Mechanics Flashcards
The horizontal component of a vector is given by ….
Fx= F cos θ
The vertical component of a vector is given by….
Fy= F sin θ
Vectors on an incline
Weight acts vertically downwards
Weight resolved along the plane = W cos θ
Weight resolved perpendicular to the plane = W sin θ
Equilibrium
Resultant force = 0
Moment = 0
Moments equation
Force x perpendicular distance between point to the line of action of the force
Couples
Equal and opposite coplanar forces
(2 forces acting in opposite directions but equal, the same distance apart)
Moment equation applied to couples
M=2Fd
Principle of moments
Sum of the clockwise moments= sum of the anti-clockwise moments
Equilibrium -> not turning
Centre of mass
Point at which weight appears to act
Centre of mass hanging objects
COM directly below the point it is hung from
Why do objects fall over?
Centre of mass is outside its base
A object is stable when….
The centre of mass lies above its base
Acceleration=
Rate of change of velocity
Velocity=
Rate of change of displacement
Displacement-time graphs
Gradient=v
Area under graph = distance
Instantaneous velocity -> tangent
Average velocity -> total displacement / time
acceleration = curve
constant gradient= constant acceleration
Acceleration- time graphs
Non horizontal lines= non uniform acceleration
Gradient= rate of change of a
Area = change in velocity
Velocity-time graphs
Area= displacement
Gradient= acceleration
curve= changing acceleration
Determination of g by free fall
Steel ball bearing and trap door
electromagnet
Light gates or pressure sensitive plates
Measure from a range of heights
Plot a graph of t2 against h
g= 2x gradient
small ball bearing low air resistance
uncertainty of ruler 1mm
Projectile motion
Horizontal v=u because a=0
Vertical u=0, a=-9.81
Parabolic projectile motion
At maximum vertical displacement, Vy =0
Time = 1/2 t
What is lift?
Upward force due to collisions with air particles
Planes push air downward during flight. Downward force on air particles, equal and opposite force upwards on wing
perpendicular to fluid flow -> shape of object causes fluid flowing over it to change direction
Terminal velocity
Initial acceleration =-9.81 ms-2
Velocity increases, collisions per second increases, air resistance increases
Constant downward force
Terminal velocity #1 air resistance = weight
Maximum speed
Parachute deployment
Air resistance increases, deceleration, force due to air resistance decreases as velocity decreases, new terminal speed
Shape of race cars
Streamlined so they can reach higher speed with the same driving force
Newton’s first law
A body will continue in a state of rest or uniform motion in a straight line unless acted on by a net external force
Newton’s second law
Σ F=Ma
A body accelerates when acted on by net external force
Newton’s third law
If object A exerts a force on object B. Then object B exerts a force on A that is equal in size and opposite in direction
two sides of an interaction viewed from different perspectives
same type
classic non-example= book on table both forces act on book, not same type
Impulse
Change in momentum
Stable equilibrium
If a body is disturbed it tends to return to its original position
Unstable equilibrium
If disturbed, a body tends to keep moving away from its original position
Impulse equation
Impulse= force x time
Conservation of momentum
Momentum before= momentum after
recoil in explosions
Elastic collisions
Momentum is conserved
Kinetic energy is conserved
Inelastic collisions
Momentum is conserved
Kinetic energy is not conserved
What type of collisions are explosions?
Inelastic
Safety features
Air bags
Seat belts
Crumple zones
Work by absorbing kinetic energy, increasing the time taken for a change in momentum to occur thus reducing force
Work equation
W= Fd
Where d is in the direction of the force
Power definition
The rate of energy transfer
Mass definition
Inertia, the ability of a body to resist acceleration by a net force
greater mass = greater resistance to change in velocity
Power equation
Force x velocity
Gravitational potential energy definition
The work an object can do by virtue of its position in a gravitational field
Kinetic energy
Work an object can do by virtue of its speed
Energy
Is the stored ability to do work
apparent weight
if an object subject to gravity is not in free fall, then there must be a reaction force to act in opposition to gravity
examples of vector quantities
displacement, velocity, acceleration, force, momentum
examples of scalar quantities
mass, temperature, distance, speed, energy
free body diagrams
show all the forces acting on a body
coplanar forces
multiple forces acting on a body in the same plane
moments in the human body
effort acts against load force
muscles, bones and joints act as levers
where joints are the pivot
stability
wide base, low centre of mass
free fall motion
a=g
all objects fall at the same rate
proof = inclined plane experiment
slows fall + reduces effect of air resistance
mass cancels
W=Fd a note
F constant or average
assume direction of force= direction of motion
W=Fs a note
cannot be used for a variable force
conservation of energy
energy cannot be created or destroyed only transferred, stored or dissipated
total amount of energy in a closed system will not change
friction in fluids
depends on viscosity
area under Ft graph
impulse
friction
force that opposes motion
convert KE into heat and sound
larger surface area= greater resistant force
W= Fscosθ
calculating work when force acts in a different direction to the direction of motion
ultrasound position detector
way of creating graphs of motion using a data logger which automatically records distance
finding centre of mass lines of symmetry
intersection between lines of symmetry
finding centre of mass irregular objects
hang from a point
draw vertical line from point of suspension (plum bob)
repeat from another point
cross over of lines= centre of mass
