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.



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.



Picture Credit : Google



 


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.




















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.


















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.