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Physics > 4: Mechanics and Materials > Flashcards

4: Mechanics and Materials Flashcards

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1
Q

What is the principle of moments?

A

-For rotational equilibrium the sum of the clockwise moments is equal to the sum of the anticlockwise moments about any given pivot

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2
Q

What is a couple?

A

-A pair of equal and coplanar forces that are in opposing directions

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3
Q

What is a moment?

A

-The force applied by an object multiplied by the perpendicular distance from the pivot

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4
Q

What is required for equilibrium?

A

-Vertical forces must be equal
-Horizontal forces must be equal
-Rotational forces must be equal

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5
Q

Describe a scalar quantiy

A

-Only has a magnitude
-Examples: time, distance, speed

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6
Q

Describe a vector quantity

A

-Has a magnitude and a direction associated with it
-Examples: velocity, displacement, force

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7
Q

What is the significance of the gradient of a force-extension graph?

A

-The gradient is equal to the spring stiffness

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8
Q

What is the significance of the area of a force-extension graph?

A

-The area underneath is equal to the strain energy

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9
Q

What is density?

A

-The ratio of the mass of a body to its volume
-Measured in kilograms per metre cubed (kg/m^3)

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10
Q

What is the elastic limit?

A

-The point at which an object will experience permanent deformation
-The object will no longer return to its original length

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11
Q

What is lift?

A

-An upward force acting upon an object in a fluid
-Caused by the object creating a change in direction of fluid flow
-Lift acts perpendiculars to direction of fluid flow

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12
Q

What is F.s.cosθ equal to?

A

-The work done on an object
-Typically for an object being pulled or pushed by an external force at an angle
Where:
F=force
s=displacement
θ=angle of force to horizontal

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13
Q

What is friction?

A

-A resistance to the motion of an object
-Friction always acts against the direction of movement

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14
Q

What is brittle behaviour?

A

-When a material fractures with little elastic deformation under stress

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15
Q

What is Hooke’s law?

A

-The extension is directly proportional to the force applied
-This applies up until the limit of proportionality

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16
Q

What is gravitational potential energy equal to?

A

Ep=m.g.h
Where:
Ep=gravitational potential energy
m=mass of object
g=acceleration due to gravity
h=height of object

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17
Q

What is kinetic energy equal to?

A

Ek=(1/2).m.v^2
Where
Ek=Kinetic energy
m=mass of object
v=velocity of object

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18
Q

How is energy related to power?

A

-Energy is the product of the power and the time elapsed

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19
Q

What is the principle of conservation of energy?

A

-Energy is neither created nor destroyed

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20
Q

What is efficiency?

A

-The ratio of the useful power output to the total power input
-Measured as a a percentage

21
Q

What is the area under a force displacement graph?

A

-Equals the work done

22
Q

What is power?

A

-The rate of doing work
-Measured in watts (W)

23
Q

What is the area under a force time graph?

A

-The change in momentum
-This is equivalent to the impulse

24
Q

What is an elastic collison?

A

-Where momentum and kinetic energy are conserved

25
Q

When is momentum conserved?

A

-Linear momentum is conserved in all collisions so long as no external forces act

26
Q

What is an impulse?

A

-The change in momentum
F.Δt=mv-mu
Where:
F=force
Δt=change in time
mv=final momentum
mu=initial momentum

27
Q

What is the fracture point?

A

-The point on a stress-strain graph at which an object breaks

28
Q

What is plastic behaviour?

A

-When a material experiences permanent deformation
-This is the elastic limit on a stress-strain graph

29
Q

What is the breaking stress?

A

-The maximum force that can be applied to an object before it breaks

30
Q

What is elastic potential energy?

A

-The energy stored in an elastic object eg. a spring
-Equal to (1/2).F.e
-Also equals (1/2).k.e^2
Where:
E=elastic potential energy
F=force
e=extension
k=spring stiffness

31
Q

How does spring stiffness combine in series springs?

A

-1/Kt=1/k1+1/k2…

32
Q

How does spring stiffness combine in parallel springs?

A

-Kt=k1+k2…

33
Q

What is a ductile material?

A

-A material that can undergo a large amount of plastic deformation before fracturing

34
Q

What is elastic deformation?

A

-When the force is removed and the object returns to its original length

35
Q

How does speed affect air resistance?

A

-As the speed increase air resistance increases

36
Q

What is terminal velocity?

A

-Point at which an object reaches its maximum velocity
-Air resistance is equal to acceleration at this point

37
Q

What are the conditions for projectile motion?

A

-Must be no air resistance
-Thus no horizontal acceleration
-Vertical acceleration is solely due to gravity

38
Q

What are the SUVAT equations?

A

v=u+a.t
s=u.t+(1/2).a.t^2
s=v.t-(1/2).a.t^2
v^2=u^2=2.a.s
s=1/2(u+v).t
Where:
s=displacement
u=initial velocity
v=final velocity
a=acceleration
t=time

39
Q

What is the centre of mass?

A

-Point at which the weight of an object acts

40
Q

Where is the centre of mass for a uniform object?

A

-At the centre of the object

41
Q

What is Newton’s first law?

A

-A body will stay at constant velocity unless acted upon by an external force

42
Q

What is Newton’s second law?

A

-The rate of change of momentum is equal to the resultant force
f=m.a
f=(mv-mu)/t=m(v-u)/t=m.a
Where:
f=force
m=mass
a=acceleration
t=time
u=initial velocity
v=final velocity

43
Q

What is Newton’s third law?

A

-When two bodies interact they apply forces opposite in direction and equal in magnitude upon one another
-Must be opposing bodies
-Must be the same type of force

44
Q

Why are loading and unloading lines on a force-extension graph parallel for a plastically deformed material?

A

-The spring stiffness hasn’t changed so forces between atoms are the same

45
Q

Why isn’t all work done stored as elastic strain energy when a stretch is plastic?

A

-Work is done to move atoms apart so energy gets dissipated as heat

46
Q

How is dissipation of energy in plastic deformation used to design safer vehicles?

A

-Crumple zones deform plastically so increase impact time which decrease the force felt as the impulse is the same
-Seat belts stretch to convert passenger’s kinetic energy into elastic strain energy

47
Q

What energy changes occur when pulling a fixed string?

A

-Work done to pull the string is converted into elastic strain energy
-When the spring is released energy is converted from elastic potential to kinetic
-The kinetic energy is eventually converted into elastic potential

48
Q

How do you calculate Young’s Modulus?

A

E= Tensile Stress/ Tensile Strain
Stress=Force/Area
Strain=Extension/Original Length
E=(ForceOriginal Length)/(AreaExtension)

Where E is the Young’s Modulus

49
Q

Describe an experiment to calculate the Young’s modulus of a material

A

-Use a micrometer to measure the diameter in several places to find a mean diameter
-Calculate the cross-sectional area by π*(diameter^2)/4
-Clamp the wire down and attach weights until the wire is taut
-Measure the distance of the wire up until the marker with a ruler-this is the original length
-Increase weights in 50g intervals and measure the extension with a ruler
-Record the extension as well as the force applied
-Get a minimum of 8 results for different weights and remove weights
-Wait until wire is slack and repeat experiment twice more to calculate mean extensions
-Plot a graph of stress on y-axis against strain on x-axis
-The gradient of this graph is the Young’s Modulus

Physics (11 decks)

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