Chapter: 04. Force
Force
Learning Outcomes
By the end of this study, you will understand:
How force can change an object’s motion. How force can change an object’s shape. What the force of friction is, with real-life examples. The different types of friction: static, sliding, and rolling. The benefits and drawbacks of friction in everyday life. What is Force?
Imagine you want to move something. Do you push it away or pull it closer? This push or pull is what we call force.
Force is the effect of an interaction that makes an object’s motion change.
Push: When you push an object, you move it away from you. Examples: Kicking a ball, hitting a nail with a hammer, pushing a car, pushing a fan switch, pushing a door open. Pull: When you pull an object, you move it closer to you. Examples: A camel pulling a cart, pulling a suitcase, pulling a door open towards you, playing tug-of-war. Effects of Force
We cannot actually see force, but we can clearly see what happens when a force is applied. Here are the main effects of force:
1. Force Can Change the Shape and Size of an Object
When you apply force on an object, its shape and size can change. Pressing an air-filled balloon changes its shape. Rolling dough to make chapatis changes its shape and size. Pulling or compressing a spring changes its shape and size. 2. Force Cannot Change the Mass of an Object
No matter how much force you apply to an object, its mass (the amount of matter in it) will always stay the same. 3. Force Can Change the State of Motion
Producing Motion in an Object at Rest: A force can make a stationary object start moving. Pedaling a bicycle exerts force, making it move. A bullock pulling a cart makes it move. Important Note: Not all forces produce motion. If the force isn’t strong enough, the object might not move. For instance, pushing a wall or a bus by yourself won’t make them move because your force isn’t sufficient. Stopping a Moving Object: If you apply force on a moving object in the opposite direction of its motion, it can stop. A cricket player stopping a ball with their hands. Applying brakes on a scooter to stop it (brakes press against wheels, applying force in the opposite direction). Applying force on the ground with your feet to stop yourself while running. Changing the Speed of a Moving Object: If you apply force in the same direction as the object’s motion, it speeds up. If you apply force in the opposite direction as the object’s motion, it slows down. Example: Pushing a moving bicycle from behind makes it go faster; pulling it from the front makes it slow down or stop. Changing the Direction of a Moving Object: If you apply force at an angle to an object’s motion, its direction changes. Kicking an incoming football changes its direction. A badminton player hitting a shuttlecock changes its direction. Steering a bicycle changes its direction. Definition of Force: Force is a physical quantity which changes or tends to change the state of rest or uniform motion of a body.
Natural Forces
Many natural things also exert force. Strong winds and storms, for example, exert a lot of force. You might notice that if the wind blows in the same direction you are moving, you speed up. If it blows against you, you slow down.
Types of Force
Forces can be divided into two main categories:
1. Non-contact Force
A force applied to an object by another object without direct physical touch. Gravitational force: The force that pulls objects towards each other, like the Earth pulling you down. (This is what makes things fall!) Magnetic force: The force between magnets, which can attract or repel without touching. Electrostatic (electrical) force: The force between electrically charged objects, like a balloon sticking to your hair after rubbing it. 2. Contact Force
A force applied to an object by another object that is in direct physical contact with it. The contact force acts at the point where the objects touch. Muscular force: The force you exert with your muscles when you push, pull, lift, or carry things. Force of friction: The force that opposes motion when two surfaces rub against each other.
Force of Friction
Have you ever wondered why a ball stops rolling after some distance, or why your bicycle stops if you stop pedaling? This happens because of the force of friction.
Sci-Vocab: Force of friction
The force acting between two surfaces in contact and trying to oppose the motion of one surface over the other.
Friction is produced when one surface moves or tries to move over another surface. It always opposes motion and tries to stop the movement. The strength of friction depends on the nature of the two surfaces (how rough or smooth they are). Rough surfaces produce more friction. Smooth surfaces produce less friction. Observing Friction and Surface Roughness
You can see this by rolling a marble:
Roll a marble on a smooth, inclined tile. Note the distance it travels. Now, place a rough blanket under the tile and roll the marble again with the same incline. Observation: The marble will travel a shorter distance on the tile placed on the blanket. Conclusion: Rough surfaces create more friction than smooth surfaces. Properties of the Force of Friction
Here are the key characteristics of friction:
Occurs During Motion or Attempted Motion: Friction is produced only when one surface moves or tries to move over another surface. Opposes Motion: It always acts in the opposite direction of motion, trying to stop the movement. Depends on Surface Nature: The amount of friction depends on how rough or smooth the surfaces are. Rough surfaces (like a concrete road) provide more friction than smooth surfaces (like a polished marble floor), making it easier to walk on concrete. Depends on Mass: The amount of friction increases with the mass of the body. It’s harder to push a heavier box than a lighter one because the heavier box has a greater frictional force acting on it. Does NOT Depend on Apparent Area of Contact: The total force of friction between two surfaces does not depend on the apparent (overall visible) area where they touch. For example, two bodies with the same mass but different total surface areas will experience the same amount of friction. Note: While the total force of friction doesn’t depend on the apparent area, practical situations like shoes with spikes or wider tires do increase grip. This is because they effectively create more points of microscopic contact and distribute weight, which helps prevent slipping by providing better interlocking with the surface.
Types of Friction
There are three main types of friction:
1. Static Friction
Sci-Vocab: Static friction
Force of friction between two surfaces in contact, with no relative motion between them.
This friction acts when an object is at rest on a surface and you try to move it, but it doesn’t budge. Example: When you try to push a heavy table, it might not move because of the strong static friction between its legs and the floor. 2. Sliding Friction
Sci-Vocab: Sliding friction
When an object slides over a surface, the friction produced between the object and the surface.
This friction occurs when one object slides over another. Example: When a flat block is pushed or pulled across a flat surface, sliding friction is at play. 3. Rolling Friction
Sci-Vocab: Rolling friction
When an object rolls on a surface, the friction produced between the object and the surface.
This friction occurs when an object rolls over a surface. Example: Moving a cart with wheels on a road, or a ball rolling on the ground. Rolling vs. Sliding Friction
Rolling friction is much less than sliding friction. It is always easier to roll an object than to slide it. This is why vehicles use wheels! Observation: If you pull a wooden block across a table (sliding), it requires more force than if you place cylindrical rods (like pencils) under it and then pull it (rolling). The spring balance reading will be much less for rolling. Advantages of Friction
Life as we know it would be impossible without friction. Here are some of its many benefits:
Writing: We can write on paper because of the friction between the pencil (or pen) and the paper. It’s difficult to write on very smooth surfaces like waxed paper because there’s very little friction. Lighting a Matchstick: A matchstick lights up due to the friction created by rubbing it against the rough surface of the matchbox. Vehicle Movement: Vehicles move on roads because of the friction between their tires and the road. Without friction, tires would just spin. Braking: Brakes in vehicles work because of friction. When brakes are applied, the brake shoes rub against the wheels, and the friction slows down or stops the vehicle. Walking: We can walk by pushing our feet against the ground, and friction allows our feet or footwear to grip the surface. It’s very difficult to walk on slippery surfaces like snow or a wet, polished floor because there’s very little friction. Life Skill: To walk on slippery surfaces, you would prefer soles with grooves. Grooves increase the roughness and effective contact points, providing more grip and friction, which prevents slipping. Smooth soles would offer very little friction. Machine Parts: Friction allows parts of machines to move against each other without slipping. Cricket: Static friction between a bowler’s hand and the ball allows them to grip and hold the ball. Without it, the ball would fall. Kinetic friction from the air opposes the ball’s speed and helps it swing or spin. Moisture: More moisture on the ground decreases friction, making it harder to spin the ball. Spikes: Sportsmen wear shoes with spikes to increase the roughness and effective surface area of their shoes, which increases their grip on the ground and prevents slipping. Disadvantages of Friction
While essential, friction also has its downsides:
Opposes Motion: Friction always works against movement, meaning you need to apply more effort to get things moving or keep them moving. Shoe soles and vehicle tires wear out over time due to constant friction with the ground. This wear can be faster on hills because of increased friction. Machine parts also experience wear and tear due to friction. Heat Production: Friction produces heat. This heat can reduce the efficiency of machines and cause them to overheat. A lot of energy is wasted in overcoming friction and dealing with the heat it generates. Environmental Impact (Eco-Soldier): During hot summer months, dry tree branches rubbing against each other can create enough friction to cause forest fires. These fires cause massive damage to forests, wildlife, and human lives, and release harmful gases into the atmosphere. Myth vs Fact:
Myth: We can completely eliminate friction. Fact: We can reduce friction to a certain extent, but we cannot make it zero. Reducing Friction:
While not explicitly detailed in the chapter, friction can be reduced to facilitate easy movement, lessen wear and tear, and decrease heat production in machines. Common ways to reduce friction include:
Lubrication: Applying oil or grease between moving parts. Using wheels/rollers: Converting sliding friction into much smaller rolling friction. Using ball bearings: Tiny balls or rollers that reduce friction in rotating parts. Polishing surfaces: Making surfaces smoother reduces friction between them.
Real-World Applications & Concepts
Reason Corner: Why Buses and Trucks Have Heavier and Wider Tyres
Buses and trucks carry heavy loads. Wider tires provide better grip and stability on the road. While the force of friction doesn’t purely depend on the apparent area, wider tires distribute the heavy weight over a larger area, reducing pressure per square inch. This helps to maintain consistent contact with the road, prevents excessive wear on small areas, and provides better overall grip and stability, especially during braking and turning, which are critical for heavy vehicles.
Inuit Sledges: Reducing Friction in the Cold
The Inuit people, living in cold regions, use sledges pulled by dogs or reindeer for transport over snow. These sledges are designed to reduce friction with the snow. The runners of the sledge are typically smooth and wide, often made of wood or bone, and sometimes coated with ice or water. This design helps to reduce the contact friction, making it easier for the sledge to glide over the smooth surface of the snow with less effort.
Learning by Doing: Static vs. Sliding Friction and Mass
When you pull a brick with a rope, you first need to apply enough force to overcome the static friction that is holding it in place. Once the brick starts moving, the friction changes to sliding friction, which is usually less than static friction. If you add more bricks, you increase the total mass. This increases the static friction, meaning you need to apply even more pulling force to get the bricks to move. This demonstrates that the amount of force required to overcome static friction increases with the mass of the object.
Higher Order Thinking Skills (HOTS) Scenarios Explained
Pulling a boat onto the beach vs. into the sea: It is more difficult to pull a boat onto the beach than into the sea because the friction between the boat and the sand on the beach is much greater than the friction between the boat and the water in the sea. Sand is a much rougher surface than water. Car A (sandy terrain) vs. Car B (highway): The tires of Car A (on sandy terrain) will wear out faster. Sandy or rough terrain generally creates more friction and resistance compared to a smooth highway. This increased friction leads to greater wear and tear on the tires. Tennis racquet handle: The handle of a tennis racquet is usually made rough to increase the friction between the player’s hand and the racquet. This provides a better grip, preventing the racquet from slipping during play, especially when swinging with force or when hands are sweaty. Driving on roads during rains: It is dangerous to drive on roads during rains because water acts as a lubricant, significantly reducing the friction between the tires and the road surface. This reduced friction makes it harder for tires to grip the road, increasing the risk of skidding and loss of control, especially during braking or turning. Kabaddi players rubbing hands with soil: Kabaddi players rub their hands with soil to increase the friction between their hands and the opponent’s body. The soil makes their hands rougher, allowing for a better grip on the opponent, which is crucial for tackling and holding during the game.
Science and Scientists
Sir Isaac Newton (1642-1727)
Sir Isaac Newton was an incredibly influential mathematician and physicist whose discoveries laid the foundation for much of modern science. He developed the laws of motion and the universal law of gravitation, which explains that every object in the universe attracts every other object. He also invented the reflecting telescope. Newton is considered one of the greatest scientists in history.
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Self Study
