Forces revision
Pressure and depth
Upthrust
Vectors and scalars
Resultant forces
Work done
Scalar quantities only have a magnitude (size) - Distance is a scalar quantity
Vector quantities have a magnitude (size) and direction - Displacement is a vector quantity
Contact & non contact forces
Contact forces occur between objects that are touching
Reaction force
Tension
Friction
Air resistance
Non-contact forces act between objects that are not physically touching
Magnetic force
Electrostatic force
Gravitational force
Gravity
All objects with mass produce a gravitational field strength
Weight is a force acting on an object due to gravity, which can be measured using a newton-meter
Gravitational field strength is measured in newtons/kg - the earth's is 9.8 N/kg
Weight = mass x gravitational field strength
Mass and weight are directly proportional
The overall force on a point or an object
Free body diagrams
Sizes of the arrows = relative magnitude
When a force moves an object through a distance, energy is transferred and work is done on an object
Work (J) = Force (N) x Distance (m)
One joule of work is done when a force of one newton causes an object to move a distance of 1 metre.
1 Joule = 1 newton Metre (Nm)
Scale diagrams to work out resultant forces
Draw all of the forces acting on an object to scale (tip to tail)
Draw a straight line from the start of the first force to the end of the last force
Measure the length of the resultant force on the diagram to find the magnitude and the and to find the direction of the force
Measure the angle of a force from north
Balancing foces
An object is in equilibrium if the forces are balanced (if all of the forces acting on it combine to give zero)
Springs
When you apply a force to an object you may cause it to stretch, compress or bend
To do this, you need more than one force acting on the object - otherwise the object would just move in the direction of the applied force
Work is done when a force stretches or compresses an object and causes energy to be transferred to the elastic potential energy store
It is is elastically deformed, all this energy is transferred to the objects elastic potential energy store
If an object has been elastically deformed, it can return to its original length and shape after the force has been removed
Objects that can be elastically deformed are called elastic objects
An object has been inelastically deformed if it doesn't return to its original shape and length after the force has been removed
Elasticity
Extension is directly proportional to force
Force (N) = Spring constant (N/m) x Extension (m)
There is a limit to the amount of force you can apply to an object for the extension to increase proportionally, this can be represented by a graph
The limit of proportionality is the point when the extension is no longer proportional to the force
Elastic potential energy (J) = 0.5 x spring constant (N/m) x extension^2 (m)
Moments
A moment is the turning effect of a force
Moment (Nm) = force (N) x distance (m)
The force on a spanner causes a turning effect of moment on the nut (which acts as a pivot). A larger force or a longer distance (spanner) would mean a larger moment.
To get the maximum moment you need to push at a right angle to the spanner. Pushing at any other angle means a smaller distance so a smaller moment
If the total anticlockwise moment equals the total clockwise moment about a pivot, the object is balanced and won't turn.
Levers make it easier to do work
Levers increase the distance from the pivot meaning that less force is needed to get the same moment.
Gears are circular discs with teeth around the edges
Their teeth interlock so that the turning one causes another to turn in the opposite direction
They are used to transmit the rotational effect of a force from one place to another
Different sized gears can be used to change the moment of the force. A force transmitted to a larger gear will cause a bigger moment, as the distance to the pivot is greater
A larger gear will turn slower than the smaller gear
As depth of a liquid increases, the number of particles above that point increase. The weight of these particles adds to the pressure felt at that point, so liquid pressure increases with depth
Fluids are substances that can flow because their patrticles can move around
As these particles move, they collide with others
Particles are light but they still have a mass and exert a force on the object they collide with. pressure is force per unit area, so the particles exert a pressure
Pressure (Pa) = Force (N) / Area (m^2)
Pressure in a liquid depends on depth and density
Pressure (pa) = height of the column of liquid (m) x density (Kg/m^3) x gravitational field strength (N/Kg)
When an object is submerged in a fluid, the pressure of the fluid exerts a force on it from every direction
Pressure increases with depth, so the force exerted on the bottom of the object is larger than the force acting on the top of the object
This causes a resultant force upwards (upthrust)
the upthrust is equal to the weight of the fluid that has been displaced by the object
If the upthrust on an object is equal to the object's weight, then the forces balance and the object floats
If an object's weight is more than the upthrust, the object sinks
An object denser than the liquid will sink and an object less dense than the fluid will float
Atmospheric pressure
Atmospheric pressure is created on a surface by air molecules colliding with the surface
As altitude increases, pressure decreases as the atmosphere gets less dense so there are less molecules to collide
There are also fewer air molecules above a surface as the height increases so the weight of the air above it decreases with altitude
Acceleration
change in velocity in a space of time
Acceleration (m/s^2) = change in velocity (m/s)/time (s)
Deceleration is negative acceleration
Uniform acceleration is constant acceleration
v^2 -u^2 =2as
Final velocity - initial velocity = 2x acceleration x distance
Distance/time graphs
Gradient = speed
Flat sections = stationart
Straight positive gradient = steady speed
Curves = acceleration / deceleration
Velocity/time graphs
Gradient = acceleration
Flat sections = steady speed
Positive gradient = acceleration
Under area of line = distance travelled
Drag
Resistance in a fluid
Increases as speed increases
terminal velocity
When falling objects first set off, the force of gravity is much more than the frictional force slowing them down, so they accelerate
As the speed increases, the friction builds up
This gradually reduces the acceleration until eventually the frictional force is equal to the accelerating force
Newton's laws
Newton's first law
If the resultant force on a stationary object is zero, the object will remain stationary. If the resultant force on a moving object is zero, it will continue moving at the same speed in the same direction
Newton's second law
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