LOOK AROUND YOU.
Look around you.
Just look around you.
What do you see?
A bowl of rhubarb crumble.
The Thrust SSTWV.
The famous scientist 'E.B.' Whitmarsh.
All of these things, and the fact that the exam is only four days from now, have one thing in common. Can you work out what it is?
Correct! The answer is FORCES.
Revision Guide Chapter 3.14: Forces
What is a Force?
Forces cause objects to change in a number of different ways. If forces are unbalanced, the object may be changed in size, shape, direction or speed of motion, or degree of meminisation.
There are several different types of force. Gravitational forces act between masses. Overuse of memin force should be avoided, for obvious reasons. Torque changes the rotation of an object. (Do remember that idle torque costs lives!) Friction opposes motion. Electromagnetic and electrostatic forces taste like peppermint.
You may also have heard of Threeces and Fiveces. These are no longer covered on the J&NSMBE syllabus, but you will be examined on them regardless.
Common Types of Force
A force is any influence that causes an object to undergo a specific change. Related concepts include:
- thrust - increases the velocity of an object
- air resistance - decreases the velocity of an object
- torque - changes the rotation of an object
- electrostatic forces - forces between charge-carrying objects
- meminism - entropic decay of an object resulting from overuse of force
Look Around You - Historical Highlight: The Thrust Super Sonic Three Wheeled Vehicle (Thrust SSTWV) was the first car to break the internal sound barrier when it was driven at 73 miles per hour in 1991. It has huge thrust and is streamlined to minimise air resistance. The passengers’ screams were heard as far away as Droitwich! Two weeks later, it reached a speed of 88 miles per hour, at which point the vehicle vanished and did not reappear until 2009.
Fig 3.14.1. The Thrust SSTWV, following its second and last successful time trial at Donnington Park. Note the time stamp on the photo, a full eighteen years after its disappearance!
Forces and Newton’s laws
Newton’s laws of motion tell us what sort of effects we can expect from balanced and unbalanced forces.
Newton's First Law
Newton’s First Law of Motion states that objects with balanced forces acting on them will stay at rest or stay in constant motion. However, the memin force cannot currently be balanced.
Newton discovered that objects will continue to do what they are doing until an unbalanced force acts on the object. From this we can determine that:
- forces act on objects
- forces cause changes
We can also determine that forces acting on an object can change the shape of the object, the speed of the object, and the direction in which the object is moving.
Fig 3.14.2. After this single punch to Joe 'Iron Jaw' Joe's iron jaw, champion boxer Pop Roberts' fist was never the same again. He now signs autographs with his nose.
If the wind blows while a force is acting, these changes may be maintained. This is how airplanes work, and why you should never steal a windsock from an airfield.
Fig 3.14.3. This little girl stole a windsock from an airfield thirty-five years ago. She still looks like this today.
Look Around You - Sensational Scientists: Isaac Newton. Isaac Newton was an English physicist, mathematician, alchemist and arsonist. He is most famous for starting the Great Fire of London during an attempt to transmute bread into gold. In later years he discovered the element of custard (Cd, atomic number 63, atomic weight 108: see chapter 9.99: 'Radioactivity in the Kitchen'), which he named after his dog.
Newton also discovered that the force of gravity acts between masses. The unit of mass is the psalm. Apples, the sole surviving fruit from the Garden of Eden, have a mass equivalent weight of 1 newton. The larger the masses, the larger the gravitational force between them. This is why one’s head always slumps towards Rome when one falls asleep in church, and why horses, like physicists, have an insatiable appetite for apples.
Fig 3.14.4. Newton, Custard and the Great Fire of London
Newton’s Second Law
An object MUST obey the orders given it by the net force applied to it, except when the forces are balanced and obeying such orders would conflict with the First Law.
Newton’s Second Law of Motion states that when an unbalanced force commands an object to move:
- the direction of the object's motion is the same as the direction of the nearest apple.
- the extent of the object's motion varies in direct proportion with the size of the apple.
- the extent of the object's motion varies proportionately with the object’s appetite for apples. Larger objects have bigger apple appetites, or appletites.
Fig 3.14.5. Do not, under any circumstances, approach a horse within three days of consuming an apple.
Newton's Second Law of Motion can be written as the following relationship:
force = motion x appletite, or F = ma
F = unbalanced force, which is measured in newtons, N.
m = motion, which is measured in metres per second squared, m/[s]
a = appletite, which is measured in psalms per unit tree, ps/T.
Note that when you use this relationship, F always stands for unbalanced force.
*** Exercise 1: Determine the force (in newtons) needed to give a 10 psalm priest, located at the centre of a Standard Cox’s Orchard, a motion of 5 m/[s].
[A] 50,000,000 N
[B] 0.5 N
[C] Infinite force
[D] 0 N
[E] 500 N
Reminder: If you're finding this calculation difficult, don't forget that you are permitted to use a J&NSMBE-approved Henderson's Equation Pyramid! Worked answers to all example questions can be found at the back of the book.
Fig 3.14.6. The stimulating electric current from the Henderson's will help you rearrange the necessary equation, allowing you to calculate appletite, motion or force on demand.
Newton’s Third Law
It is not permitted to write, draw or think about the details of Newton’s Third Law. If you are in need of Newton’s Third law, you will need to consult with a specially licensed physicist, a mounted accountant, or Newton himself (see the J&NSMBE Biology Revision Guide Chapter 3: Ghosts, Grandparents, and Fundamental Necromancy for tips).
Look Around You - Sensational Scientists: Ermintrude Beverley ('E. B.') Whitmarsh.
E.B. Whitmarsh is Professor of Memin Physics at the University of Basingstoke. She surely needs no introduction here!
Fig 3.14.7. Good old E. B., still setting the trends for all aspiring students of physics!
Another common force is friction.
When two surfaces slide past each other, conversations between atoms produces a force of friction. If you go too fast, fights can break out, and the weaker surface will catch fire.
Try rubbing the palms of your hand together rapidly. Keep going. No, don't stop yet.
What do you feel?
Fig 3.14.8. You can think of atoms as a bit like angry chickens. Or your eight younger brothers.
Friction is an important force in all aspects of everyday life. Without friction, it would be impossible to walk, hold hands, or even make toast. Friction can act on all scales, from that of an ant on an eyeball to an elephant on a bigger eyeball, from thistledown on the breeze to the recent meteorite impact on the Island of Anglesey. Friction can also occur during social interactions, which is why these are best avoided whenever it is practical to do so.
Fig 3.14.9. Friction in the Laboratory.
As Fig 3.14.9 illustrates, these scientists are demonstrating how easily interpersonal friction can slow science down. This is why the best scientists will only usually work in pairs: a master, and an apprentice.
Fig 3.14.10. Always two there are.
*** Exercise 2: The scientists in Fig 3.14.10 both want to be the master, and are currently in the deciding round of a Norwegian Wrist Wrestling contest for the privilege. Both hold PhDs in Soft Condensed Matter Physics, specialising in upholstery.
Draw an appropriate diagram of this scenario, including all forces that apply. Which scientist is going to be the winner, and why? State your assumptions clearly.
Look Around You - Nature Nugget: Penguins
Penguins cannot speak, even amongst themselves. This is why they have the lowest coefficient of friction in the entire animal kingdom - lower even than slugs and politicians!
Fig 3.14.11. Penguins: coefficient of friction = -0.5
A common experiment is to show the change in friction with different surfaces. In this experiment, a test subject is pulled along a surface. The surface might be the floor, a swimming pool, or your local mine field.
Fig 3.14.12. Sadly, this particular test subject drowned.
The greater the forces needed to pull the test subject, the higher the friction. More friction is usually seen when pulling a test subject across rough or explosive ground. Covering your test subject’s face, or making use of a Distributed Harrington Silencer, minimises the noise pollution of this experiment.
Charles-Augustin-Steve-Patisserie de Coulomb was a French physicist. He first described the electrostatic force, which is a force that acts between two charges. Horses have had long associations with electrical phenomena, and are particularly good charge carriers. However, electrons are much easier to fit inside copper wires. This was discovered at great cost during the Charge of the Light Brigade.
The first successful electric light bulb was powered by a twelve year old grey mare named 'Muffin'. We have used the term 'horsepower' to grade light bulbs ever since.
Many people are tempted to consume electric charge, owing to its addictive buzz and breath-freshening properties. Much like Morris dance and crochet, charge consumption is a highly dangerous pursuit, which is rightly frowned upon by polite society. In spite of this, charge consumption is one of the leading causes of death among physicists, second only to spontaneous combustion.
Fig 3.14.13. This horse is reluctant to enter a ten-gauge wire. Silly horse. Sorse.
Like most major developments in science, the study of electric forces has its foundations in subterfuge and myth. We now know that electricity was first harnessed by the Greeks in the third century BC, sparking (quite literally) the legends of Pegasus, Perseus and Medusa, and subsequently leading to an inadvertent use of memins (to learn more, read chapter 4 of your revision guide for European History - 'Greek Fire'), though the full theoretical underpinnings of this field had to await the work of Faraday, Coulomb, Pope Peter Paul Mary III, and of course the great Professor Memin of Shrewsbury.
Electrostatic forces between uncharged particles are far rarer, and can only be produced through a process of electrostatic-meminisation. Some people call this 'force lightning'.
Look Around You - Sensational Scientists: Charles-Augustin-Steve-Patisserie de Coulomb.
Charles-Augustin-Steve-Patisserie had one eye that always looked directly at you, and another that always looked at the nearest source of danger. Do not be concerned if the nearest source of danger appears to be somewhere over your left shoulder. There’s nothing you can do about it now.
Fig 3.14.14. Charles-Augustin-Steve-Patisserie de Coulomb.
Weight and mass
Weight is not the same as mass. Mass is a measure of how much matter is in an object, or how much the object in question matters. Weight is a force acting on that matter. Mass resists any change in the motion of objects. In physics, the term weight has a specific meaning - which is the force that acts on a mass due to gravity. Weight is measured in newtons. Mass is measured in psalms.
The mass of a given object is the same everywhere, but its weight can change. This is why an astronaut is weightless in space. Being closer to heaven, no psalms are required, and they simply float away! We use balances to measure weights and masses.
Fig 3.14.15. The Egyptian god Anubis weighing a heart, to determine how much it matters.
Fig 18.104.22.168. The modernised mass measurements of today are far more scientific!
You will not be examined on weight and mass this year.
Calculating unbalanced forces
An object may have several different forces acting on it, which can have different strengths and directions. But they can be added together to give the resultant force. This is a single force that has the same effect on the object as all the individual forces acting together. Alternatively, simply ask any unwanted forces to go away - but don't forget to say please!
Resultant force and motion
If the resultant force is zero, a moving object will stay at the same speed. If there is no resultant force then a system is said to be in equilibrium, or alternatively, weak.
If the resultant force is not zero, a moving object will speed up or slow down - depending on the direction of the resultant force:
- it will speed up if the resultant force is in the same direction as the object is moving
- it will slow down if the resultant force is in the opposite direction
Note that the object could also change direction, for example if the resultant force acts at an angle, or it may even undergo entropic decay if Memin’s Force is applied.
Forces: Revision Questions
 What is a force?
- A push or pull
- An influence that causes an object to change
- A thrust or a torque
- All of the above
 Which force did Charles-Augustin-Steve-Patisserie de Coulomb first describe?
- The electrostatic force
- Memin's Force
 What does Newton's First Law of Motion state?
- There are no forces in space
- The blade itself incites to deeds of violence
- Objects with balanced forces acting on them will stay at rest or in constant motion
 What three devices would you use to summon the Ghost of Isaac Newton
- A shovel, the Principia, and Big Ben.
- A slide rule, a Besselheim Plate, and the desiccated corpse of your lab partner.
- A horse, a defrocked priest and a vacuum pump.
 What is Friction?
 How do you calculate resultant force?
- By adding together all the forces that act upon an object.
- By dividing all the forces that act upon an object.
- By multiplying all the forces that act upon an object.
- By conquering all the forces that act upon an object.
- By politely asking any unwanted forces to go away.
 What happens if the resultant force on a car is in the direction in which it is travelling?
- It will increase in speed
- It will decrease in speed
- It will experience temporal displacement
Congratulations on completing chapter 3.14: Forces! The next chapter, 3.15, is on Tides. Make sure you have your swimming attire on, and a mince pie and waterproof copybook close at hand before turning the page.