Forces

Force

a push or pull that acts on an object due to the interaction with another object

all forces are either non-contact or contact

Non-Contact force

objects are physically seperated

e.g electrostatic: charges cause a force of attraction/repulsion

gravitational attraction: mass creates a force of attraction

Contact force

the objects are physically touching

e.g Normal contact force: felt in opposite direction to contact

Friction: surfaces and their roughness cause friction when moved in contact

Scalar

Magnitude (size) but no direction

Generally cannot be negative

Vector

Magnitude (size) and direction.

Can be represented by arrows (length / size = magnitude)

Displacement

a vector quantity that means the distance travelled in a straight line from the start to the finish AND the direction of that straight line

Typical speed of a person walking

1.5 metres per second

Typical speed of a person running

3 metres per second

Typical speed of a person cycling

6 metres per second

Typical speed of sound waves in the air

330 metres per second

Typical speed of a car

25 metres per second

Typical speed of a train

55 metres per second

Typical speed of a plane

250 metres per second

Velocity

Vector quantity that is speed in a given direction

When an object moves in a circle

it may travel at constant speed BUT the velocity will

be constantly changing because velocity is a vector quantity that depends on speed and direction

Distance-Time Graph: Curved upwards line means

the object is accelerating and a tangent must be drawn to find speed

DTG vs VTG gradient.

DTG - Speed
VTG - Acceleration

Stopping Distance

Thinking Distance + Breaking Distance

Gravity

All matter has a gravitational field, and attracts all other matter

The larger the mass, the stronger the field, the greater the attraction

Weight

The force exerted on a mass by the gravitational field, in Newtons

considered to act as object’s centre of mass

What is weight measured by?

Measured by a force meter (also known as calibrated spring-balance)

Weighing scale measures the force you exert, and then divides by 10 to give mass

Gravitational field strength (n/kg)

for each kg of mass, experiences n of force

gravitational field strength (g) on earth

9.8

If the same person was on two different planets..

-mass = same

-g will be different

-so weight different

Why does acceleration in free falling occur?

gravity

acceleration = g

resultant force

single force representing the sum of all the forces acting on an object

more than one force on a straight line, find resultant by adding (act in same direction) or subtracting (act in opposite direction)

terminal velocity

max. velocity

Work done (motion)

when energy is transferred from the object doing the work to another form

Work Done = Force × Distance

where ditstance = distance moved along the line of action of the force

To stretch, bend or compress an object..

more than one force has to be applied

If a single force is applied to an object…

it will just move in that direction

-if pulled on opposite sides of object = stretch

-if fixed at one point then stretched, force still being applied by fixed point

deformation

changing shape

elastic deformation

object returns to original shape when load removed

e.g elastic band

plastic deformation

object does not return to original mistake when load removed

e.g spring when pulled too far

hooke’s law

The extension of an elastic object, such as a spring, is directly proportional to the force applied, provided that the limit of proportionality is not exceeded.

When a force stretches/compresses a spring

spring does work

Elastic potential energy is stored in the spring

if it doesn’t inelastically deform, work done in spring = elastic potential energy stored

pivot point

a point which it can rotate about, but cannot move away from

Moment of a Force

force × perpendicular distance

Example of moment

Bike Riding – pressing your foot down on the pedal, causes a moment about the pivot, turning the pedal arms.

Equilibrium (motion)

sum of anticlockwise moments = sum of clockwise moments

Pivots

-if force applied along line passing through pivot, object held still

-if distance between pivot and line of action of force, object rotates about pivot in direction of force applied

-if force applied isn’t perpendicular, we have to consider perpendicular distance from pivot to line of force

Gears can change…

speed, force or direction

by rotation

Pressure produces a net force at..

right angles to any surface

When does an object float?

if it’s weight is less than the weight of the water it displaces

e.g 1000kg boat will sink until it has displaced 1000kg of water (if it doesn’t submerge, it will float)

What does pressure in a liquid vary with?

depth and density. this leads to an upwards force on a partially submerged object

The buoyancy force

the upwards force that counteracts the weight of the floating object

equal to the weight of the fluid displaced by the object

Why does a ping pong ball float on water?

-its density is less than the water

-so, for volume displaced, weight of equivalent amount of water is GREATER than weight of ball

-resultant force is buoyancy, so it floats

increasing depth

increases pressure (greater weight of water above you)

upthrust

partially / totally submerged object

experiences greater pressure on bottom surface than top surface

this creates a resultant force upwards

known as upthrust

what is earth’s atmosphere?

A thin layer (relative to size of the earth) of air around the Earth.

atmosphere gets less dense with increasing altitude

Why does earth’s atmosphere get less dense with increasing altitude?

-because it is the total weight of the air above a unit area at a certain altitude

-weight of air IS force that causes pressure

-so, higher elevation = fewer air molecules above unit area than the same at lower heights, so smaller weight, less pressure

idealised assumptions for a simple model of the atmosphere

Isothermal, so it is all at the same temperature

Transparent to solar radiation

Opaque to terrestrial radiation