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Topic 5 Forces By Bethan Poole (5.3 Force and Elasticity (Limit of…
Topic 5 Forces
By Bethan Poole
5.1 Forces and their interactions
5.1.1 Scalar and vector quantities
Scalar
Magnitude only
Vector
Magnitude and direction
Arrows used to represent vector quantities
length of arrow shows magnitude
Arrow points in the direction that the vector quantity is acting
Forces
Vector quantities
5.1.2 Contact and non-contact forces
Force occurs when two or more objects interact
Either
Non-contact
Objects not touching
e.g. Gravitational force Electrostatic force Magnetic force
Contact
Objects are touching
e.g. Friction, Air-resistance, Tension, Normal contact force, Upthrust
5.1.3 Gravity
Force of attraction between all masses
Force of gravity close to Earth due to the gravitational field around the planet
Mass
Relates to the amount of matter it contains
Is constant
Weight
Amount of force acting on a object due to gravity
Depends on the gravitational field strength - directly proportionate to mass
Proportionate symbol
5.1.4 Resultant forces
When more than one force acts on an object
These forces can be seen as a single force that has the same effect as the forces acting together-
Resultant force
Vector diagrams
Free body diagrams
Can be used to show difference forces acting on an object
Scale vector diagrams are used to show the overall effect of more than one force acting together
Forces added together- get a single resultant force (magnitude and direction)
Vectors added head to tail and a resultant force arrow drawn
F1+F2=Fr
Scale vector diagrams
Used when a force is acting in a diagonal direction
Expresses the diagonal force as two forces at right angles
Fr broken down int F1 and F2
F1= same length as Fr in horizontal direction
F2= same length as Fr in vertical direction
Fr is vectors found by adding F1 and F2 head to tail
Work done and energy transfer
When a force causes an object to move work is done on an object
As energy is required to move the object
1 Joule of work is down when a force of 1N causes a displacement of 1M
1Joule=1newton metre
When work is done energy transfers take place within the system
5.3 Force and Elasticity
To change an objects shape more than one force must be applied
Elastically deformed
When the forces acting on a object are removed and it goes back to it's original shape
Plastically deformed
When the forces acting on an object are removed and it doesn't go back to its original shape
Extension
Directly proportionate to the force applied
Limit of proportionality
doubling the force won't double the extension
Relationship becomes non-linear
force-extension graph will stop being a straight line
Also applies to the compression of an elastic object
Spring constant
Indicates how easy it is to stretch or compress the spring
Higher the spring constant the stiffer the spring
A force that compresses or stretches a spring store elastic potential energy in the spring
Amount of work done and energy stored are equal- providing it hasn't passed the limit of propotionality
5.4 Moments, levers and gears
When a force causes an object to rotate about a pivot point, turning effect is called the moment of a force
If an object is balanced, total clockwise moment about the pivot equals the total anticlockwise moment about that pivot
F1 x d1= F2 x d2
Leavers and gears can be used to
transmit the rotational effects of forces
Magnify either the size of the applied force or the distance the force moves over
When the applied force moves further than the transmitted force, force is increased
When the applied force is bigger that the transmitted force, distance is increased
5.5 Pressure and pressure differences in fluids
5.5.1 Pressure in a fluid
Fluid can be a liquid or a gas
As particles move around in a fluid, they collide with the surface of objects in the fluid or the surface of the container
Collisions create a force normal (right-angle) to the surface
If the pressure acts on a bigger area, it will produce a larger force
5.5.1.2 Pressure in a fluid 2
Pressure at a point in the liquid is dependent on:
Height of column above a point
Density of liquid
The higher the column and the denser the liquid...
the greater the weight above the point
the greater the force on the surface at that point
the greater the pressure
Upthrust
When an object is submerged in a liquid there is a greater height of liquid above the bottom surface that above the top of the surface
Greater pressure at bottom that top- creates a resultant fore upwards
The upward force is called upthrust
Objects float when weight is equal to upthrust and sink when weight is greater that upthrust
Object less dense than liquid
displaces a volume of liquid greater than its own weight so rises to surface
will float with some of the object remaining below surface
displaces liquid of equal weight to the object
Object with a low density, more will remain above surface
Object denser than liquid cannot displace enough liquid so sinks
No matter the density, the size of upthrust equals the weight of liquid displaced
Pressure higher at bottom of column as the particles above are exerting a force on the particles below it
5.5.2 Atmospheric pressure
Atmosphere- relatively thin layer of air around the earth
Greater the altitude, the less dense the atmosphere and the lower the atmospheric pressure
High altitude- less air above the surface than at lower altitude so smaller weight of air acting on the surface, therefore lower pressure
5.6.1 Forces and motion
5.6.1.1 Distance and displacement
Distance
Scalar quantity
It is how far an object moves
Doesn't take into account the direction or if it ends up back where it started
Displacement
Vector quantity
Has a magnitude- describes how far an object has travelled
Has direction
5.6.1.2 Speed
Does not involve direction
Measures how fast something is going
Scalar quantity - measured in metres per second (m/s)
Typical speeds
Walking- 1.5 m/s
Running-3m/s
Cycling- 6m/s
City driving- 12m/s
High speed trains - 75m/s
Commercial aircraft- 250m/s
Speed of sound in air 330m/s
Most things do not travel at constant speed so average speed is often used
2.6.1.3 Velocity
Vector quantity
Speed of an object in a given direction
If travelling in a straight line at constant speed you have constant velocity
Not in straight line
speed can still be constant
velocity will change as direction as changed
Moving in a circle
Constantly changing direction so constantly changing velocity
It's accelerating even if travelling at constant speed
Orbiting plants for example, force of gravity causes acceleration
2.6.1.4 The distance-time relationship
Distance-time graph used to represent the motion of an object travelling in a straight line
Gradient= speed of the object
Stationary object= horizontal line
Diagonal line= constant speed
Object accelerating- distance time graph will be a curve
Accelerating object find out speed by drawing a tangent to the curve at the point in time, working out tangents gradient
5.6.1.5 Acceleration
Measure of how quickly something speeds up, slows down and changes direction
When an object slows down, the change in velocity is negative (negative acceleration)
Velocity-time graphs
Gradient used to find objects acceleration
Total distance travelled = area under graph
Terminal velocity
When object falls through a fluid
1st object accelerated due to gravity
As it speed up resistance increases
Resultant force reaches 0 when resistive force equals the force of gravity- terminal velocity
Near to earth's surface acceleration due to gravity= 10m/s 2
5.6.2 Forces and motion
5.6.2.1 Newtons first law
An object will remain in the same state of motion unless acted on by an external force
When the resultant fore acting on the object is zero
Object is stationary if previously
Constant speed if moving
This tendency for objects to continue in the same state of motion is called
Inertia
Velocity or speed will change if a force is acting on it
5.6.2.2 Newton's second law
The acceleration is proportional the resultant force acting on an object and inversely proportionate to the mass
If the resultant force is doubled, acceleration will double
If the mass doubles, acceleration will half
Mass is a measure of inertia
Describes how difficult it is to change the velocity of an object
Inertial mass given by the ration of force over acceleration (m=f/a)
Larger mass, bigger force needed to change velocity
5.6.2.3 Newtons 3rd law
For every action there is an equal and opposite reaction
Whenever one object exerts a force on another it exerts a force back
Reaction force is the same type and equal but in the opposite direction
5.6.2 Forces and motion
5.6.2.1 Stopping distance
Depends on
The thinking distance
The braking distance
The greater the speed of the car the longer the sopping distance
Thinking distance directly proportional to speed
Doubling the speed increases braking distance by factor 4
5.6.2.2 Reaction time
Varies from person to person- typical is between 0.2-0.9 seconds
Can be affected by tiredness, drugs and alcohol
Distractions i.e Phones also affect ability to react
Measuring reaction time
use lights or sound as start symbol and use an electronic timing system to measure how long someone takes to react
Can be measured by dropping a ruler and catching it as it falls- distance ruler falls without catching it is time it took to react
5.6.2.3 Factors affecting braking distance
Can be affected by
Weather Conditions
Wet, icy/ snowy roads
Road Conditions
The Vehicle
Including worn brakes or tyres and over or under-inflated tyres
To stop the brakes must apply force to the wheel
Greater the breaking force= greater the deceleration of the vehicle
Work Done transfers the kinetic energy of the vehicle into heat energy- increasing temp. of breaks
If breaking force is too large, breaks may overheat or the tyres may lose traction= Car will skid on the road
The faster the vehicle the larger the braking force needed to stop within a certain distance
To work out braking distance use W =fd
For a given braking distance
Doubling mass doubles force required
Doubling speed quadruples force required
5.7 Momentum
5.7.3. Changes in momentum
All moving objects have momentum
When an unbalance force acts on a moving or able to move object a change in momentum occurs
Force equals the rate of change of momentum
Important fact when designing many safety devices
These devices reduce force by increasing the time over which the change of momentum takes place
E.G. Crash mats increase the amount of time it takes someone to rest when they fall onto the floor
5.7.2 Conservation of momentum
In a closed system the total momentum before is equal to the total momentum after
If calculating velocity after
1) Calculate total momentum before
2) Remember momentum before= momentum after
3) Substitute values of momentum and mass into Momentum= mass x velocity
4) Rearrange equation to find velocity
