1.0 Introduction to Force
In our daily life, we perform many actions like opening a door, kicking a ball, or lifting a school bag. All these actions involve either a push or a pull. In Physics, this push or pull exerted on a body is called Force.
What is Force?
A Force is a push or pull upon an object resulting from the object's interaction with another object. It can change the state of rest, motion, shape, or size of a body.
Effects of Force
A force cannot be seen, but its effects can be observed. A force can:
- Move a stationary object: Like pushing a toy car.
- Stop a moving object: Like a goalkeeper catching a football.
- Change the speed of an object: Like pedaling a cycle faster.
- Change the direction of motion: Like a batsman hitting a ball.
- Change the shape or size: Like squeezing a sponge or stretching a rubber band.
SI Unit of Force
$$Newton (N)$$
1 kgf (kilogram-force) ≈ 10 N
1.1 Concept of Net Force
When multiple forces act on an object at the same time, the combined effect is called the Resultant Force or Net Force.
Two boys are pushing a heavy box. Boy A pushes with 50 N and Boy B pushes in the same direction with 30 N. What is the total force?
Solution:
1. Force A ($F_1$) = $50\,N$
2. Force B ($F_2$) = $30\,N$
3. Since they are in the same direction, we add them:
4. Calculation: $Net\,Force = F_1 + F_2 = 50 + 30 = 80\,N$
Result: The net force is $80\,N$ in the direction of the push.
If two forces act in opposite directions, the net force is the difference between them. For example, if you pull with 10 N to the left and your friend pulls with 10 N to the right, the net force is 0 N (Balanced Force).
Sir Isaac Newton formulated the Laws of Motion. The unit of force is named 'Newton' in his honour. 1 Newton is approximately the weight of a small apple!
2.0 Types of Forces
In nature, forces are broadly classified into two main categories based on whether the interacting bodies are touching each other or not: Contact Forces and Non-contact Forces.
2.1 Contact Forces
These forces act only when there is a physical touch between two objects. Major examples include:
- Muscular Force: The force applied by the muscles of our body (e.g., lifting a bucket, pedaling a cycle).
- Frictional Force: The force that opposes the motion of an object over a surface. It always acts in the opposite direction to motion.
- Mechanical Force: Force generated by machines (e.g., a crane lifting a car).
2.2 Non-Contact Forces
These forces act through a distance or space without any physical contact between the objects. They are also called Action-at-a-Distance forces.
- Gravitational Force: The attractive force exerted by the Earth on all objects (e.g., a ball falling down).
- Magnetic Force: The force exerted by a magnet on iron pieces or another magnet.
- Electrostatic Force: The force exerted by a charged body on another charged or uncharged body (e.g., a comb attracting bits of paper).
Gravity vs. Mass
While Mass is the amount of matter, Weight is actually the Gravitational Force acting on that mass. Weight is measured in Newtons (N) or Kilogram-force (kgf).
Weight Formula
$$W = m \times g$$
Where: $W$ = Weight, $m$ = Mass, $g$ = Acceleration due to gravity ($\approx 9.8\,m/s^2$)
If a boy has a mass of 40 kg, what is his weight on Earth? (Take $g = 10\,m/s^2$)
Solution:
1. Given: $m = 40\,kg$, $g = 10\,m/s^2$
2. Formula: $W = m \times g$
3. Calculation: $W = 40 \times 10 = 400\,N$
Result: The weight of the boy is $400\,N$.
Magnetic and Electrostatic forces can be both attractive (pull) and repulsive (push). However, Gravitational force is always attractive.
3.0 Friction: The Opposing Force
When you stop pedaling a bicycle, it slowly comes to a halt. When a ball is rolled on the ground, it eventually stops. This happens due to a contact force called Friction. Friction is the force that develops between two surfaces in contact and opposes the relative motion between them.
Cause of Friction
No surface is perfectly smooth. Even surfaces that look smooth have tiny microscopic ups and downs called irregularities (hills and valleys). Friction is caused by the interlocking of these irregularities when two surfaces slide over each other.
Factors Affecting Friction
The strength of the frictional force depends on two main factors:
- Nature of the Surfaces: Rough surfaces have more irregularities and thus offer more friction than smooth surfaces.
- The Force Pressing the Surfaces: The heavier the object, the harder it presses against the surface, leading to stronger interlocking and higher friction.
3.1 Static and Sliding Friction
Friction is not always the same; it changes based on whether the object is just about to move or is already moving.
| Type of Friction | Description |
|---|---|
| Static Friction | The friction that acts when you try to move an object at rest. It is the strongest type. |
| Sliding Friction | The friction that acts when an object is already sliding over a surface. It is slightly less than static friction. |
| Rolling Friction | The friction that acts when an object rolls over a surface. It is the weakest of all. |
Why is it easier to pull a luggage bag with wheels than one without wheels?
Solution:
When a bag has wheels, the friction acting on it is Rolling Friction. Since rolling friction is much smaller than Sliding Friction (which acts on a bag without wheels), less force is required to move the bag.
Many students think friction is only bad because it slows things down. However, without friction, we wouldn't be able to walk, write with a pen, or even stop a car using brakes!
4.0 Friction: A Necessary Evil
Friction is often called a "Necessary Evil" because while it causes wear and tear and wastes energy, it is also absolutely essential for most of our daily activities. We can increase or decrease friction depending on our needs.
4.1 Advantages of Friction
- Walking: Friction between our shoes and the ground prevents us from slipping.
- Writing: Friction between the pen tip and paper allows the ink to stick.
- Braking: Friction between brake pads and wheels allows vehicles to stop.
- Lighting a Matchstick: The heat produced by friction ignites the chemicals on the matchstick.
4.2 Disadvantages of Friction
- Wear and Tear: It wears out the soles of shoes, tires of vehicles, and moving parts of machines.
- Heat Production: Excessive friction produces heat which can damage sensitive machine parts.
- Energy Loss: A lot of energy is wasted in overcoming friction, reducing the efficiency of machines.
4.3 Methods to Change Friction
How to Control Friction?
Ways to Reduce Friction:
- Polishing: Making the surfaces smoother.
- Lubrication: Using oil, grease, or graphite to fill the irregularities.
- Using Ball Bearings: Converting sliding friction into rolling friction.
- Streamlining: Giving a special shape to boats and airplanes to reduce friction with water or air.
Ways to Increase Friction:
- Treading of Tires: Making patterns on tires to provide a better grip.
- Spikes on Shoes: Athletes use spikes to increase friction with the track.
Why do we sprinkle fine powder on a carrom board before playing?
Solution:
Sprinkling powder fills the microscopic irregularities on the surface of the carrom board. This makes the surface smoother and significantly reduces friction, allowing the striker and coins to slide easily.
On a rainy day, it is harder to stop a car because the layer of water acts as a lubricant between the tires and the road, reducing the friction required for the brakes to work effectively.