Force and Pressure
Focused last-minute revision notes with clear concepts, exam traps, key comparisons and compact practice.
Chapter Snapshot
- A force is a push or pull arising from interaction between objects.
- Force can change motion, speed, direction or shape.
- Contact forces include muscular force and friction.
- Non-contact forces include magnetic, electrostatic and gravitational forces.
- The effect of force depends on both magnitude and direction.
- Pressure is force acting per unit area.
- For the same force, smaller area produces greater pressure.
- Liquids exert pressure on the walls and bottom of containers and pressure increases with depth.
- Air exerts atmospheric pressure in all directions.
What You Will Learn
- Define force and identify its effects
- Classify contact and non-contact forces
- Explain balanced and unbalanced effects conceptually
- Define pressure and use its formula
- Relate area to pressure
- Explain pressure in liquids and atmosphere
Last-Minute Chapter Map
Force can act by contact or at a distance; pressure depends on how force is distributed over area.
1. Force as Push or Pull
A force is a push or pull produced by interaction. A force can start or stop motion, change speed, change direction or deform an object. A force has both magnitude and direction.
2. Contact Forces
Muscular force is applied by muscles and requires contact. Friction acts between surfaces in contact and opposes relative motion or the tendency to move.
3. Non-contact Forces
Magnetic force acts between magnets or magnetic materials, electrostatic force acts between electric charges, and gravitational force attracts masses. These forces can act without direct physical contact.
4. Pressure
The same force can have different effects depending on area. A sharp knife cuts better because the force is concentrated over a small area. Pressure is force per unit area.
5. Pressure in Liquids
Liquids exert pressure on the bottom and walls of their containers. At greater depth, liquid pressure is generally greater. This is why lower holes in a water bottle can produce stronger jets.
6. Atmospheric Pressure
Air has mass and exerts pressure in all directions. This atmospheric pressure explains phenomena such as drinking through a straw and the difficulty of pulling apart suction cups.
Contact vs Non-contact Forces
| Feature | Contact Force | Non-contact Force |
|---|---|---|
| Need physical contact? | Yes | No |
| Examples | Muscular force, friction | Magnetic, electrostatic, gravitational |
Essential Formula
Pressure
P = pressure, F = force, A = area
Exam Focus
Must Revise Before the Exam
- Force = push/pull
- Effects of force
- Contact vs non-contact
- Muscular, friction, magnetic, electrostatic, gravitational
- Pressure formula
- Area and pressure
- Liquid pressure
- Atmospheric pressure
Common Mistakes
Key Terms
Key Terms
Quick Revision
Quick Revision
- Force is a push or pull due to interaction.
- Force can change speed, direction, state of motion or shape.
- Contact: muscular, friction.
- Non-contact: magnetic, electrostatic, gravitational.
- Pressure = force / area.
- Same force on smaller area → greater pressure.
- Liquid pressure increases with depth.
- Atmosphere exerts pressure in all directions.
Practice Questions
Which is a non-contact force?
A. friction
B. muscular force
C. magnetic force
D. push by hand
View Solution
Correct Answer: C. magnetic force
Magnetic force can act without direct contact.
For the same force, pressure is greatest when area is:
A. largest
B. smallest
C. unchanged only
D. zero in all cases
View Solution
Correct Answer: B. smallest
Pressure is inversely related to area for a fixed force.
A sharp needle pierces easily because it has:
A. large contact area
B. small contact area and high pressure
C. no force
D. no pressure
View Solution
Correct Answer: B. small contact area and high pressure
The force is concentrated over a small tip area.
Liquid pressure generally becomes greater with:
A. decreasing depth
B. increasing depth
C. zero gravity only
D. colour
View Solution
Correct Answer: B. increasing depth
Pressure in a liquid increases with depth.
Why do school bags have broad shoulder straps?
View Solution
Broad straps increase the contact area, reducing pressure on the shoulders for the same weight.
Give two examples each of contact and non-contact forces.
View Solution
Contact: muscular force, friction. Non-contact: magnetic force, electrostatic force or gravitational force.
Why is the wall of a dam generally thicker near the bottom?
View Solution
Water pressure increases with depth, so lower parts of the dam experience greater pressure and require greater strength.
Chapter Summary
Chapter Summary
- Force is a push or pull due to interaction.
- Force can change speed, direction, state of motion or shape.
- Contact: muscular, friction.
- Non-contact: magnetic, electrostatic, gravitational.
- Pressure = force / area.
- Same force on smaller area → greater pressure.
- Liquid pressure increases with depth.
- Atmosphere exerts pressure in all directions.