u) - Greek letter - coefficient of friction, how rough the surfaces are
(N) - Normal force, the perpendicular reaction force between the surfaces
What is the reaction force or normal force (N) usually equal to?
What is different if the surface is sloped?
Typically its weight, mg (given by mass x acceleration due to gravity)
If the surface is sloped, then you’ll need to work out the component of the force that acts at right angles to the surface
What is the difference between friction and gravity?
Unlike gravity, friction is a resistance rather than a force in its own right.
If the applied force is less than the force due to stiction, the object remains stationary, it doesn’t accelerate in the opposite direction
