How We Came to Know the Cosmos: Space & Time

Discover How We Came to Know the Cosmos

Chapter 4. Force, Momentum, and Energy

4.1 Force

4.1.1 Aristotle and the five elements

The concept of force has been used since the first civilisations invented simple machines like levers and ramps. Simple machines allow less force to be used to do the same amount of work.

The ancient Greek philosopher Empedocles first suggested that there are four elements - water, earth, fire, and air - in about 475 BCE. Aristotle popularised this idea about 100 years later.[1] Aristotle also claimed that space contains a fifth element, the aether,[2] an idea first suggested by Plato.[3]

Aristotle claimed that all of these elements have a natural state and that they will tend towards this state if left alone. He believed that it’s natural for water and earth to be motionless on the ground, for example, and that unnatural motion is required to move them. He argued that this requires a force to be applied and that it must continue to be applied as long as the object is not in its natural state.[2] Aristotle suggested that in the case of objects like arrows, which continue to stay above the ground even when nothing is touching them, the force must be provided by displaced air.[4]

4.1.2 Archimedes and the law of the lever

The ancient Greek philosopher Archimedes of Syracuse studied how pulleys, screws, and levers work in about 260 BCE.[5] Levers include objects like wheelbarrows and tongs. Archimedes showed that a small weight could balance a larger one if the smaller weight were placed further from the pivot. This is why he is quoted as saying,

give me a place to stand on, and I will move the earth.[6]

We would now say that a lever will balance if the torques on both sides of the pivot are equal, where

τ = Fd = mgd (4.1)

Here, τ is the torque, F is the force, and d is the distance from the pivot. The force is due to the object’s weight, which was later shown to be a description of the force of gravity (discussed in Chapter 5). F = mg, where m is mass and g is acceleration due to gravity.

g will be the same for both sides, and so for a lever in balance

m1d1 = m2d2 (4.2)
A diagram showing how torques balance on balancing scales or a seesaw.

Figure 4.1
Image credit

A lever remains in balance if the torques on both sides are equal where, torque = mass × distance from the pivot.

4.1.3 Archimedes’ principle of buoyancy

Archimedes also discovered that if an object is immersed in a fluid, like water or air, then it will experience an upward force, known as buoyancy.[7] For an object to float, the buoyant force must be equal to, or greater than, the weight of the object. If the weight of the object is greater than the buoyant force, then the object will sink.

A diagram showing that if the force of weight is less than, or equal to, the force due to buoyancy, then an object will float.

Figure 4.2
Image credit

If the weight of an object is less than the force due to buoyancy, the object will float.

A diagram showing that if the force of weight is greater than the force due to buoyancy, then an object will sink.

Figure 4.3
Image credit

If the weight of an object is greater than the force due to buoyancy, the object will sink.

The force of buoyancy is equal to the weight of the fluid that the object displaces. This means that to increase buoyancy, an object needs to either lose weight or displace more fluid, which it can do by increasing in volume (V). If an object increases in volume, then it will also increase in surface area (A). This means it will exert less pressure (Pr) on the fluid because

Pr = F/A (4.3)

If an object increases in volume while remaining the same mass (m), then it will also decrease in density (ρ) because

ρ = m/V (4.4)

4.1.4 Hero and the first steam engine

In about 60 CE, the ancient Greek engineer Hero of Alexandria described six simple machines: the lever, the windlass (a type of winch), the pulley, the wedge, the screw, and a primitive steam engine called an aeolipile.

An aeolipile is composed of a sphere that is placed above a container of water and positioned so it can rotate on its axis. When the container of water is heated, steam rises through tubes attached to the sphere. The steam is allowed to escape through holes on the top and bottom of the sphere and this causes it to rotate. Hero used the power of steam to create automated machines, which he used to put on plays.[8]

A diagram of an aeolipile.

Figure 4.4
Image credit

An aeolipile is a simple steam engine, which was first created by Hero of Alexandria in about 60 CE.

4.1.5 Early forms of Newton’s laws

The Iranian polymath Abū Rayḥān al-Bīrūnī realised that acceleration is related to non-uniform motion in about 1021.[9] Al-Bīrūnī also noted that everything on Earth seems to be attracted to the Earth’s centre,[10] and was one of the first people to suggest that friction, the force that resists motion, can cause heat.[11] About 100 years later, the Iraqi philosopher Abu’l-Barakāt al-Baghdādī discovered that force is proportional to acceleration,[12] a precursor to Newton’s second law of motion (discussed in Chapter 5).

The Andalusian polymath Ibn Bâjjah (also known as Avempace) was the first to suggest that for every force there is a reaction force in around 1120.[13] This was a precursor to Newton’s third law of motion (also discussed in Chapter 5).

4.1.6 Leonardo da Vinci and capillary action

The Italian polymath Leonardo da Vinci designed several advanced machines, including a helicopter, an aeroplane, a tank, a parachute, and a hang glider, in the late 15th century, although not all of them were built. Da Vinci’s machines used levers, pulleys, gears, and cranks - an arm attached at right angles to a rotating shaft, like those used to manually open a car window.

Da Vinci utilised the laws of friction,[14] momentum,[15] centripetal force[16] - the force that causes rotating objects to move in a circle, and capillary action.[17] This is the ability of water to flow against gravity in narrow spaces, like when liquid is drawn up between the hairs of a paintbrush. Capillary action occurs because of surface tension and adhesive forces.

Surface tension can be seen when something denser than water floats on the surface. This extra surface pressure occurs because the atoms on the surface are pulled inwards. This is because, unlike all the other atoms in the liquid, they do not have atoms above them to balance the force of those below.

Adhesion is the tendency of dissimilar surfaces to cling to each other, like dew attached to a spider’s web. Adhesion can be caused by many things including chemical bonding, opposite charges, and mechanical bonding.

A diagram showing molecules on the surface of water experience an uneven force.

Figure 4.5
Image credit

The forces on molecules of liquid, which cause surface tension.

4.1.7 Simon Stevin’s inclined plane

The Italian mathematician Jordanus de Nemore had already shown that the ramp could be described as a simple machine in the 13th century. He did this by describing why moving an object up a slope requires less force than lifting it straight up.[18]

This idea did not become popular until the 1580s when people like Galileo Galilei and the Flemish mathematician Simon Stevin published their own proofs.[18]

Stevin imagined a loop of string with weights attached that are placed equally apart. If the string is placed over a small ramp, it will balance due to the tension in the string at the top, point T in Figure 4.6.

If you cut the string below the bottom of the triangle, at points S and V in Figure 4.6, then the string will still balance even though there is more weight on the longer slope. There must be an equal force on both sides for the string to balance, and so it takes less force per mass to push something up a longer and hence less steep slope.

A diagram showing that moving an object vertically requires less force if you use a ramp.

Figure 4.6
Image credit

Forces on a ramp by Stevin.

4.1.8 Galileo and the laws of motion

Galileo was one of the first modern scientists to state that the laws of nature are mathematical. He made observations and then tried to determine the mathematics that explained them. In 1623, he stated,

Philosophy is written in this grand book, the universe, which stands continually open to our gaze. But the book cannot be understood unless one first learns to comprehend the language and read the letters in which it is composed. It is written in the language of mathematics, and its characters are triangles, circles, and other geometric figures without which it is humanly impossible to understand a single word of it; without these, one wanders about in a dark labyrinth.[19]

Galileo’s student, the Italian mathematician Vincenzo Viviani, claimed that Galileo had dropped balls of different weights from the Leaning Tower of Pisa in 1589.[20] He is said to have done this to demonstrate that they would fall at the same rate as long as air resistance was negligible. This contradicted Aristotle’s belief that more massive objects would fall faster. Although there’s no proof that Galileo performed this experiment, Stevin may have performed the experiment from the church tower in Delft in the Netherlands in 1586.[21]

There’s evidence that Galileo did conduct experiments to prove that bodies fall at the same rate, whatever their mass, but he did this by timing how long it took balls to roll down a ramp. Galileo showed that the time it took the balls to fall was proportional to the square root of the distance they travelled, something that had first been discovered by the French mathematician Nicole Oresme in the 14th century.[22]

Δt ∝ √d  (4.5)

This equation is analogous to the equation linking the period and length of pendulums (discussed in Chapter 2), and Galileo became the first person to suggest using a pendulum to measure time in 1602.

The fact that different masses fall at the same rate was later explained by combining Newton’s second law of motion with his theory of universal gravitation. This was proven in 1971 when Apollo 15 astronaut Commander David Scott dropped a feather and hammer at the same time on the Moon (discussed in Chapter 5).

In 1632, the same year Galileo published his theory of relativity (discussed in Chapter 3), he contradicted Aristotle once again by arguing that once in motion, objects will remain in motion, travelling in the same direction at a constant speed unless they are acted on by an outside force, like friction.[23] This influenced Newton’s first law of motion, which states that objects continue to move in a state of constant velocity, which can be zero, unless acted upon by an external force.

Newton published his laws of motion and universal gravitation in 1687 (discussed in Chapter 5). In the 20th century, forces were explained in terms of Einstein’s theory of general relativity (discussed in Chapter 8) and quantum field theories (discussed in Book II).

4.2 Momentum

The Alexandrian philosopher John Philoponus criticised Aristotle’s theory of motion in the 6th century. Philoponus modified Aristotelian physics to account for the motion of arrows by stating that a hurled object acquires a temporary motive power.[24]

In the 14th century, the French philosopher Jean Buridan improved upon Philoponus’ ideas with his theory of impetus. Buridan described impetus as a force that enables an object like an arrow to continue moving in the direction it is fired. This force is opposed by air resistance and the pull of the Earth, which is why the arrow does not fly forever.

Buridan stated that impetus is equal to weight multiplied by velocity[25] - an early form of the equation for momentum (p).

p = mv (4.6)

The French natural philosopher Rene Descartes devised a similar equation in the 17th century.[26] The English mathematician John Wallis was one of the first people to suggest that momentum is conserved during collisions in 1668, along with Christiaan Huygens and the English architect and astronomer Christopher Wren.[27] This means that the total momentum of two objects will be the same before and after a collision.

Newton proved this and showed the link between force and momentum in 1687 (discussed in Chapter 5). Newton also considered angular momentum (L), which describes objects that are moving in a circle.[28]

L = pr (4.7)

The conservation of angular momentum means that the angular momentum of a spinning object will remain the same if the radius decreases. This means that either the mass or velocity must increase, and explains why ice skaters spin faster when they hold their arms close to their body, as well as Kepler’s second law (discussed in Chapter 3), which states that planets orbit at a higher velocity the closer they are to the Sun.

The conservation of momentum

The conservation of momentum shows that the total momentum of two objects will be the same before and after a collision.

If two objects are moving in the same direction,

A diagram showing the momentum of two objects before and after a collision. Momentum equals mass × velocity.

Figure 4.7 Image credit

p = 20 × 10 p = 5 × 30
p = 200 kg km/s p = 150 kg km/s p = 200 + 150
p = 350 kg km/s
p = mv,
and so
v = p/m v = 350/25
v = 14 km/s

If two objects are moving in opposite directions,

A diagram showing the total momentum after two objects collide while moving towards each other.

Figure 4.8 Image credit

p = 20 × 10 p = 5 × -30
p = 200 kg km/s p = -150 kg km/s p = 200 - 150
p = 50 kg km/s
v = 50/25
v = 2 km/s

4.3 Energy

Forces transfer energy in a process known as ‘work’, where

ΔE = W = Fd (4.8)

Here, E is energy and W is work.

Galileo suggested that simple machines might not create energy but instead transform it from one form to another in 1638.[29] German mathematician Gottfried Leibniz devised the first mathematical theory of the conservation of energy in the 1670-1680s. Leibniz showed that a force known as vis viva, which is Latin for living force, is conserved during collisions.[29]

In 1740, the French mathematician Émilie du Châtelet combined the theories of Leibniz and the Dutch natural philosopher Willem ’s Gravesande, to show that the energy of a moving object is proportional to the square of its velocity.[30] Finally, in 1829 the French mathematician Gustave-Gaspard Coriolis showed that[31]

KE = 1/2mv2 (4.9)

Here, KE is kinetic energy, the energy of a moving object. The equation for kinetic energy can be derived from Isaac Newton’s second law of motion, which was published in 1687 (discussed in Chapter 5).

The German natural philosophers Julius Robert von Mayer and Hermann von Helmholtz and the British natural philosopher James Prescott Joule formed the law of conservation of energy in the 1840s, although their theory was stated in terms of vis viva rather than energy.[29]

The term ‘energy’ was first used by Thomas Young in 1807 in reference to kinetic energy. The term was popularised in the 1850s by the British natural philosopher William Thomson (also known as Lord Kelvin) and the British engineer William Rankine..[32,33] Rankine coined the term ‘potential energy’ in 1853.[32] Gravitational potential energy (GPE) refers to the energy that’s needed to keep an object from falling from a height (h), where

GPE = mgh (4.10)

Here, (g) is acceleration due to gravity (first discussed in Chapter 2).

An object like a pendulum converts gravitational potential energy into kinetic energy as it swings back and forth. Potential energy and the law of pendulums were later explained by Newton’s theory of gravitation, which was published in 1687 (discussed in Chapter 5).

In the 20th century, energy was explained in terms of Einstein’s theory of special relativity (discussed in Chapter 7) and quantum mechanics (discussed in Book II).

4.4 References

Back to top