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Force – A Push or a Pull
How Objects Move
- Movement and Speed: We can tell which object moves faster by the distance it covers in a unit of time.
- Changes in Movement: Objects like a rolling ball slow down and may change direction. Have you wondered why?
Everyday Examples
- Making Objects Move:
- Kicking a football.
- Making a moving ball go faster.
- Stopping a ball like a goalkeeper.
- Changing the direction of a ball with a hockey stick.
- Stopping a ball hit by a batsman.
- Force: In these actions, we say a force is applied (kicked, pushed, thrown, flicked).
Force – A Push or a Pull
- Common Actions: Picking, opening, shutting, kicking, hitting, lifting, flicking, pushing, pulling.
- Push or Pull:
- All these actions can be seen as either a push or a pull.
- In science, these are called forces.
- Examples:
- A magnet attracts iron (a pull).
- Repulsion between similar poles of magnets (a push).
Forces are due to an Interaction
- Interaction Needed:
- A stationary car doesn’t move just because a man stands behind it.
- It moves when he pushes it.
- Examples:
- Two girls pushing each other.
- Two girls pulling each other.
- A cow and a man pulling each other.
- Conclusion:
- At least two objects must interact for a force to exist.
Exploring Forces
- Tug-of-War:
- Two teams pull a rope in opposite directions.
- The team that pulls harder wins.
- Adding Forces:
- Forces in the same direction add up.
- Forces in opposite directions subtract.
- If equal, they cancel each other out.
- Net Force:
- The effect on an object is due to the net force (total force considering direction and magnitude).
Key Points
- Magnitude and Direction: The strength of a force is shown by its magnitude and direction.
- Net Force: The overall effect on an object is due to the net force acting on it.
Think It Over
- Equal Opposite Forces: If two equal forces act in opposite directions, the net force is zero.
More to Know
- Multiple Forces: Usually, more than one force acts on an object, but the effect is from the net force.
Force and Motion
A Force can Change the State of Motion
- Effect of Force on Motion:
- Example: Penalty kick in football.
- Before being hit, the ball is at rest (speed is zero).
- Force from the kick makes the ball move towards the goal.
- Goalkeeper applies force to stop or deflect the ball.
- Changing Speed:
- Force in the direction of motion: Speed increases.
- Force opposite to motion: Speed decreases.
- Example:
- Pushing a rubber tire or ring makes it move faster.
- Changing Direction:
- Volleyball: Players push the ball to teammates or smash it.
- Cricket: Batsman hits the ball with the bat, changing its direction.
- State of Motion:
- Change in speed or direction, or both.
- A force can change an object’s state of motion.
- No Change in Motion:
- Sometimes, force doesn’t change motion (e.g., pushing a heavy box or a wall).
- State of Motion Definition:
- Described by speed and direction.
- Rest: Zero speed.
- Motion: Can be at rest or moving.
Force can Change the Shape of an Object
- Changing Shape:
- Pressing an inflated balloon changes its shape.
- Rolling a ball of dough changes its shape.
- Pressing a rubber ball on a table changes its shape.
- Effects of Force:
- May make an object move from rest.
- May change the speed of a moving object.
- May change the direction of motion.
- May change the shape of an object.
- May cause one or more of these effects.
- Remember:
- Actions like movement, speed change, direction change, or shape change need a force.
- An object cannot move, change speed, change direction, or change shape by itself.
Contact and Non-contact Forces
Contact Forces
Muscular Force
- Examples:
- Pushing or lifting a book.
- Lifting a bucket of water.
- Source: The force comes from muscles in our body.
- Animals: Bullocks, horses, donkeys, and camels use muscular force for tasks.
- Definition: Muscular force is a contact force, meaning it requires contact to be applied.
Think it over!
- Muscular force helps us move and bend.
- It also helps in digestion (moving food) and breathing (lungs expanding and contracting).
Friction
- Examples:
- A rolling ball slows down and stops.
- A bicycle slows down when we stop pedaling.
- A car or scooter stops when the engine is off.
- A boat stops when we stop rowing.
- Cause: Friction is the force that slows down and stops these objects.
- Direction: Friction always acts opposite to the direction of motion.
- Type: Friction is a contact force because it arises from the contact between surfaces.
Non-contact Forces
Magnetic Force
- Observation:
- Magnets can attract or repel each other without touching.
- Nature: This force acts without contact, making it a non-contact force.
- Examples:
- Like poles of magnets repel, unlike poles attract.
- A magnet can attract a piece of iron without touching it.
Electrostatic Force
- Example:
- A charged straw can attract or repel another object.
- Nature: The force acts even when bodies are not in contact.
- Definition: Electrostatic force is a non-contact force exerted by charged bodies.
Gravitational Force
- Examples:
- Objects like coins or pens fall to the ground when dropped.
- Leaves and fruits fall from trees.
- Nature: Gravity is an attractive force that acts without contact.
- Effect: Gravity pulls objects towards the earth.
- Fact: Gravity is a universal force; every object exerts a gravitational force on every other object.
Do You Know?
- Gravity is not just an earth property; it’s a force that exists between all objects in the universe.
Pressure
Understanding Pressure
- Relation between Pressure and Force:
- Pressure is the force acting on a unit area of a surface.
- Formula: Pressure=Force/ Area
- Examples:
- Pushing a nail into wood is easier with the pointed end than the head.
- Cutting vegetables is easier with a sharp knife than a blunt one.
- Key Idea:
- Smaller area means larger pressure for the same force.
Think it over!
- Porters use a cloth on their heads to spread the load over a larger area, reducing pressure.
- Explanation:
- Smaller the area, greater the pressure.
- Sharp tools like knives have small areas to increase pressure and make cutting easier.
- Broad straps on bags reduce pressure on shoulders by spreading the load.
Pressure Exerted by Liquids and Gases
Activity 8.8: Pressure in Liquids
- Materials: Transparent tube, rubber sheet, water.
- Observation: Rubber sheet bulges more as the water height increases.
- Conclusion: Pressure at the bottom of a container depends on the height of the water column.
Activity 8.9: Pressure on Container Walls
- Materials: Plastic bottle, glass tube, rubber sheet.
- Observation: Rubber sheet on the side of the bottle bulges as water is added.
- Conclusion: Liquids exert pressure on the sides of a container as well.
Activity 8.10: Equal Pressure at Same Depth
- Materials: Plastic bottle with holes near the bottom.
- Observation: Water streams from the holes fall at the same distance.
- Conclusion: Liquids exert equal pressure at the same depth.
Think it over!
- Water leaks from pipes due to pressure exerted by water inside.
Pressure in Gases
- Examples:
- Inflating a balloon and observing it deflate when the mouth is open shows that air exerts pressure.
- Bicycle tubes deflate when punctured, indicating air pressure on the inner walls.
Conclusion:
- Both liquids and gases exert pressure on the walls of their containers.
Atmospheric Pressure
What is Atmospheric Pressure?
- Atmosphere:
- The air surrounding us is called the atmosphere.
- It extends many kilometers above the earth’s surface.
- Definition:
- The pressure exerted by this air is known as atmospheric pressure.
- It is the force per unit area exerted by the weight of the air above.
Understanding Atmospheric Pressure
- Example:
- Imagine a long cylinder standing on a unit area filled with air.
- The force of gravity on this air is the atmospheric pressure.
- Magnitude:
- Atmospheric pressure is quite large.
- For example, the force due to air on a 15 cm x 15 cm area is equal to the force of gravity on a 225 kg object (2250 N).
Activity: Sucker Example
- How it works:
- When you press a sucker, air escapes between the cup and the surface.
- The sucker sticks because atmospheric pressure acts on it.
- To remove the sucker, you need to apply a force larger than the atmospheric pressure.
- Observation:
- It is very hard to pull the sucker off if there is no air between it and the surface.
- This shows how strong atmospheric pressure is.
Balancing Pressure
- Why we are not crushed:
- The pressure inside our bodies equals the atmospheric pressure.
- This balance prevents us from being crushed by the atmospheric pressure.
Think it over!
- If your head’s area is 15 cm x 15 cm, imagine the force air would exert on it!
Historical Example: Otto von Guericke’s Experiment
- Otto von Guericke:
- A German scientist from the 17th century.
- Invented a pump to extract air from a vessel.
- Experiment:
- Joined two hollow hemispheres of 51 cm diameter.
- Pumped air out, then used eight horses on each hemisphere to try to pull them apart.
- The force of air pressure was so great that the hemispheres could not be separated.
Chapter Summary:
- Force could be a push or a pull.
- A force arises due to the interaction between two objects.
- Force has magnitude as well as direction.
- A change in the speed of an object or the direction of its motion or both implies a change in its state of motion.
- Force acting on an object may cause a change in its state of motion or a change in its shape.
- A force can act on an object with or without being in contact with it.
- Force per unit area is called pressure.
- Liquids and gases exert pressure on the walls of their containers.
- The pressure exerted by air around us is called atmospheric pressure.
KEYWORDS
- ATMOSPHERIC PRESSURE
- CONTACT FORCE
- ELECTROSTATIC FORCE
- FORCE
- FRICTION
- GRAVITATIONAL FORCE
- GRAVITY
- MAGNETIC FORCE
- MUSCULAR FORCE
- NON-CONTACT FORCE
- PRESSURE
- PULL
- PUSH
