Chapter 3 Flashcards
How can you get closer to instantaneous speed?
Closer the time interval, otherwise everything is normally an AVERAGE speed
Instsneous speed?
Velocity?
Gradient tangent
Again gradient of tangent
What gives something infinite acceleration?
If it turns around and switches direction of velocity completely!
- this because you cant find a time for this change so acceleration is change of velocity and it goes all the way
Change direction = accelerate
WHAT IS DISPLACEMENT ANYWAYS
Remember
It’s the MIMIUM DISTANCE BETWEEN TWO POINTS , SO IF IT IS CIRCLE, STRAIGHT LINE
-“ vector that travels directly between two points
What is acceleration
The rate if change if velocity
What is uniform acceleration?
When a body changes it’d speed at a constant rate, whilst travelling in a STRAIGHT LINE.
So like changes by 2ms every second
- rather if it changed different each second that’s variable and dynamics equations do not apply
How to find acceleration that is variable ?
Instantaneous gradient of velocity time graph
Displacement on velocity time graph?
Area
Velocity time graphs how to describe motion
Say acceleration or whatever
Always mention the stationary point at 0
Displacement / disrsnce for velocity time graph?
What do you have to remember
Displacement is area above - area below,
Distance is area above + area below
BE CAREFUL TO DRAW YKUR SHALES INKY WHERE THE DOTS CONNECT OR ANSWERS WILL BE WRONG
Suvat 4
S= Ut+ 1/2at2 S= 1/2 (U+V) t
V2=u2 + 2as
V=u + at
Another one in case?
S = vt-1/2at2
Important to remeber for SUVAT about acceleration direction
If by going your acceleration decreases (deceleration), then it will be negative 9.81
But if you are going down where acceleration is increasing, use positive 9.81
Now with acceleration there will always be final and initial velocity,
IMPORTANT FOR SUVAT TIME
If it says time of flight but inky wants halfway make sure to use half the time!
What about acceleration in Air due to gravity
If you pick one as direction it will ALWAYS BE CONSTANT, so doesn’t change direction, is never maximum or minimum or 9 but always 9.81
What is thinking , breaking and stopping distance
Thinking distance is distance travelled when driver is reacting and thinking about stopping
Breaking is the distance once driver actually hits breaks
Stopping is thinking + breaking
So how calculate thinking distance and what affects it
Factors affecting driver = drugs alcohol distraction tired etc
This is dependent on reaction time and speed
So if speed = distance/ time, distance = speed x reaction time
2) Thus higher speeds and higher reaction times = more distance
Breaking distance!
Here you are going from an injtsl velocity to final velocity of 0, so use SUVAT
Factors affecting braking
Weather conditions (icy etc)
Worn tyres
Good brakes
Bad roads
As these are factors affecting the car and its ability to brake in quick time
What is free fall
The fall of a body through a medium without a drag producing device
How is acceleration derived without any resistance ?
F= ma
A= F/M
The only force acting is the body weight
A= W/m
A= mg/m
A= G = 9.81
Thus an object released into the earth will accelerate at 9.81 if no Over force is acting on it
What about with resistance
A= F/m
A=W-drag/M
A= mg/m -d/m
A=g- d/m, the DRAG MASS RATIO
Thus it is on the drag mass ratio that depends
Why do we ignore drag?
When do we not
Thus you can use SUVAT
It is normally 3 orders less than our weight
However we don’t when
1) body mass is very small compared to its surface area (LIKE A FEATHER)
2) A body is falling at a considerable velocity (like parachutist) because then drag is there (increase velocity = increase drag^2)
How to do experiment to calculate g
1) attach weights to tapper which hits a mark at a steady frequency
2) let go, measure the distance dots went and count them multiply by 1/f to get time
3) you can now do suvat with #= ut +1/2t2 and get g
How to do the electromagnet and trapdoor experiment
Why inaccurate and how improve
1) electromagnets holds a ball above trapdoor
2) when current switched off, timer starts, it demagnetises and falls
3) then it hits trapdoor breaking circuit and ending the time
4) using distance and time and initial velocity you can work it out
- inaccurate because REISTANCE IS PLAYING A PART, and also a fixed time for the electromagnet to demagnisitse
- use heavier ball and longer distance so that the error uncertainty doesn’t make much of a difference
Using a camera and then graph
Takes rapid pictures of a ball dropping next to distance and time
Plot graph of regular intervals of DISTANCE/ TIME2
The gradient will be g/2 so multiply by 2
What do you have to remember about saying when drag is involved
Drag is NEGLIGIBLE
With air resistance , when is the only time acceleration is maximum, 9.81
At the start, after that resultant force begins to decrease
When bodies are connected what do you have to do about mass?
Add them up and use them as a whole
What is projectile motion
What is ignored in projectile motion
What happens in both planes
Something that is thrown at angle to horizontal
- drag
- in horizontal the only force present is at the start, but after that there is no longer any force acting on it. Newton law means 0 resultant force= 0 acceleration and moves at its velocity
- for vertical gravity acts. If go up then negative
Thus suvat equations used here
But time is linked in both!
How to calculate say final velocity of something
What about time
1) use one plane to get the time
2) put this in the other plane to get final velocity in that plane
3) get final velocity in horizontal
4) now do Pythagorean and get final velocity overall
1) for time remember if it’s symmetrical calculate half way and multiply by 2 or take final velocity to be negative initial
Calculating final velocity requires WHAT TOO
The angle IT IS AT USE SOH CAH TOA
Remember for projectile motion , do objects with same one component hit the ground at the same time?
Yes if they have same one component it means it will have the same time everywhere.
What changes is the SPEED
Circular motion what direction has the force got to be so that it is the same size but different direction!
Perpendicular
If stuck
Use 5th equation
Or try find initial / final velocity with other equations,
Particularly v=u+at
Why is there an error if something is proportional to something in a graph
Because IT DOESNR GO THROUGH ORIGIN , THIS PROBABLY BECAUSE MISTAKE IN measurements !
Drawing graphs to represent acceleration in distance time?
Acceleration means changing velocity, represented by gradient
If this decreases, think about it as decrease of gradient
