Chapter: 04. Force
REASON CORNER
When a force is applied along the ground on a stationary ball, it makes the ball To slow down a moving object, in which direction should the force be applied? a. Opposite to the direction of the motion of object. Which of the following cannot be changed in an object by applying force? LIFE SKILLS
What will you do to untie a knot on a string: push or pull?
To untie a knot on a string, you would typically use a pulling force (or a combination of pushing and pulling specific strands to loosen them). Pushing might tighten the knot, while pulling strategically helps separate the loops. LIFE SKILLS
Suppose you have to design shoes to walk on slippery surfaces. What type of soles would you prefer?
soles with a smooth surface soles with grooves
Give reasons for your answer. soles with grooves. Grooves increase the roughness of the sole’s surface, which creates more friction with the slippery ground, providing a better grip and preventing slipping. CHECK YOUR PROGRESS
Friction is greater when the surfaces are ECO-SOLDIER
Discuss ways in which a large forest fire harms the environment. Also suggest ways in which you would help protect a forest.
Harms to the environment: Forest fires destroy trees and vegetation, leading to habitat loss for animals, loss of biodiversity, soil erosion, and increased carbon dioxide in the atmosphere (contributing to climate change). They also release harmful smoke and particulates that degrade air quality. Ways to help protect a forest: Be careful with fire (extinguish campfires completely, avoid throwing cigarette butts), report unattended fires, participate in community clean-ups to remove dry brush, and support organizations focused on forest conservation and fire prevention. PREPPING FOR PISA
On a rainy day, the force of friction between the ground and the ball: Tick (✓) the incorrect option.
Sportsmen often wear shoes with spikes to: b. decrease the roughness. In which of the following cases will the box be easiest to push? c. (The image depicting the box with cylindrical rods/rollers underneath, as rolling friction is less than sliding friction). SCIENCE TALK
Talk about the ways friction can be reduced to facilitate easy movement, less wear and tear, and reduced heat production.
Friction can be reduced by: Lubrication: Applying oils, greases, or other lubricants between moving surfaces to create a smooth layer, separating them and reducing direct contact. Using Wheels, Rollers, or Ball Bearings: Converting sliding friction into rolling friction, which is much less. This is why vehicles have wheels and machines use ball bearings. Polishing Surfaces: Making surfaces smoother reduces the interlocking of irregularities, thereby reducing friction. Using Air Cushions: Like in hovercrafts, where a layer of air separates the object from the surface. EVALUATION
A. Choose the correct option.
Force applied on an object can change its Which of the following is an example of a contact force? When we apply the brakes on a moving scooter, it will The tyres of vehicles get worn out with time because of constant friction between the tyres and the Which of the following is not a type of friction? Ritu has to go to school and has four paths that she can take. One of the paths is laid with concrete, the other with pebbles, the third is covered with wet polished surface and the fourth is covered with dry polished surface. The path on which she is most likely to slip is B. Name the following:
the force produced when an object rolls over a surface the force produced when an object is at rest on a surface the force produced when an object slides over a surface a force which opposes motion of one surface over the other Force of friction (or Frictional force) C. Write true (T) or false (F) against the following statements.
If we apply force at an angle to the direction of motion of an object, the object changes its direction. When a force is applied to pull an object, the direction of motion is always away from us. We cannot see force but we can see the effects of force. Strong winds and storms do not exert force. It is easier to run on a dry floor than on a wet one because friction is less on a dry floor. Rolling friction is always greater than sliding friction. D. Fill in the blanks.
When we run, we use muscular force. When chapatis are being rolled, the shape and size of the dough changes. The frictional force always opposes the motion of one surface over the other. Muscular force is an example of a contact force. E. Short answer type questions.
Name any two types of contact forces. Muscular force, Force of friction. How does wind affect our movement? Strong winds can exert force on us. If the wind blows in the same direction as our movement, it can make us move faster. If it blows in the opposite direction, it can slow us down. Define force of friction. Give two examples. Force of friction is the force acting between two surfaces in contact that opposes or tries to oppose the motion of one surface over the other. Examples: The force that stops a rolling ball, or the force that allows us to walk on the ground. Give two examples where you think friction is useful. Friction is useful for writing on paper with a pencil, and for vehicles to move and brake on the road. F. Long answer type questions.
State and explain any four effects of force on an object. Force can change the shape and size of an object: For example, pressing a balloon changes its shape. Force can produce motion in an object at rest: For example, pedaling a bicycle makes it move. Force can stop a moving object: For example, applying brakes on a scooter stops it. Force can change the speed of a moving object: For example, pushing a moving swing in the direction of its motion makes it go faster. Force can change the direction of a moving object: For example, kicking a football changes its direction. What are the various effects of friction? Advantages of friction: Enables us to walk, write, light matchsticks, allows vehicles to move and brake, and prevents machine parts from slipping. Disadvantages of friction: Opposes motion (requiring more effort), causes wear and tear on surfaces (like shoe soles and tires) and machine parts, and produces heat (wasting energy and affecting machine efficiency). Higher Order Thinking Skills (HOTS)
Assertion (A): It is more difficult to pull a boat onto the beach than to pull it into the sea.
Reason ®: The friction between the boat and the sand on the beach is greater than that between the boat and the sea. a. Both A and R are true and R is the correct explanation of A. In which of the following cars will the tyres wear out faster and why? Car A (on sandy terrain) will wear out faster. This is because sandy or rough terrain creates more friction compared to a smooth highway (Car B), leading to increased wear and tear on the tires. Why is the handle of a tennis racquet usually made rough? The handle of a tennis racquet is made rough to increase friction between the player’s hand and the racquet. This provides a better grip, preventing the racquet from slipping out of the hand during powerful swings or when the player’s hands are sweaty. Why is it dangerous to drive 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 vehicle’s tires and the road surface. This reduced grip makes it much harder to control the vehicle, especially when braking, turning, or accelerating, increasing the risk of skidding and accidents. Why do kabaddi players rub their 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, which provides a better and firmer grip on the opponent, essential for tackling and holding them effectively during the game. 
Answers to textbook exercises