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chap 14: interactions through the applications of forces - Coggle Diagram
chap 14: interactions through the applications of forces
forces
contact forces
whenever an object is touched, contact forces act on it. for example, the tyres of a car rest on the road and exert a contact force on the road
the contact forces that acts on two materials rubbing against each other is called friction
when we walk, the rough soles of our shoes rub against the ground. we depend on friction between our feet and the ground to push our body forward.
non-contact forces
a force that acts on an object without touching it is called a non-contact force. gravitational force, electrostatic force and magnetic force are three examples of non-contact forces.
gravitational force is the force that attracts objects to planets. this force affects the earth, the moon, and the planets in the solar system.
we see the effect of gravitational force in the movement oh high and low tides in harbours, ports, and coastal areas. the tides on earth are caused partly by the moon's gravity pulling on the waters of the oceans. the force of gravity pulls paperweights down on loose sheets of paper, preventing the paper from being blown away by wind.
we can feel electrostatic force when we use a transparent sheet of plastic to wrap a book. the hair on our arms and the dust on the table are attracted to the plastic wrap because of electrostatic forces. an
electric charge
can be positive or negative.
magnetic force, or magnetism, is the force exerted by a magnet on magnetic materials such as iron and steel.
every magnet has a north pole and a south pole.
unlike poles attract each other. the force acting on the poles is called the magnetic force of attraction. like poles repel each other. the force acting on the poles is called the magnetic force of repulsion.
a force is either a 'push' or a 'pull'. we interact with forces whenever we grip the poles or hold hand straps in a bus.
we usually apply forces on an object by touching it with our body, or by touching it with another object. however, forces can also act when objects do not touch.
there are two types of forces: contact forces and non-contact forces
measuring force
in everyday language, the weight of a rock-climber is measured in kilograms. however, weight is in fact the force that the earth exerts on objects. therefore, weight should be measured as a force. the SI unit of force is
newton
(N)
weight:
newton, N
the spring balance is an instrument that is used to measure force. there are two types of spring balance: the extension spring balance and the compression spring balance
the spring in an extension spring balance stretches when a pulling force acts on it.
the spring in a compression spring balance is compressed when a pushing force acts on it
in both types of spring balance, a pointer attached to the spring indicates the amount of force acting on the spring.
mass:
kilogram, kg
weighting scale
the force of gravity is different on different massive objects :
the weight of a person on earth is greater than his weight on the moon. this is due to stronger gravity on earth.
mass on earth=60kg, weight on earth=600N
mass on moon=60kg, weight on moon=100N
effects of forces
although we cannot see forces, we can see their effects on objects. the large forces acting during a crash test change the shape of a car and stop its motion. forces also make the car begin to move from rest or move faster
effects on the size/shape
when we apply force on an object, its physical dimensions may change
when we apply a force to knead a lump of plasticine, the shape of object changes.
when we mould a lump of wet clay, the size and shape of the object changes.
effects of forces on the motion of an object.
during a hockey game, each player uses his/her stick to apply forces on the ball. these forces affect the motion of the ball
the ball is stationary. by striking the ball with her stick, player A applies a force on the stationary ball to make it move.
player B touches the ball with her stick. the force changes the direction of the ball and moves it towards player C.
player C now hits the ball with her stick.the force changes the direction of the ball again and moves is to player D.
player D hits the ball with her stick with such force that it moves quickly towards the goal. she hopes that the ball moves quickly for the goalkeeper to stop it.
player E, the goalkeeper, puts her foot out and exerts a force on the ball to stop it.
speed
in the example of a hockey game, we learnt that forces can cause an object to move faster or move slower. in short, forces can change the speed of a moving object
speed= distance travelled/time taken
speed is the distance travelled per unit time.
turning effects of forces on an object
levers are tools that helps us do things more easily. by applying a force to one point of a lever, we overcome the turning effect on another force
a person applies a force on the handle of a crowbar. the crowbar turns and pulls out the nail from the wooden plank
a person applies a large force on the fishing rod handle to lift the fishing rod. the fish that is caught at the other end of the rod moves over a large distance.
a person applies an upward force on the handles of a wheelbarrow. the heavy load becomes easier to move.
pressure
pressure ia an effect of a force acting on an object. the amount of pressure exerted depends on the amount of force and the area on which the force acts.
if we measure the amount of force applied on a surface an the area of the surface, we can calculate the pressure.
pressure is the amount of force acting perpendicularly on a unit area.
pressure= force/area
pressure and force
when dennis uses a plastic bag to carry some durians home, his fingers hurt.
the weight of the durians exerts a large pressure on his fingers
dennis could reduce the pain in his fingers by carrying fewer durians.
the weight of the fewer durians causes a smaller force to b exerted on his hand. the pressure exerted on his fingers are smaller.
pressure and area
although dennis's fingers hurt, he does not want to carry fewer durians
the handles of the plastic bag that dennis uses are narrow and thin. the weight of the durians acts on a small area on his hand. the pressure exerted on his fingers is large
dennis can reduce the pain in his fingers by padding the handles with a thick cloth
the handles now rest on a larger area. the pressure exerted on his fingers becomes smaller
pressure in everyday life
the effects of pressure can be advantageous or disadvantageous to us in our everyday life
we use our fingers to apply force on the head of a pin. a large pressure is created at the sharp point. this allows the pin to pierce paper and wooden boards. the sharp tip of a pin exerts a large pressure on notice boards.
the cutting edges on knives and scissors have very small areas. a small force produces a large pressure on the cutting edges. this allows the edges to cut through objects easily. the sharp edge of a knife blade exerts a large pressure on food.
the spikes on the soles of football shoes have small areas. the large pressure produced by the spikes increases the shoes' grip on the ground
when a car tyre rolls over the sharp pointed end of a nail, the nail exerts a large pressure on the tyre as the tip of the nail has a very small area. this may puncture the tyre.
when we hammer a nail, we may accidentally hammer on our finger. this causes a large force to act on a small area of our finger. the large pressure on our finger bursts the blood vessels and produces a bruise.
stiletto heels often leave unsightly marks on carpets. this is because the weight of the wearer exerts a large pressure on a small area of ground
atmospheric pressure
the air around us exerts pressure on all objects that are exposed to air. the pressure exerted by the air in the earth's atmosphere is called atmospheric pressure.
when we drink liquid using a drinking straw, we make use of atmospheric pressure to push water up the tube
by sucking through the straw, we lower the air pressure in the straw
the higher atmospheric pressure outside the straw pushes the liquid up the drinking straw
pressure in liquids
when an object is immersed in a liquid, the liquid exerts pressure on it. the deeper the object is immersed, the greater the liquid pressure
submarines dive to great depths underwater. the rigid metal body of a submarine is built to withstand the very high pressure deep underwater.