Topic 2/6 - Mechanics / Further Mechanics Flashcards
mass
the amount of matter something contains, kg
weight
the force due to gravity pulling two objects together, N
g
The acceleration due to gravity
normal contact force
a force perpendicular to a surface stopping two objects crushing together.
Tension
a force which can be due to compression of stretching of a material
it always comes in pairs
Tension due to stretching
the materials molecules are pulled apart and an attractive electromagnetic force between the molecules pulls them back together
Tension due to compression
the material molecules are pushed together and a net repulsive electromagnetic force tries to push them further apart
Friction
a force that acts against motion between two surfaces in contact.
parallel to motion in the opposite direction
What does the size of a frictional force depend on
- type of surface
- how hard the surfaces are pressed together
moment
turning effect on an object. It is the product of the distance perpendicular to the force and the force applied.
moment of a force about a point (Nm) =
force (N) * perpendicular distance from the force (m)
couple
two forces that are equal and opposite do not act along the same line meaning there is no resultant force but still a turning effect.
principal of moments
when an object is in equilibrium the sum of clockwise moments about any point equals the sum of anticlockwise moments about the same point
centre of gravity
the point at which we can take its entire weight to act, THERE IS NO MOMENT DUE TO WEIGHT
centre of mass
the point at which we can take the entire mass to be concentrated
Stable object
the centre of mass and line of action lies between the objects base/supports
locating the centre of mass
hang the shape from a pivot point with a plum line and nail
trace along the plum line
repeat from at least 2 more pivots
where all the lines cross is the CoM
(the CoM must be along the line of action because there are no resultant moments)
Distance
how much ground an object has covered during its motion (scalar quantity)
Displacement
how far out of place an object is and the overall change in the objects position (vector quantity)
speed
rate of change of distance
velocity
rate of change of displacement
Acceleration
rate of change of velocity
straight lines on a displacement time graph
constant velocity (if flat the velocity = 0)
instantaneous velocity from a dt graph
draw a tangent and find the gradient
distance of journey from a dt graph
sum of slope height (not inc negatives)
displacement of journey from a dt graph
sum of slope heights inc negatives
acceleration from a dt graph
find the initial and final velocity at two points by finding the gradient
divide by the time between the two points to get acceleration
gradient of a vt graph
acceleration
area under a vt graph
distance/displacement depending on negatives
when can suvat equations be used
when an object moves with constant uniform acceleration
Suvat equation: X = X + XX
v = u + at
Suvat equation: X^n = X^n + nXX
v^2 = u^2 + 2as
Suvat equation: X = XX - nXX^n
s = ut - 1/2at^2
Suvat equation: X = n(X +X)X
s = 1/2(u + v)t
free fall
the only force acting on an object is gravity. Accelerate downwards at the same rate
terminal velocity
the highest attainable velocity of an object as it falls through a fluid. the drag = weight.
terminal velocity can be different speeds i.e. a skydiver will have two different terminal velocities depending on if the parachute is open.
projectile motion
an object follows a parabolic path
approaching a projectiles question
resolve the initial velocity into horizontal and vertical components
acceleration only acts on the vertical component and is g.
horizontal distance can be easily calculated using the equation s = vt
Newtons first law
an object will continue to move with uniform velocity unless acted upon by a resultant external force
Newtons second law
the rate of change of momentum of an object is directly proportional to the resultant force acting on it and will act in the same direction as that force
F = (mv-mu)/t or F=ma
Newtons third law
If object A exerts a force on object B then object B exerts an equal and opposite force back on A
Momentum
a vector quantity that is the product of the mass and velocity of an object
momentum, p (kgm/s) =
mass(kg)*velocity(m/s)
To have a newtons third law pair:
- forces must have the same magnitude
- forces act along the same line bu in opposite directions
- forces act for/at the same time
- forces act on a different object
- forces are the same type
impulse
the increase/decrease in momentum
change in momentum =
Force * time
area under a force time curve
the total impulse acting and the total change in momentum produced.
principal of conservation of momentum
in a closed system, the total momentum is constant so the momentum before a collision is equal to the momentum afterwards.
recoil
the movement backwards due to conserved momentum
work done
the energy transferred whenever a force moves an object over a distance. scalar quantity measured in joules.
work done, w (J) =
force, F (N) * displacement in the direction of the force, s (m)
if the force and displacement are not in the same direction when calculating work done:
resolve the force
W = Fcosx * s
energy
the ability to do work
1 joule
the work done when a force of 1N moves through a distance of 1m in the direction of the force
power
rate of doing work
power, P (W) =
= work done, w(J) / time takens, t (s)
= force, F (N) * velocity, v (m/s)
Kinetic energy
the energy an object has because of its motion
K.E (J) =
1/ 2 * m* v^2
Potential energy
the energy stored in an object due to its position, state or shape
e.g. gravitational, chemical, elastic, electrical or nuclear
GPE =
mgh
Elastic potential energy =
= 1/2Fx
= 1/2 kx^2
constant velocity
no resultant force and no work is done
principle of conservation of energy
energy can be transferred from one form to another, bu it cannot be created or destroyed. The total amount of energy always remains the same.
efficiency
the proportion of energy that is usefully transferred is called the efficiency of the machine
efficiency =
useful out/total in
linking k.e. and momentum
K.E. =
p^2 / 2m
statics
objects in equilibrium
dynamics
objects that are accelerating/decelerating
The resultant vector must be drawn where…
the bases of the other two vectors meet
Why is calculated momentum not always correct?
At high speeds mass increases and the equations are only valid for non relativistic speeds.
elastic collision
100% of energy and momentum conserved
impulse
change in momentum
newtons first law
an object will continue to move with uniform velocity unless acted upon by a resultant external force
newtons second law
the rate of change of momentum of an object is directly proportional to the resultant force acting on it and will act in the same direction as that force
F = (mv-mu)/t or F=ma
newtons third law
If object A exerts a force on object B then object B exerts an equal and opposite force back on A
conditions for a newtons third law pair
- forces must have the same magnitude
- forces act along the same line bu in opposite directions
- forces act for/at the same time
- forces act on a different object
- forces are the same type
