Motion Flashcards
What is the definiton for speed?
and
What is the equation for Average speed?
The rate of change of distance
Average speed = distance travelled/time taken
v = x/t
Distance time graphs
Which axis is distance plotted on?
Which axis is time plotted on?
What is speed?
Distance is plotted on the Y - axis
Time is plotted on the X - axis
Speed = gradient of a distance time graph
How is instantaneous speed determined on a distance time graph?
Instantaneous speed is the speed of something over a very short time interval.
on a d/t graph, instantaneous speed can be determined by drawing a tangent to the graph at a specific time and calculating the gradient.
What is acceleration?
On a velocity - time graph?
Acceleration is defined as the rate of change of velocity.
a = change in v / time taken
on a velocity time graph it is the gradient of the graph.
Displacement is the area of the velocity time graph.
What is the equation of motion without the ‘s’?
How is it devired?
v = u + at
a = v / t = v - u / t –> v= u + at
What is the equation of motion without ‘v’?
How do you derive it?
s = ut + 1/2 at^2
Derived as a triangle and rectangle
Rectangle area = ut
Triangular area = 1/2 x (v - u)t
sub in (v - u) = at to get 1/2 at^2
total area together =
s = ut + 1/2 at^2
What is the equation of motion without ‘a’?
How is it derived?
s = u + v /2 x t
Treating area under graph as trapezium
What is the equation of motion without ‘t’?
How is it derived?
v^2 = u^2 + 2as
Use equation 1 & 2 together:
t = v - u / a –> s = 1/2 (u+v)t —> s = (u + v / 2) x (v - u / a)
—> v^2 = u^2 + 2as
What is:
Stopping distance?
Thinking distance?
Breaking distance?
Stopping distance is the distance traveled from when the driver first sees the reason the stop to when the vehicle stops. SD = TD + BD
Thinking distance is the distance traveled between the moment you first see a reason to stop to the moment when the break is applied. (reaction time is too vauge) = speed x reaction time
Breaking distance is the distance travelled form the time the break is applied to when the vehicle stops.
By what factor does breaking distance increase if speed is doubled?
Would increase by a factor of four
Because E = Fx and E = 1/2mv^2 hence Fx = mv^2
What is Free fall?
Free fall is when an object is accelerating under gravity, with no other force acting on it.
Acceleration of free fall is denoted by g (ms^-2)
What is the value of g?
The value varies depending on altitude, latitude and area geology but a value of
9.81 ms^-2
is typically used.
What are the 4 main methods of determining g?
1) Dropping a heavy ball a set distance & calculating time
2) Electomagnet & Trap door
3) Light gates
4) Taking pictures - either stroboscope and long exposure or video
Determining g - Method 1 - Electromagnet and trap door
An electromagnet holds a small steel ball above a trap door.
- when current is switched off, a timer is triggered, electromagnet demagnitised and the ball falls
- When it hits the trapdoor, the electrical current is broken and the timer stops. Height and time can be used to find g.
What may the inacuraces be caused by?
The electomagnet will remain magnitsed for a finite amount of time so it will not immediatly fall.
May be improved by using heavier ball and larger drop.
Unlikely to be used as requires a lot of precise equipment to be set up.
Determining g - Method 2 - Light gates
Light gates eliminate the tiny timing delays found with electromagnet and trap door.
- Two beams of light, one above the other, a set distance apart with detectors are connected to a timer.
- When the ball falls through the first beam, it interrupt the light and the timer starts. When it falls the the second the timer stops.
Why may this reading differ from the ‘timer value’
AIR RESISTANCE
however, this is virtually unavoidable
