mechanics Flashcards
displacement
change in position from one point to its equilibrium position
velocity
rate of change of displacement
acceleration
rate of change of velocity
describe how to find velocity from a linear displacement vs time graph
find the gradient of the graph using as much of the graph as possible
(change in displacement and in time are as big as possible to minimise percentage uncertainty)
describe how to find acceleration from a non-linear velocity vs time graph
draw a tangent to the graph at the point you want to find the acceleration and find the gradient of that tangent
describe how to find displacement from a velocity vs time graph
find the area between the graph and the axis during the period of time.
(above x-axis is positive displacement, below x-axis is negative displacement)
describe how to calculate the final velocity of an object from an acceleration vs time graph that had initial velocity u
find the area between the graph and the x-axis during the period of time using an approximation method e.g. counting squares or trapezium rule.
the area is equal to the change in velocity so add the change to the initial velocity to get the final velocity. (v = u + area)
describe how light gates can be used to measure velocity of a trolley
measure a length of card and fix it to the top of the trolley. using a datalogger, record the time taken for the card to pass through the light gate.
velocity = card length / time
describe how 2 light gates can be used to find average acceleration
measure a length of card and fix it to the top of the trolley. Using a datalogger, record the time taken for the car to pass between the 2 light gates and the time taken to pass through each
describe an experiment to determine g in a normal school laboratory as accurately as possible
an electronic trap door method should be used to eliminate any error starting and stopping the timer correctly and a ball with a small surface area use to minimise the impact of air resistance
the time should be measured for the ball to fall from heights of 20cm, 30cm up to 80cm which will be measured with a metre ruler with each measurement of time being repeated twice and an average time calculated to reduce the effects of any random error
plot a graph of height vs time squared and determine the gradient. g will be equal to the gradient x2
state an equation to calculate the thinking distance (distance travelled by a vehicle before the driving presses the brake)
s = ut
state an equation to calculate the braking distance (the distance travelled when the driver applies a constant driving force)
v^2 = u^2 + 2as
=> v = 0
=> s = - (u^2 / 2a)
=> mu^2/fm
state an equation for the stopping distance (total distance travelled)
s = ut + (mu^2/2f)
explain why the vertical and horizontal motion of a projectile are indpendent
a projectile experiences a weight force vertically downward. the component of weight force horizontally = mgcos90 = 0N
if horizontal force = 0N, according to Newtons first law, horizontal velocity must be constant and therefore be unaffected.
describe the condition that lead to a drag force
an object colliding with the particles of fluid (liquid or gas) that it is travelling through. the drag force will always act in the opposite direction to velocity
describe the factors that affect the size of a drag force
the speed at which the object is travelling and its surface area perpendicular to its velocity (F ∝ Av^2)
describe the conditions at terminal velocity
drag force is equal to weight force so the resultant force is zero. Therefore acceleration is zero and velocity remains constant
describe an experiment that could be performed in a school laboratory to demonstrate and measure terminal velocity
get a 1 litre measuring cylinder and fill it with water. Then measure the distance between the divisions using a 30cm ruler and finally set up a slow motion camera to record.
drop a ball with higher density than water and cross sectional area just less than the measuring cylinder into the water and start recording
record the location of the ball each 0.1 seconds and use the earlier measurement to calculate the total distance travelled at that time.
define a moment of a force
product of force and perinduclar distance between the line of action of the force and the pivot point
define a couple
2 forces of equal magnitude but opposite and direction acting to rate an object in the same direction
define a torque
the product of the magnitude of the 2 forces that are a couple and the perpendicular distance between them
state the principle of moments
if an object is in equilibrium, the total moment clockwise about a pivot is equal to the total moment anti - clockwise about the same pivot
state the principle of moments
if an object is in equilibrium, the total moment clockwise about a pivot is equal to the total moment anti - clockwise about the same pivot
define centre of mass
a point on an object where all the mass can be modelled to be concentrated
define centre of gravity
a point on an object where the weight force acts from
experiment to determine centre of gravity
freely hang the object from a point and use a plumb line to draw a vertical line downwards from the pivot.
freely hang the object from another point and use a plumb line to draw a vertical line downwards from the pivot
if the object is 3D, repeat this step again
the centre of gravity is located at the intersection of these lines
define work done
product of force and displacement parallel to the force
conservation of energy
energy cannot be created or destroyed, only transferred
facts about the equation below:
W = Fscos θ
θ is the angle between the force and displacement
this equation can be used when force is constant
mechanical energy
the sum of an systems kinetic energy, gravitational potential energy and elastic potential energy
conservation of mechanical energy
if no external force acts on a system, its total mechanical energy is constant
calculate work done from conservation of energy
W = ∆KE +∆GPE + ∆EPE
kinetic energy
the work done to accelerate an objects from stationary
calculate kinetic energy
KE = 0.5 x m x v^2
define and calculate gravitational potential energy
the work done against an objects weight force to increase the height of an object from a point defined as zero GPE
GPE = mgh
define power
the rate at which work is done
calculate power
W / t
Calculate instantaneous power
P = Fv
Calculate efficiency from power or energy
Useful energy output / total energy input = useful power output / total power input
define tensile force
a force acting to increase the length of an object
define compressive force
a force acting to decrease the length of an object
state Hooke’s law
force is directly proportional to extension up the limit of proportionality
describe techniques to investigate the force-extension characteristics of other materials
measure the original length of the material using a metre ruler and then fixed one end to a clamp with stand which has been G-clampled to the desk to prevent it toppling
hang 100g on the wire using fixed masses and measure the extended length and calculate the extension
describe techniques to investigate the force-extension characteristics of other materials
measure the original length of the material using a metre ruler and then fixed one end to a clamp with stand which has been G-clamped to the desk to prevent it toppling
hang 100g on the wire using fixed masses and measure the extended length and calculate the extension
repeat this process with 200g, 300g, up to 700g and plot a graph of force vs extension for the material
define elastic potential energy
the work done to change the shape of a material from undeformed
calculate elastic potential energy from a material obeying Hooke’s law
EPE = 0.5 x F x X
F = kx
=> EPE = 0.5 x K x X^2
define stress
the force divided by the cross-sectional area
define strain
the extension divided by the original length
define Ultimate tensile stress
the maximum tensile stress that a material can undergo
define young modulus
the ratio of tensile stress to tensile strain
equations to calculate Young Modulus
E = (F/A) / (x/L) = (FL) / (Ax)
describe techniques to determine young modulus
measure the original length of the material using a metre ruler and then fixed one end to a clamp stand which has been G-clamped to the desk to prevent it toppling
hang a 1kg mass on the wire and use a Vernier scale to measure the extension of the test wire
increase the mass to 2kg, 3kg up to 7kg and measure the extension for each
to calculate the stress, first measure the diameter of the wire in several location to get an average diameter and calculate the cross-sectional area
plot a graph of stress vs strain and measure the gradient in the straight line section to find the young modulus
define elastic deformation
when a material is stretched below its elastic limit and will return to its original shape once the force is removed
define plastic deformation
when a material is stretched above its elastic limit and will be permanently deformed once the force is removed
newtons 1st law
velocity is constant unless a resultant force acts
newtons 2nd law
resultant force is equal to the rate of change of momentum
newtons 3rd law
every action has an equal and opposite reaction
(the force will be equal in magnitude, opposite in direction and of the same type)
define linear momentum
the product of mass and velocity
calculate linear momentum
p = mv
calculate resultant force
F = ∆mv / ∆t
define impulse
the change in momentum of an object
calculate impulse from graphs and equations
impulse is calculated by the area under a force vs time graph (if force is variable)
impulse is calculated using I = Ft (if force is constant)
