Force and Pressure

A force is a push or a pull on an object, and it always arises from the interaction of at least two objects. In this chapter you will learn that a force has both magnitude and direction, that forces acting in the same direction add up while those in opposite directions give a net force equal to their difference, and that a force can change the speed, the direction of motion or the shape of an object. You will study contact forces, such as muscular force and friction, and non-contact forces, such as magnetic force, electrostatic force and the force of gravity. The second part introduces pressure, the force acting per unit area, measured in newtons per square metre or pascals. You will see why a sharp knife cuts better than a blunt one and why porters use a round cloth on their heads. You will learn that liquids exert pressure on the bottom and walls of a container, increasing with depth, that gases exert pressure, and that the air around us exerts atmospheric pressure.

Detailed Worksheet: Force and Pressure Section A - Definitions (10 marks) 1. Define force. Why do we say that a force arises from an interaction between two objects? (2 marks) 2. List the effects that a force can produce on an object, with one example of each. (2 marks) 3. Distinguish between contact forces and non-contact forces, giving two examples of each. (2 marks) 4. Define pressure and write its SI unit. How does pressure change when the area is reduced for the same force? (2 marks) 5. What is atmospheric pressure? Why are we not crushed by it? (2 marks) Section B - Calculations and Applications (15 marks) 6. A box weighing 600 N rests on the floor. Find the pressure it exerts when it rests (i) on a face of area 1.5 m^2 (ii) on a face of area 0.5 m^2. On which face is the pressure greater, and why? (3 marks) 7. Two children pull a box in the same direction with forces of 30 N and 45 N, while a third child pulls in the opposite direction with 50 N. Find the net force and its direction. What would happen if the third child pulled with 75 N? (3 marks) 8. A girl of weight 450 N stands on both feet, each of area 150 cm^2. Find the pressure she exerts on the floor in N/m^2. What will the pressure be if she stands on one foot? (3 marks) 9. The atmospheric pressure is about 1,00,000 N/m^2. Find the force exerted by the air on the top surface of a table of area 1.2 m^2. Why does the table not collapse under this force? (3 marks) 10. A nail has a pointed tip of area 0.5 mm^2 and a head of area 50 mm^2. A force of 20 N is applied on the head. Find the pressure at the head and at the tip in N/mm^2, and explain why nails are made pointed. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of the activity in which a plastic bottle with holes at different heights is filled with water. Show the streams of water and explain what they show about pressure at different depths. (4 marks) 12. Draw diagrams showing two forces acting on a box (i) in the same direction (ii) in opposite directions. Mark the forces with arrows and write the net force in each case for forces of 10 N and 6 N. (3 marks) 13. Draw a labelled diagram showing why the wall of a dam is made much thicker at the bottom than at the top. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain, using the idea of pressure, why (i) school bags have wide straps (ii) a sharp knife cuts vegetables more easily than a blunt one (iii) porters place a round piece of cloth on their heads (iv) camels can walk easily on sand (v) the tip of a drawing pin is sharp. (5 marks) 15. When a rubber sucker is pressed firmly on a smooth surface, it sticks and is difficult to pull off. Explain why, and describe what happens if a small hole is made in the sucker. Explain how a straw is used to drink juice, using the idea of atmospheric pressure. (5 marks) 16. A ball rolling on the ground slows down and stops; a goalkeeper changes the direction of a moving football; a lump of dough is flattened into a chapati. Identify the force involved and its effect in each case. Explain why a force does not always produce motion, using the example of pushing a heavy wall. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Write the formula and units in every calculation, and show forces with arrows in all diagrams.