Pariksha SaathiLesson 3 of 8 · Science and its Pedagogy (Paper II)
Moving Things, People and Ideas: Measurement, Motion, Force, Pressure and Friction
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Measuring distance
Measurement compares an unknown quantity with a known fixed quantity called a unit. The handspan, cubit, footstep and fathom differ from person to person, so standard units are needed; the International System of Units (SI), adopted in 1960, uses the metre for length, with 1 m = 100 cm, 1 cm = 10 mm and 1 km = 1000 m. To measure correctly, place the scale along the length, keep the eye exactly above the point being read, and if the zero mark is broken start from a full mark such as 1 cm and subtract it at the end.
Types of motion
An object is in motion when its position changes with time. Motion along a straight line, as of a marching soldier or a falling stone, is rectilinear; motion along a circle, as of a point on a fan blade or a stone whirled on a thread, is circular; motion that repeats after a fixed interval, as of a pendulum, a swing or a sewing machine needle, is periodic. A body can show more than one kind at once: a bicycle wheel spins on its axle while moving forward.
Speed and time
Speed is the distance covered in unit time, speed = distance / time, measured in m/s or km/h; a bus covering 120 km in 2 hours has an average speed of 60 km/h. Motion along a straight line at constant speed is uniform; with changing speed it is non-uniform. A simple pendulum, a metal bob hung by a thread, has a time period (the time for one oscillation) that stays the same for a given length at a place, which is why pendulums were used in clocks; Galileo noticed this from a swinging lamp in a church. The second is the basic unit of time. A speedometer shows speed, an odometer records distance, and the distance-time graph of uniform motion is a straight line.
Force as a push or pull
A force is a push or a pull that arises from the interaction of at least two objects. It has magnitude and direction: forces in the same direction add up, opposite forces subtract, and balanced forces give a net force of zero. An unbalanced force changes an object's speed or its direction of motion, and a force can also change its shape. Contact forces need touch, such as muscular force and friction; non-contact forces act from a distance, such as magnetic force, electrostatic force (a comb run through dry hair attracts bits of paper) and gravitational force, by which the Earth pulls everything towards it. The SI unit of force is the newton.
Pressure
Pressure is the force acting on unit area, pressure = force / area, so the same force exerts more pressure on a smaller area: knives are sharpened and nails pointed, while porters place a round cloth on their heads and school bags have broad straps to spread the load. Liquids exert pressure on the bottom and walls of their container, and the pressure increases with depth, so water comes out with more force from the lowest hole of a bottle and dams are built wider at the base. The air exerts atmospheric pressure; a rubber sucker sticks to a smooth wall because the air presses it there, and our bodies are not crushed because the pressure inside them balances the air outside. The SI unit of pressure is the pascal (1 Pa = 1 N/m^2).
Friction
Friction opposes relative motion between two surfaces in contact. It arises from the interlocking of tiny irregularities, so it depends on the nature of the surfaces and on how hard they are pressed together. Static friction is greater than sliding friction, and rolling friction is the smallest, which is why wheels and ball bearings make moving loads easier. Friction is a necessary evil: without it we could not walk, write, light a matchstick or stop a vehicle, yet it wears out shoe soles, tyres and machine parts and wastes energy as heat. It is increased by treads on tyres, spikes on athletes' shoes and the coarse powder gymnasts rub on their hands, and reduced by lubricants (oil, grease, graphite), ball bearings, polishing and air cushions. Air and water exert fluid friction, or drag, so birds, fish, boats and aircraft have streamlined shapes.
Common confusions and classroom ideas
Many children believe that a moving object needs a continuous push to keep moving, or that a heavier object falls faster; both ideas go back to Aristotle and were challenged by Galileo. Rolling the same ball over a carpet, a cloth and a smooth floor, and seeing it travel farthest on the smoothest surface, shows that friction, not the absence of a push, stops it. Another is thinking that a book lying on a table has no force acting on it, when balanced forces act on it. Making a pendulum from a stone and a thread to measure its time period for different lengths, and plotting a distance-time graph for a toy car, turn this unit into hands-on inquiry.
Key facts
- 1 m = 100 cm; 1 cm = 10 mm; 1 km = 1000 m; the SI unit of length is the metre; body measures such as the handspan, cubit and footstep are not standard.
- Rectilinear: marching soldier, falling stone; circular: a point on a fan blade; periodic: pendulum, swing, sewing machine needle; a bicycle wheel shows rotation and rectilinear motion together.
- Speed = distance / time; 120 km in 2 hours = 60 km/h; uniform motion gives a straight-line distance-time graph; the second is the basic unit of time.
- Time period of a simple pendulum = time for one oscillation; it is constant for a given length at a place; Galileo noticed it from a swinging lamp in a church.
- Force is a push or pull arising from the interaction of two objects; it can change speed, direction of motion or shape; SI unit newton.
- Contact forces: muscular force, friction. Non-contact forces: magnetic, electrostatic, gravitational.
- Pressure = force / area; sharp knife and pointed nail (small area, more pressure); broad bag straps and a porter's cloth roll (large area, less pressure); SI unit pascal.
- Liquid pressure increases with depth, so dams are wider at the base; atmospheric pressure holds a rubber sucker to a wall.
- Friction: static > sliding > rolling; increased by tyre treads and shoe spikes; reduced by lubricants, ball bearings and polishing; streamlined shapes reduce drag.
Quick revision
1 m = 100 cm, 1 km = 1000 m; rectilinear, circular and periodic motion; speed = distance / time in m/s or km/h; pendulum time period constant for a given length; force is a push or pull that can change speed, direction or shape; contact (muscular, friction) and non-contact (magnetic, electrostatic, gravitational) forces; pressure = force / area; liquid pressure rises with depth; static > sliding > rolling friction; lubricants and ball bearings reduce friction.