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Balanced Forces The famous English scientist, SIR ISAAC NEWTON
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who lived between the 17th and 18th century, devised THREE LAWS OF MOTION. The first law
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states that if all the FORCES acting on an object are BALANCED there will be NO ACCELERATION
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meaning either CONSTANT VELOCITY or the object remaining STATIONARY. But what do we mean
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by balanced forces? Let's look at the example of a horse. The
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first force we'll consider is the force of GRAVITY. This acts towards the centre of
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the Earth. The size of the force of gravity is the horse's WEIGHT. An arrow is drawn
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downwards from the horse towards the centre of the Earth. If this were the only force acting
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on the horse it would sink into the ground. As this is not the case there must be a force
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opposing the force of gravity. An arrow is drawn in the opposite direction but equal
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in size. This force is called the REACTION FORCE. As the reaction force is the same size
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but acts in the opposite direction to the horse's weight, these two forces are a BALANCED
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PAIR OF FORCES. There are other forces acting on the horse.
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The horse is moving forward, and therefore, there must be a DRIVING FORCE from the horse's
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legs. The driving force, though, is opposed by forces trying to slow the horse down. And
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if the horse is travelling at a CONSTANT VELOCITY, the force of FRICTION and AIR RESISTANCE are
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equal in size but opposite in direction to the driving force. The driving force is now
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balanced by friction and air resistance, a BALANCED PAIR OF FORCES. And the horse now
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travels at a CONSTANT VELOCITY. If, though, the horse ACCELERATES, the driving
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force must be greater than friction and air resistance. The driving force is now no longer
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balanced by friction and air resistance. These forces are now UNBALANCED.
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So in summary, when the forces are BALANCED on an object, the object will either remain
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STATIONARY or travel at a CONSTANT VELOCITY. When the forces are unbalanced, the object
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will either ACCELERATE meaning an increasing velocity or DECELERATE meaning a decreasing
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velocity.
