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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