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Category Forces in action

IS FRICTION A USEFUL FORCE?

Friction is a force that slows the motion of two surfaces when they move across each other. An engine has many moving parts. If they rub against each other, creating friction, the efficiency of the engine is affected. The friction creates heat and the engine needs more energy to work. The parts also wear down as they come into contact. To reduce the friction in an engine, a lubricant, such as oil, is put between the moving parts.

There are also times when friction is useful. For example, if there were no friction between our feet and the ground, we would fall over. This can happen when a floor is polished or there is ice on the ground. There is less friction between our feet and these surfaces, so we easily slip.

The grooves on a tyre help to push water on the surface of the road out of the way. This means that there is more friction between the tyre and the road, preventing the car from skidding.

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WHAT IS INERTIA?

Inertia is the resistance of any physical object to any change in its velocity. This includes changes to the object’s speed, or direction of motion. An aspect of this property is the tendency of objects to keep moving in a straight line at a constant speed, when no forces act upon them.

Inertia comes from the Latin word, iners, meaning idle, sluggish. Inertia is one of the primary manifestations of mass, which is a quantitative property of physical systems. Isaac Newton defined inertia as his first law in his Philosophiae Naturalis Principia Mathematica, which states:

The vis insita, or innate force of matter, is a power of resisting by which every body, as much as in it lies, endeavours to preserve its present state, whether it be of rest or of moving uniformly forward in a straight line.

In common usage, the term “inertia” may refer to an object’s “amount of resistance to change in velocity” or for simpler terms, “resistance to a change in motion” (which is quantified by its mass), or sometimes to its momentum, depending on the context. The term “inertia” is more properly understood as shorthand for “the principle of inertia” as described by Newton in his first law of motion: an object not subject to any net external force moves at a constant velocity. Thus, an object will continue moving at its current velocity until some force causes its speed or direction to change.

On the surface of the Earth, inertia is often masked by gravity and the effects of friction and air resistance, both of which tend to decrease the speed of moving objects (commonly to the point of rest). This misled the philosopher Aristotle to believe that objects would move only as long as force was applied to them.

The principle of inertia is one of the fundamental principles in classical physics that are still used today to describe the motion of objects and how they are affected by the applied forces on them.

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WHAT IS A FORCE?

There are forces acting on us — and everything on our planet — all the time. The push and pull of forces is what keeps things where they are or starts them into motion. Forces enable something to stay the same size and shape or to change size and shape. They can slow down a moving object or speed it up, or change the direction of its motion. Whenever energy is being used, forces are at work.

In physics, a force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity. It is measured in the SI of newtons and represented by the symbol F.

The original form of Newton’s second law states that the net force acting upon an object is equal to the rate at which its momentum changes with time. If the mass of the object is constant, this law implies that the acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object.

Concepts related to force include: thrust, which increases the velocity of an object; drag, which decreases the velocity of an object; and torque, which produces changes in rotational speed of an object. In an extended body, each part usually applies forces on the adjacent parts; the distribution of such forces through the body is the internal mechanical stress. Such internal mechanical stresses cause no acceleration of that body as the forces balance one another. Pressure, the distribution of many small forces applied over an area of a body, is a simple type of stress that if unbalanced can cause the body to accelerate. Stress usually causes deformation of solid materials, or flow in Fluids.

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ARE THERE DIFFERENT KINDS OF FORCE?

There are many different kinds of force. They are affecting everyday objects around us all the time. Here are just some of the many forces that we experience.

An applied force is a force that is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is applied force acting upon the object. The applied force is the force exerted on the desk by the person.

The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity that is directed “downward” towards the center of the earth. The force of gravity on earth is always equal to the weight of the object as found by the equation:

The normal force is the support force exerted upon an object that is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. On occasions, a normal force is exerted horizontally between two objects that are in contact with each other. For instance, if a person leans against a wall, the wall pushes horizontally on the person.

The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. There are at least two types of friction force – sliding and static friction. Though it is not always the case, the friction force often opposes the motion of an object. For example, if a book slides across the surface of a desk, then the desk exerts a friction force in the opposite direction of its motion. Friction results from the two surfaces being pressed together closely, causing intermolecular attractive forces between molecules of different surfaces. As such, friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together.

The air resistance is a special type of frictional force that acts upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object. This force will frequently be neglected due to its negligible magnitude (and due to the fact that it is mathematically difficult to predict its value). It is most noticeable for objects that travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas.

The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire.

The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it. An object that compresses or stretches a spring is always acted upon by a force that restores the object to its rest or equilibrium position. For most springs (specifically, for those that are said to obey “Hooke’s Law”), the magnitude of the force is directly proportional to the amount of stretch or compression of the spring.

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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)

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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.

Fixed

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.

Moving

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.

Compound

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.

Cone

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.

Gears

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.

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OVERCOMING FORCES

‘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.

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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.

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WHAT IS GRAVITY?

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.

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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.

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