Forces

Menu What is a force? ‘Drawing’ forces What can a force do? Terminal Velocity The Laws of Motion Gravity Weight, Mass & Gravity Stopping Distances Work Done Power

What is a Force? A force is a PUSH Or a PULL

There are 6 different forces….. Gravity (always straight downwards) Reaction force (from a surface) Thrust/ Push Drag (air resistance/ friction) Lift (aeroplane wing) Tension (rope / cable) Return to Menu

How do we ‘draw’ a force?

We use arrows to show the size & direction of a Force This box has a force of 2N acting on it to the right. It is an unbalanced force.

In these examples, more than one Force is acting In these examples, more than one Force is acting. What is the RESULTANT FORCE? Resultant 2N to left 4N 2N Zero Resultant 2N 2N Resultant 2N to right 3N 1N

Are the forces ’Balanced’ or ‘Unbalanced’ Are the forces ’Balanced’ or ‘Unbalanced’? State whether the cars are at a steady speed or will accelerate Unbalanced Accelerate A Balanced steady Speed B Unbalanced Accelerate (slow down / decelerate) C Return to Menu

A Force can…. …..Start something moving …..Stop something moving

….Change an objects shape …..Change an objects direction …..and change an objects speed

Look how this golf ball changes shape the split second it is hit! Return to menu

The Terminal Velocity of a falling object depends on their shape and area The accelerating force acting on all falling objects is gravity and it would make them all fall at the same rate if it wasn’t for air resistance

This means that 2 objects of different mass but the same shape and area would fall at the SAME rate However, a feather & a hammer would fall at different rates because of air resistance

What would happen on the Moon? A hammer and feather would fall at exactly the same rate because there is NO air and therefore NO AIR RESISTANCE!

The sky diver explains AIR RESISTANCE and TERMINAL VELOCITY perfectly! ….so the weight arrow will be the same size! Note: their weight doesn’t change… WEIGHT

Just Jumped AIR RESISTANCE Initially there is little air resistance and they are unbalanced The result is that they accelerate WEIGHT

As the speed increases – the air resistance also increases until it BALANCES the WEIGHT And there is NO acceleration TERMINAL VELOCITY has been reached WEIGHT

The Parachutes have opened! AIR RESISTANCE The air resistance has now increased. The forces are unbalanced They decelerate (slow down) WEIGHT Return to Menu

The Laws of Motion This is Sir Isaac Newton He is responsible for the 3 Law’s of Motion

Newton’s 1st law of Motion BALANCED FORCES mean NO CHANGE IN VELOCITY This car is travelling at a steady speed – it will not change direction or speed!

Newton’s 2nd law of Motion ‘If there is an UNBALANCED FORCE then the object will ACCELERATE IN THAT DIRECTION’ The size of the acceleration depends on the formula: F = ma

Newton’s 3rd law of Motion If object A EXERTS A FORCE on object B….. Then object B exerts the EXACT OPPOSITE FORCE on object A …OR… ‘for every action there is an equal & opposite reaction’

Questions on:- Force, mass and acceleration A force of 3000N acts on a motor bike to make it accelerate by 2m/s2. What is the mass of the motor bike? A car accelerates at a rate of 7m/s2. If it has a mass of 700kg how much driving force is the engine applying? A force of 10N is applied by a boy while lifting a 15kg mass. How much does it accelerate by? F m x a Return to Menu

Gravity is the force of attraction between all masses It makes all objects accelerate to the ground It gives everything its ‘weight’

Weight & Mass are NOT the same! MASS is the amount of matter in an object (for any given object this will be the same ANYWHERE in the Universe) WEIGHT is caused by the pull of gravity Return to Menu

Mass Weight & Gravity are all related with this formula:- W = m x g Weight = mass x g Mass is in kg Weight is in Newtons This represents the strength of the gravity (on Earth it is 10N/kg)

Weight vs. Mass On Earth the Gravitational Field Strength is 10N/kg This means:- a 1kg mass is pulled downwards by a force of 10N. What is the weight on Earth of a book with mass 1.5kg? What is the weight on Earth of an apple with mass 120g? Debbie weighs 700N. What is her mass?

What is the weight, in newtons of a 5kg mass, both on the Earth & on the Moon? (On Earth g = 10 N/kg. On the Moon g = 1.6 N/kg) On Earth:- W = m x g = 5 x 10 = 50N On Moon:- W = m x g = 5 x 1.6 = 8N 5kg Return to Menu

Stopping distance of vehicles is affected by…. The Thinking Distance And the… Braking Distance

What affects the ‘Thinking Distance? Too much Alcohol Tiredness Thinking Distance Poor Visibility Drugs

What affects the ‘Braking Distance? Icy Roads Wet Roads Braking Distance Driving Too Fast Brakes/ Tyres Worn

Identify whether the ‘Thinking distance’ or ‘Braking distance’ is affected in each case

Stopping Distances Increase alarmingly with extra speed (because velocity is squared!) Kinetic Energy Transferred = Work Done by Brakes ½ mv2 = f x d v = speed of car f = maximum braking force d = braking distance Return to menu

Work Done = Force x Distance Work Done whenever a force moves an object (whenever energy is transferred) Work Done = Force x Distance (Nm or J) (N) (m) 1 joule of work is done whenever a force of 1 newton moves a distance of 1 metre Wd F x d

Work Done = FORCE x DISTANCE = 60 N x 5 m = 300 Nm Question: Calculate the work done by a pupil who pulls a car tyre 5 metres with a force of 60 N. Answer: Work Done = FORCE x DISTANCE = 60 N x 5 m = 300 Nm We could also give our answer as 300 J ( As 1Nm = 1J) Return to Menu

1) Calculate the work done when the broom moves a 3N force 6.5 metres 2) Calculate the distance moved by the broom if it does 24J of work moving a 10N force Return to Menu

The unit of power is the watt (W) What is POWER? Power is the ‘rate of doing work’. A powerful machine is one that can transfer a lot of energy in a short space of time. Work Done J Power = Time Taken s W WD The unit of power is the watt (W) P x t

To measure your power output Measure the vertical height of the stairs (metres) Measure your force (Newtons) Measure the time it takes you to run or walk up the steps (seconds) Calculate the WORK DONE (Nm) Calculate the POWER (W)

Results Table for Power Name Force (N) Distance (m) Work done (J) Time (s) Power (W) Return to Menu