Motion Flashcards
Distance
How far an object moves
But not what direction it moves in
What quantity is distance?
Scalar
It has a measurement of how far it moved but not what direction
Displacement
The distance an object moves from start to finish
Only shown as a straight line, not the turns it could take
Include direction if that line eg north east or west
What quantity is displacement?
Vector
Because it is a measure of how far it travelled and the direction
Speed
Tells us the distance something travelled in a given time
What quantity is speed?
Scalar because it doesn’t have a direction
How to find speed (m/s)
Distance (m)
—————
Time (s)
What’s different about the speed/distance/time equation?
Distance represented by symbol S
Speed represented by symbol V
Normal walking speed
1.5m/s
Running speed
3m/s
Cycling speed
6m/s
What do non mechanical speeds (walking, running etc) depend on?
Who the person is, a fitter/younger person may move quicker than older/unfit
Terrain, people move quicker on flat ground
Than uphill
Distance travelled, people move quicker at the beginning of a long distance route when more energy
Car on main road speed
13m/s
Fast train in Uk speed
50m/s
Cruising aeroplane speed
250m/s
Speed or sound in air
330m/s
Is speed of a losing object always constant?
No
It changes on its journey ie a car slowing down and speeding up
Even though speed is usually not constant, how do we find a value for calculations?
We use the average speed over the cause of the journey
Equation for distance
Speed (m/s) x time (s)
Velocity
An objects speed in a certain direction
For example it says north, east, south, west
Speed vs velocity
For example:
Speed = 20m/s
Velocity = 20 m/s east
What quantity is velocity?
Vector
Because it gives us a measure of speed and the direction it is in
How do we calculate velocity?
The same way we find speed
(Distance/time)
But you MUST write the direction NEXT to it
When is the velocity value different?
When objects move in a circle
How does velocity change in objects moving in a circle?
If it moves at a constant speed in a circle/ part of a circle
The velocity constantly changes
Because it’s direction changes although the speed is the same
This is because to calculate the velocity, distance is a value you need to know as well
Distance time graphs
How we can show a distance an object travelled in a journey
Over the course of a given time
Also can calculate speed using it
When can we draw a distance time graph?
If the object moves in a straight line
How do distance time graphs look?
DISTANCE (m) on y axis
TIME (s) on x axis
What happens when someone stops walking in a distance time graph?
The line is straight
There is a point when they first stop
Connected to another point when they finish stopping
Straight line because the distance doesn’t increase = they have stopped
What do you need to be sure of when plotting distance time graph?
Make sure you add the time to the time
and the distance to the distance
DO NOT MIX THEM UP AND ADD DISTANCE TO TIME
On a distance time graph what does the gradient show?
The speed!!!!!
If the distance time graph is an upwards slope, what does it tell us?
It is constantly increasing/ decreasing speed
Aka it’s accelerating or decelerating
How to find the speed at a specific point if the distance time graph is a slope?
Put a point in the graph at the specified point
Draw a tangent as large as possible
Use that to make a triangle and do distance / time = speed
Acceleration
Tells us the change in velocity of an object over a given time
Unit for acceleration
m/s²
How do we find the acceleration?
Change in velocity (m/s)
————————————
Time (s)
What does acceleration tell us?
Every second, the object increased by the acceleration value every second over the given time period
What happens if the starting velocity is more than the end velocity when calculating acceleration
The velocity decreases
Thus the change is negative
So the acceleration is negative =deceleration
How else can we find the acceleration not using equations?
Velocity time graphs
Velocity time graphs
Shows us the objects change in speed in a certain direction over a period of time
How do velocity time graphs look like?
Y axis = velocity (m/s)
X axis = time (s)
What does a changing gradient on a distance-time graph show?
THE ACCELERATION!!!
if it’s straight = constant speed. Not acceleration
Positive gradient = acceleration = upward slope
Negative gradient = deceleration = downward slope
If the line is flat on a velocity time graph what does that show.
That it’s moving at a constant velocity
It isn’t accelerating
NOT THAT IT HAS STOPPED, OTHERWISE IT WOULD GO DOWN TO 0 ON THE Y AXIS!!!!
What does the area below the graph on a velocity time graph show?
The distance travelled in a specific direction = displacement
In m
What equation can we use if an object accelerates at a constant rate?
V² - u² = 2Ad
V = final velocity
U = initial velocity
A = acceleration
D = distance
If an object falls to the surface of the Earth, what is the initial acceleration and why?
9.8m/s²
Because of the force of gravity acting on the object
As an object falls to the surface of the Earth, why does the initial acceleration change?
As it falls it experiences upward force of friction due to air particles
Aka air resistance
And the force of air resistance balances the force due to gravity
Therefore it stops accelerating and moves at a constant velocity
Terminal velocity
When the force of gravity balances the force of air resistance as an object falls to surface of the Earth
Thus it moves at a constant velocity and doesn’t accelerate
What does terminal velocity apply to?
Objects in liquid
Objects falling to surface of the Earth (liquid is air)
What determines the terminal velocity of an object?
How much friction an object experiences (based on shape)
Greater force of friction = lower terminal velocity
Newtons laws of motions
Describes the effects that forces have on the motion of an object
Newtons FIRST law of motion
If the resultant force acting on a STATIONARY object is zero, it will stay STILL
If the resultant force acting on a MOVING object is zero, it continues moving in the SAME DIRECTION at the SAME SPEED (SAME VELOCITY)
So how will the velocity of an object change?
If a resultant force of above 0N acts on the object
If a car is moving at a constant speed what is its resultant force?
0N
Resistive forces
Forces that act against the forward force of an object
Examples of resistive forces
Friction on the ground
Air resistance in air
If an object is moving to the right and a resultant force is applied to the RIGHT, what happens?
It will accelerate to the right
Aka it’s velocity will increase
In an object is moving to the right and a resultant force is applied to the LEFT, what happens?
It will decelerate but continue moving to the right
Because the resultant force still pushes to the right
Aka it’s velocity will decrease
If an object is moving up and a resultant force pushes upwards, what happens?
It will accelerate upwards
Because resultant forces can also change an objects direction
Because it affects velocity that affects direction
Newtons SECOND law
The acceleration of an object = proportional to the resultant force
And inversely proportional to mass of object
What does Newton’s second law mean in practise?
Bigger resultant force = bigger acceleration
Smaller resultant force = smaller acceleration
Which is directly proportional
Bigger mass = smaller acceleration
Smaller mass = bigger acceleration
Inversely proportional
How to find force needed to accelerate an object?
Force = mass x acceleration
Speed of cars on main road in uk
13 m/s
Speeed of cars on motorway
30m/s
What do cars need to accelerate from a main road to motorway?
2m/s^2
What force would need to accelerate a typical family car by 2m/s^2
2000N
Inertia
The property of mass that obeys newtons 1st law
That they will keep still or keep the same motion if you apply resultant force
Inertial mass
Measure of how difficult it is to change the velocity of an object
Large inertial mass
Requires large force to produce a given acceleration
Because it has more inertia
So it’s harder to apply enough force to overpower the resistive forces
Newtons THIRD law
Whenever 2 objects interact the forces they exert on each other are equal and opposite
(Write this word for word in exam)
What does Newton’s third law mean in practise?
If you exert a force on an object, it will exert a force on you back which is equal in size
But opposite in direction
Inertial mass equation
Resultant force
————————
Acceleration
