Category Forces of nature

How do we measure force and work?

Measuring forces

You can use a device called a force meter to measure the size of a force. Most force meters have a hook that you can use to hang or pull on something. This will cause a spring to move and show you how much force is being applied.

We measure forces using a unit called Newtons. They get this name from one of the most famous scientists of all time – Isaac Newton. He was the first person to describe the force that we know as gravity.

A Newton can be described in another way, measured in kg*(m/sec2). You would read this as ‘kilograms times meters divided by seconds squared.’ This is because:

Force = Mass * Acceleration

Newton described this in his laws about physics which tell us that motion is created by unbalanced forces. He realized that objects that are not moving will remain still, and objects in motion will stay in motion unless a force interferes.

Force, mass, and acceleration are interrelated. If we know any two out of the three, we can find the third.

  • Acceleration = Force / Mass
  • Mass = Force / Acceleration

Example: Imagine two vehicles driving along the road with the same acceleration. One is a lorry and the other is a small car. The lorry has the larger force because it has a greater mass than the car. Now imagine two identical cars with the same mass. They move slowly and then gradually speed up. But one of the cars gets faster more quickly than the other one and overtakes it. We say that this car has greater acceleration. The car with the higher acceleration has the greater force.


Measuring work

In physics, work is defined as a force causing the movement—or displacement—of an object. In the case of a constant force, work is the scalar product of the force acting on an object and the displacement caused by that force. Though both force and displacement are vector quantities, work has no direction due to the nature of a scalar product (or dot product) in vector mathematics . This definition is consistent with the proper definition because a constant force integrates to merely the product of the force and distance.

To raise an object you have to do work to it. The work you have to do depend on the force on the object you are lifting and the distance you are going to raise it. Work is measured in units called JOULES.

Work = force x distance

A box exerts a force of 50 newtons on the ground. You want to lift it onto a table 1 metre high. The amount of work you have to do is 50 joules.

50 newtons (force) x 1 metre (distance) = 50 joules (work)

What are pulleys and gears used for?

Pulleys are machines that we use to lift heavy objects. They are made from a number of wheels and a long piece of rope or cable. The cable is wound around each of the wheels in turn, and the whole system is attached to a weight.

By pulling the cable, the weight can be raised easily. The more wheels in a pulley system, the easier the lifting becomes. When three pulleys are used, the weight is shared between three stretches of cable and the force you need is only a third of what you would need to lift the weight by yourself. If four were used, the force you would need would be reduced to a quarter.


The wheel of a fixed pulley system is attached to a solid structure such as a wall or a floor, while the rope is free. This means the pulley itself is stationary. A fixed pulley offers no mechanical advantage but does allow a person to redirect the force. So rather than directly lifting a heavy object up, a person can use a pulley to instead lift the object by pushing down on the rope.


The wheel of a moving pulley is not attached to any particular surface; instead, the rope of the pulley is attached to a stationary surface. Unlike a fixed pulley, a moveable pulley does offer a mechanical advantage. A heavy load is attached to the wheel rather than the rope, and as the rope is pulled the wheel slides up the rope, bringing the load with it. This requires less work than lifting the load directly would require.


A compound pulley consists of both a fixed pulley and a moveable pulley. This combines the benefits of both a fixed and a moveable pulley. In a compound pulley the weight is attached to the wheel of a moveable pulley, which itself is strung to a rope attached to a fixed pulley. With a compound pulley you can redirect the required direction of the force as well as the total workload for the force.

Block and Tackle

A block and tackle is a specialized form of compound pulley that can dramatically lessen the required amount of work to move a heavy object. A block-and-tackle pulley system consists of several fixed and moveable pulleys arranged parallel with one another; fixed pulleys aligned with fixed and moveable pulleys with moveable. Each compound pair is attached to the next pair, and each set reduces the total work required. This pulley system is popularly attributed to Archimedes, the famous ancient inventor and mathematician.


The cone pulley is another specialized pulley system that incorporates the basic mechanics of a pulley system while allowing for mechanical adjustments. A cone pulley is essentially multiple pulley wheels of decreasing circumferences stacked on top of one another, forming a cone shape. This cone shape allows the pulley operator to shift the speed of the pulley’s movements, with a smaller circumference requiring less work but also producing less work. Multi-gear bicycles essentially operate on this same system; the bicyclist can easily shift between smaller gears that move the bike less, and higher gears that require more effort but move the bike a greater distance per revolution.


Like pulleys, gears make work easier. Gears are objects which are used to move force from one place to another. The most common gear is the cog — a wheel with teeth. In cars and bicycles, gears are used to help turn the wheels. On a bicycle, a chain moves around two cogs — a large cog attached to the pedals and a smaller cog attached to the back wheel.

As the pedals turn, the large cog rotates, the chain turns and the smaller cog makes the back wheel rotate quickly. If the large cog has twice as many teeth as the small cog, the back wheel will turn twice as quickly when you pedal. The smaller the cog at the rear, the faster you will travel (a high gear). The larger the cog at the rear, the lower the gear.

Picture Credit : Google



‘Weight’ is the force exerted by gravity on a body. To lift something up, you must exert a greater upward force to overcome the downward force. The amount of ‘work’ you have to do to achieve this depends on the weight of the object and the distance you have to move it. Some things are too heavy for you to lift alone, and you need help — another person or a machine perhaps. Machines make our lives easier by doing work or helping us to do work.

A lever is a simple machine which can help you lift things, like the lid of a tin. Ramps can also overcome force — it is easier to roll an object than to lift it.

The way levers work is by multiplying the effort exerted by the user. Specifically, to lift and balance an object, the effort force the user applies multiplied by its distance to the fulcrum must equal the load force multiplied by its distance to the fulcrum. Consequently, the greater the distance between the effort force and the fulcrum, the heavier a load can be lifted with the same effort force.

A wedge and an inclined plane are similar. An inclined plane is also known as a ramp. A ramp is a flat surface with one end higher than the other. Gravity makes it easier to move a heavy load up and down an inclined plane than to move that same load straight up or down without the help of a simple machine. A wedge is two inclined planes placed back to back and put into action.

What are some examples of centripetal force?

Any net force causing uniform circular motion is called a centripetal force. The direction of a centripetal force is toward the center of curvature, the same as the direction of centripetal acceleration. 

It is important to understand that the centripetal force is not a fundamental force, but just a label given to the net force which causes an object to move in a circular path. The tension force in the string of a swinging tethered ball and the gravitational force keeping a satellite in orbit are examples of centripetal forces. Multiple individual forces can even be involved as long as they add up to give a net force towards the center of the circular path.

A moving object always travels in a straight line unless a force acts upon it. When a weight is spun round quickly on a string, it moves in a circle. This means that a force must be making the weight change its direction all the time. As the object spins you can feel the string pulling on your fingers. The string also pulls on the weight. It is this pull that makes the weight change its direction — a ‘centripetal’ force.

When you sit in a ride at a funfair, or in a car moving fast around a roundabout, you will also feel the effects of centripetal force. As the car turns, it pulls you with it, exerting centripetal force on you as it does so.


Gravity is a force of attraction that exists between any two masses, any two bodies, and any two particles. Gravity is not just the attraction between objects and the Earth. It is an attraction that exists between all objects, everywhere in the universe. Sir Isaac Newton (1642 — 1727) discovered that a force is required to change the speed or direction of movement of an object. He also realized that the force called “gravity” must make an apple fall from a tree, or humans and animals live on the surface of our spinning planet without being flung off. Furthermore, he deduced that gravity forces exist between all objects.

Newton’s “law” of gravity is a mathematical description of the way bodies are observed to attract one another, based on many scientific experiments and observations. The gravitational equation says that the force of gravity is proportional to the product of the two masses and inversely proportional to the square of the distance between their centers of mass. 

The effect of gravity extends from each object out into space in all directions, and for an infinite distance. However, the strength of the gravitational force reduces quickly with distance. Humans are never aware of the Sun’s gravity pulling them, because the pull is so small at the distance between the Earth and Sun. Yet, it is the Sun’s gravity that keeps the Earth in its orbit! Neither are we aware of the pull of lunar gravity on our bodies, but the Moon’s gravity is responsible for the ocean tides on Earth.

If you jump in the air you will soon fall back down to the ground again. Snowboarders and skiers can jump high, but only for a moment. Sky-divers will also fall towards the Earth at a great speed. This is because the Earth has its own pulling forces called ‘gravity’. The pull of gravity gradually becomes weaker as you move further away from the Earth’s surface.

Like Earth, the Moon, the stars and other planets also have a gravitational pull of their own. Jupiter is much larger than the Earth so it has a stronger gravitational pull. The Moon is smaller than the Earth, so its force of gravity is weaker than the Earth’s.

What is Elastic Force?

We have seen that forces can change the shape of things. But sometimes the changes are not always permanent. A rubber band will stretch, but as soon as you let go it will return to its original shape. As it does so, it exerts a force — called an ‘elastic force’.  Metals are harder to stretch but they can exert a greater elastic force. Metals are often coiled to make springs —to be used for machinery parts or trampolines, or to make seats and mattresses more comfortable for example. Elastic forces are also used to absorb a large force — like breaking the fall of a bungee jumper.

When a rubber ball is dropped onto the ground, it is squashed. The ground exerts a force which pushes the ball upwards and back into the air. The ball then returns to its original shape.

As you stretch or compress an elastic material like a bungee cord, it resists the change in shape. It exerts a counter force in the opposite direction. This force is called elastic force. The farther the material is stretched or compressed, the greater the elastic force becomes. As soon as the stretching or compressing force is released, elastic force causes the material to spring back to its original shape.

After the bungee jumper jumps, he accelerates toward the ground due to gravity. His weight stretches the bungee cord. As the bungee cord stretches, it exerts elastic force upward against the jumper, which slows his descent and brings him to a momentary stop. Then the bungee cord springs back to its original shape and the jumper bounces upward.

Bedsprings provide springy support beneath a mattress. The spring in a door closer pulls the door shut. The spring in a retractable ballpoint pen retracts the point of the pen. The spring in a pogo stick bounces the rider up off the ground.

Reducing Friction

Friction makes it difficult to rub two dry, rough surfaces together. The thin blades of ice skates move easily on ice because there is little friction between the two smooth surfaces. Sometimes, friction can be used to make a surface smoother. For example, sandpaper is rubbed over wood to wear away the rough edges.

Friction can also wear away moving parts in a machine, eventually ruining them. To prevent this, a lubricant, such as oil, is used. Oiled door hinges will move against each other easily and there will be little wear. Some machines, like aeroplanes and cars, are also designed to reduce friction between the body and surrounding air particles.

If you magnify two surfaces which look smooth you can see that they are actually quite rough. As you rub the surfaces together they scrape against each other. Friction slows down their movement and wears them away.

Putting oil between surfaces (like the parts in a car engine) helps to make them smooth and wet. The materials can now be moved quickly and easily against each other, which reduces friction, and prevents the surfaces from being worn away.

Sports cars have a streamlined design to reduce friction between the moving vehicle and air particles.

What is friction?

When two objects rub against each other they cause ‘friction’. Friction is vitally important in our lives. Friction between our shoes and the ground stops us from slipping over when we walk. Friction between tyres and the road allows cars and Lorries to move forwards and prevents them from skidding. Friction also causes heat — you can start a fire by rubbing two sticks together.

Friction also slows things down. A ball rolling along the ground will gradually get slower until it stops, because of friction between the ball and the ground. Car and bicycle brakes also use friction to slow a moving vehicle down.

Friction is the resistance to motion of one object moving relative to another. It is not a fundamental force, like gravity or electromagnetism. Instead, scientists believe it is the result of the electromagnetic attraction between charged particles in two touching surfaces.

Scientists began piecing together the laws governing friction in the 1400s, but because the interactions are so complex, characterizing the force of friction in different situations typically requires experiments and can’t be derived from equations or laws alone.

For every general rule about friction, there are just as many exceptions. For instance, while two rough surfaces (such as sandpaper) rubbing against each other sometimes have more friction, very smoothly polished materials (such as plates of glass) that have been carefully cleaned of all surface particles may actually stick to each other very strongly. 

There are two main types of friction, static friction and kinetic friction. Static friction operates between two surfaces that aren’t moving relative to each other, while kinetic friction acts between objects in motion. In liquids, friction is the resistance between moving layers of a fluid, which is also known as viscosity. In general, more viscous fluids are thicker, so honey has more fluid friction than water.

The atoms inside a solid material can experience friction as well. For instance, if a solid block of metal gets compressed, all the atoms inside the material move, creating internal friction. In nature, there are no completely frictionless environments: even in deep space, tiny particles of matter may interact, causing friction.

Forces at work

A force can change the shape of an object and the greater the force, the greater the change it brings about. Although you can dent a wall with a hammer, a metal ball could knock a whole building down.

When you lift yourself up onto a wall you push down on the top of the wall with your hands. As you push, you exert a force which acts downwards on the wall, and the wall pushes against your hands, lifting you upwards. This force is equal to the force exerted by your hands, but it acts in the opposite direction. Gymnasts use a similar action to perform. Opposite forces in action can also be seen when a rocket takes off.

The study of rockets is an excellent way for students to learn the basics of forces and the response of an object to external forces. The motion of an object in response to an external force was first accurately described over 300 years ago by Sir Isaac Newton, using his three laws of motion. Engineers still use Newton’s laws to design and predict the flight of full scale rockets.

Forces are vector quantities having both a magnitude and a direction. When describing the action of forces, one must account for both the magnitude and the direction. In flight, a rocket is subjected to four forces; weight, thrust and the aerodynamic forces, lift and drag. The magnitude of the weight depends on the mass of all of the parts of the rocket. The weight force is always directed towards the center of the earth and acts through the center of gravity, the yellow dot on the figure. The magnitude of the thrust depends on the mass flow rate through the engine and the velocity and pressure at the exit of the nozzle. The thrust force normally acts along the longitudinal axis of the rocket and therefore acts through the center of gravity. Some full scale rockets can move, or gimbal, their nozzles to produce a force which is not aligned with the center of gravity. The resulting torque about the center of gravity can be used to maneuver the rocket. The magnitude of the aerodynamic forces depends on the shape, size, and velocity of the rocket and on properties of the atmosphere. The aerodynamic forces act through the center of pressure, the black and yellow dot on the figure. Aerodynamic forces are very important for model rockets, but may not be as important for full scale rockets, depending on the mission of the rocket. Full scale boosters usually spend only a short amount of time in the atmosphere.

In flight the magnitude, and sometimes the direction, of the four forces is constantly changing. The response of the rocket depends on the relative magnitude and direction of the forces, much like the motion of the rope in a “tug-of-war” contest. If we add up the forces, being careful to account for the direction, we obtain a net external force on the rocket. The resulting motion of the rocket is described by Newton’s laws of motion.

What can forces do?

Forces change the way things move. The force of the wind will alter the direction of a hot air balloon. A moving ball, with no forces acting on it, will continue moving in the same direction and at the same speed until a force acts upon it.  A force can also change the shape of an object. A giant crusher can change the shape of a car — even your hand can exert a force to shape and mould certain objects. Whenever we find that the speed or direction of a moving object is changing, or the shape is changing, we say that forces are acting to cause these changes.

Forces change motion and shape. The force of a foot kicking a ball speeds the ball up. The force of a parachute on a skydiver slows the skydiver down. The force of a string on a whirling ball constantly changes the direction of motion, keeping it moving in a circle. Combinations of forces applied to materials can stretch, twist, and crush them.

            Illustrates the force exerted on a steel ball by the flick of a finger. We can see that the force sets the ball moving, and when the force stops, the ball continues in a straight line at a constant speed, unless another force acts on it. When a magnet is held near the moving ball, it exerts a pulling force on it – changing the direction of the ball. This is because magnets attract steel.