Question 5
In the following situations, identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.
(a) Squeezing a piece of lemon between the fingers to extract its juice.
(b) Taking out paste from a toothpaste tube.
(c) A load suspended from a spring while its other end is on a hook fixed to a wall.
(d) An athlete making a high jump to clear the bar at a certain height.
| S. No. | Agent | Object | Effect | Reason |
|---|---|---|---|---|
| a. | Fingers | Lemon | Shape of lemon changes | Muscular force is exerted on the lemon in order to extract its juice. As a result, the shape of the lemon changes. |
| b. | Fingers | Tooth paste tube | Shape of tooth paste tube changes | Muscular force is exerted on the tube in order to take out paste. As a result, the shape of the tube changes. |
| c. | Suspended load | Spring | The spring gets stretched. | The suspended load pushes the spring downwards. Hence, its shape changes. |
| d. | Athlete | Ground | Pushes the ground backwards and as a result takes a high jump to clear the bar at a certain height. | When the athlete pushes the ground with his feet, his feet exert a muscular force on the ground. Same magnitude of force is applied back by the ground on the athlete. This force allows them to jump over the bar. As a result, their state of motion gets changed. |
Chapter Overview: Force and Pressure
This physics chapter introduces the concept of force (push/pull) and its effects, classifies forces as contact and non-contact, defines pressure as force per unit area (P = F/A), and explores atmospheric pressure and liquid pressure with everyday applications.
Board Exam Weightage: 5-7 marks | Difficulty: Moderate to High
Key Formulas & Concepts
| Concept | Details |
|---|---|
| Pressure | P = F / A (Force divided by Area); Unit: Pascal (Pa) or N/m² |
| 1 Pascal | 1 Pa = 1 N/m² (force of 1 Newton on area of 1 square metre) |
| Net Force | Forces in same direction add; forces in opposite directions subtract |
| Atmospheric Pressure | ~101,325 Pa at sea level; decreases with altitude |
Must-Know Concepts
- Force can change state of motion, speed, direction, or shape of an object
- Contact forces (muscular, friction) need physical contact; non-contact forces (gravity, electrostatic, magnetic) act at a distance
- Pressure increases when area decreases for the same force (sharp knife, pointed nail)
- Atmospheric pressure acts in all directions and is demonstrated by suckers, straws, and syringes
- Liquid pressure increases with depth and acts in all directions
- Dams are thicker at the bottom because water pressure is greatest there
Contact vs Non-Contact Forces
| Contact Forces | Non-Contact Forces |
|---|---|
| Need physical contact to act | Act at a distance without contact |
| Muscular force (pushing, pulling) | Gravitational force (falling objects) |
| Friction (opposes motion) | Electrostatic force (charged objects) |
| Normal force (surface pushing back) | Magnetic force (magnets) |
Common Mistakes to Avoid
- Thinking force always causes movement (balanced forces produce no motion)
- Confusing force (Newtons) and pressure (Pascals)
- Forgetting to convert cm² to m² in pressure calculations
- Thinking atmospheric pressure acts only downward (it acts in all directions)
Scoring Tips
- Practice 5 numerical problems on P = F/A with unit conversion
- List 5 everyday examples each of atmospheric and liquid pressure
- Remember: same force, smaller area = greater pressure
- Draw diagrams showing forces acting on objects in different situations
Frequently Asked Questions
Q: How many marks does this chapter carry in the exam?
A: Approximately 5-7 marks in the annual exam.
Q: What question types are commonly asked?
A: MCQs (1 mark), Very Short Answer (2 marks), Short Answer (3 marks), and Long Answer / Diagram (5 marks).
Q: Is this chapter important for competitive exams?
A: Yes, concepts from this chapter appear in NTSE, Olympiad, and other science competitions.