Mechanics Flashcards
(22 cards)
what does it mean if object is in equilibrium
there is no resultant for or vector sum =0
how can an object be shown to be in equilibrium
- adding horizontal and vertical components and showing they equal zero
- if there are three forces on a scale diagram they will form a closed traingle
under what condition is momentum conserved
when there are no external forces applied
in the equation W=Fcosθ does θ represent
the angle between the direction of motion and direction of force
equation for efficiency
useful power input / total power output
what is the principle of moments
for an object in equilibrium the sum of anticlockwise moments equals the sum of the clockwise moments about a fixed pivot
difference between elastic and inelastic collisions
elastic : momentum and kinetic energy are conserved
inelastic : inly momentum is conserved (kinetic energy is converted to other unuseful stores like heat)
how to find the moment of a couple
multiply one of the forces by the perpendicular distance between the lines of action of the forces
what is a couple
2 coplanar forces equal in magnitude acting in opposite directions about a fixed pivot
what is Newtons first law
object will remain in at rest or at a constant velocity unless acted on by a resultant force
what is Newtons second law
F=ma acceleration of an object is proportional to the resultant force it experiences
What is Newtons 3rd Law
For every force experience by an object the object exerts an equal and opposite force
in inelastic collisions will particles stick together or rebound
stick together
speed of someone walking, running and cycling
walking - 1/2 ms
running - 3 ms
cycling - 6 ms
what is the resultant force of a couple
0
what is hooks law
extension is directly proportional to force applied up to the limit of proportionality
what is the limit of proportionality and the elastic limit
- the point after which hooks law is not obeyed and the object undergoes elastic deformation
- the point at which the material starts to plastically deform therefore will not return to its original shape
what is the breaking stress
the value of stress at which after a material will break
it is dependent on external conditions like stress
what is the difference in energy transfers when a material is undergoing elastic and plastic deformation
- Elastic - all work done is transferred to elastic strain energy
- plastic - work is done to move atoms apart so is not stored and is dissipated as thermal energy
why are crumple zones plastically deforming
when the car crashes the crumple zones deform plastically therefore dissipating some of the cars kinetic energy through thermal energy. Because workdone = FD, if the car is doing less work because its total energy has decreased then it will exert lower force on the occupents
the bumpers increase time over which the collision takes place therfore reduces the force on the occupents
deceration will be lower leading to a lower force on the care
what are the three types of material
ductile, brittle, plastic
What is the young modulus
when a material is obeying hooks law tensile stress its experiencing is proportional to its tensile strain , the young modulus measures how much stress an object can take per unit strain (how strong/stiff it is)
