Lesson 11: Force and Pressure

Rule 1: Effects of force — We apply force to push, pull, stop, throw, pick and press objects. A force is needed to move a stationary object and also to stop a moving object.

• Easy : Pressing a switch: force.
• Medium : Stopping a rolling ball with the foot: force stops a moving object.
• Exam-type : Lifting a school bag from the floor needs force to move a stationary object.

Rule 2: Muscular and mechanical force — Muscular force is applied with the help of bones and muscles, as in work, games and agriculture; animals also use it. Mechanical force is applied by a machine, though some machines still need muscular force.

• Easy : A bullock pulling a cart: muscular force.
• Medium : A crane lifting a load: mechanical force.
• Exam-type : A cycle needs muscular force on the pedals, so both forces can work together.

Spot the difference

Rule 1: Gravitational force — The earth attracts all objects. A ball thrown upward slows down and comes back. Gravity acts even on a stationary object on the ground.

• Easy : A dropped pen falls down.
• Medium : A stone thrown up slows down and then falls.
• Exam-type : A book on the table is pulled down by gravity but the table pushes it up, so it stays at rest.

Rule 2: Scientists and magnetic force — About 950 years ago Bhaskaracharya (II) wrote in Siddhant Shiromani that the earth pulls objects towards itself, though he did not use the word gravity. Sir Isaac Newton lived about 350 years ago and made discoveries on motion and gravity. Magnetic force is used in many appliances.

• Easy : Bhaskaracharya came about 950 years ago.
• Medium : Newton came about 350 years ago, so Bhaskaracharya was earlier by 950 − 350 = 600 years.
• Exam-type : A magnet attracts an iron nail without touching it: magnetic force.

Spot the difference

In the figure : In the figure a 6 kg box is on a slope of 30°. Its weight is 60 N, the normal force is 51.96 N, and the friction of 18 N acts up the slope, against the sliding.

Rule 1: Friction and surfaces — Friction reduces the speed of carrom coins and marbles; it makes writing, erasing and brakes work. Friction between rough surfaces is greater than between smooth surfaces.

• Easy : A rolling marble slows down because of friction.
• Medium : Walking on a smooth wet floor is difficult because friction is less.
• Exam-type : Brakes work because friction between the brake pad and the wheel stops it.

Rule 2: Reducing friction and heat by friction — Lubricants and grease reduce friction, so machine wheels speed up and wear and tear decreases. A meteor or a returning spacecraft burns in the air because of tremendous friction with the atmosphere. Columbia, with seven astronauts including Dr Kalpana Chawla, flew a 16-day mission in January 2003 (252 orbits) and broke up on re-entry on 1 February 2003.

• Easy : Oil on a cycle chain reduces friction.
• Medium : Machines with grease wear out less.
• Exam-type : The spacecraft returning to the earth gets very hot because of friction with the air.

Spot the difference

Rule 1: Electrostatic force — Rubbing glass, plastic, rubber or ebonite produces a temporary electric charge on it. It attracts light objects like paper pieces and threads. In the rainy season, nylon or terrycot clothes may crackle and give small sparks.

• Easy : A rubbed plastic scale picks up paper bits.
• Medium : A nylon shirt may crackle in the rainy season.
• Exam-type : The charge stays only for some time; it is temporary.

In the figure : In the figure the push is 35 N and the friction is 15 N. The forces are unbalanced and the net force is 35 − 15 = 20 N in the direction of the push.

Rule 2: Unbalanced forces — Forces in opposite directions that are not equal; the effect of the greater force is seen, as in the winning team in tug of war. Net force = greater force − smaller force.

• Easy : 40 N pull and 25 N pull in opposite directions: net = 40 − 25 = 15 N.
• Medium : Three children pull with 30 N each (total 90 N) against two children with 40 N each (total 80 N): net = 90 − 80 = 10 N towards the three.
• Exam-type : Same-direction forces add up: 20 N + 30 N = 50 N in the same direction.

In the figure : In the figure the push and the friction are both 18 N. They are equal and opposite, so the forces are balanced and the net force is zero.

Rule 3: Balanced forces — Equal forces in opposite directions: the combined effect is zero and the object stays at rest.

• Easy : 50 N and 50 N in opposite directions: net force 0.
• Medium : Tug of war with equal pulls: the rope stays still.
• Exam-type : A book on the table: weight down and push of the table up are equal, so it is at rest.

Spot the difference

In the figure : In the figure a force of 60 N presses on an area of 0.5 m², so the pressure is 60 ÷ 0.5 = 120 pascal.

Rule 1: Using the formula — Pressure = Force ÷ Area. More area gives less pressure; less area gives more pressure. A nail's tip goes into wood easily because the area is very small.

• Easy : 100 N on 5 m²: pressure = 100 ÷ 5 = 20 pascal.
• Medium : 200 N on 0.5 m²: pressure = 200 ÷ 0.5 = 400 pascal.
• Exam-type : The same 60 N on 0.1 m² gives 600 pascal, but on 0.3 m² gives 200 pascal: 3 times more area gives 3 times less pressure.

In the figure : In the figure the air gets thinner as we go up, and the air pressure is smaller at the higher places of the mountain.

Rule 2: Atmospheric pressure — The atmosphere is the envelope of air around the earth. Air pressure decreases as we go higher. Air flows from higher to lower pressure, which can cause large storms. In a straw the air pressure inside falls, and the higher atmospheric pressure pushes the water in.

• Easy : Air pressure is less on a high mountain than at sea level.
• Medium : Air moves from high pressure to low pressure.
• Exam-type : Drinking with a straw: pressure inside the straw falls and outside air pushes the juice up.

Spot the difference