Lesson 5. Friction

Friction is an opposing force against an object in motion. It is a both friend and a foe. It plays a crucial role in our everyday lives and in many scientific applications, from walking without slipping to the functioning of machines. Friction arises due to the small irregularities on surfaces of objects . The interactions between these surfaces cause a resistance which is a frictional force. There are different types of friction, including static, kinetic (sliding), and rolling friction. While friction can be helpful—such as in providing grip—it can also cause energy loss in the form of heat, leading scientists and engineers to study and manage it carefully in various technologies.

Some examples of Friction

1. Walking or Running
   When you walk or run, the friction between your shoes and the ground prevents you from slipping. Without this friction, your foot would slide backward every time you tried to move forward. This type of friction is essential for movement, as it provides the necessary grip for your feet to push against the surface.
2. Writing with a Pen or Pencil
   The tip of a pen or pencil rubs against the paper, and friction helps transfer ink (in pens) or graphite (in pencils) onto the paper. If there were no friction, the tip would simply glide over the surface without leaving a mark. Controlled friction ensures neat and legible writing.
3. Striking a Matchstick
   When you drag a matchstick across a rough striking surface, friction generates heat. This heat is enough to ignite the chemicals on the match head, producing a flame. This is a perfect example of friction being used to create fire through heat.
4. Using Sandpaper
   Sandpaper has a rough surface that creates high friction. When rubbed on wood or metal, friction wears away uneven parts, smoothing the surface. It’s commonly used in carpentry and metalwork to prepare materials for painting or finishing.
5. Climbing a Ladder
   When you climb a ladder, your hands and feet rely on friction between them and the rungs to avoid slipping. If the ladder or your shoes are slippery, there’s a risk of falling. High friction ensures grip and safety during climbing.
6. Opening a Drawer
   As you pull or push a drawer, friction between the sliding parts (usually the tracks or runners) either resists or controls the motion. Too much friction can make the drawer hard to open, while too little can cause it to slide too quickly or fall out.
7. Wearing Out Shoes or Tires
   Over time, the constant rubbing between shoes or tires and the ground wears them down. This is due to friction. It gradually removes material from the soles or treads, leading to smooth surfaces that can reduce grip and increase the chance of slipping or skidding.
8. Rubbing Hands Together
   When you rub your hands together, friction between your palms creates heat. This is especially useful in cold weather to warm your hands naturally. The faster and harder you rub, the more heat is generated.
9. Skating or Sliding
   Ice, polished floors, or slides have low friction surfaces, which is why skating or sliding feels smooth and fast. Here, reduced friction allows objects or people to glide with minimal resistance, making movement more fluid and enjoyable (or sometimes dangerous if not controlled).

Factors Affecting Friction:

1. Nature of the Surfaces in Contact
   The roughness or smoothness of surfaces plays a crucial role.
   • Rough surfaces have more microscopic bumps and irregularities, which interlock and increase friction.
   • Smooth surfaces have fewer irregularities, so there is less resistance to motion.
   • Example: Rubber on concrete creates more friction than rubber on ice.
2. Force (Weight)
   Friction increases with the force pressing the surfaces together, known as the normal force.
   • A heavier object pushes more strongly against the surface, increasing the contact and friction.
   • Formula: Frictional force = coefficient of friction × normal force.
   • Example: A loaded cart is harder to push than an empty one due to greater friction.
3. Type of Motion
   Friction varies based on how motion occurs.
   • Static friction prevents an object from starting to move; it is usually higher than other types.
   • Kinetic (sliding) friction occurs once an object is in motion.
   • Rolling friction (e.g., wheels or ball bearings) is much less than sliding friction.
   • Example: It’s harder to push a stationary box than to keep it moving, but rolling it on wheels is easiest.
4. Presence of Lubricants
   Lubricants like oil, grease, or wax form a thin, smooth layer between surfaces, reducing direct contact.
   • This lowers friction, allowing smoother movement and preventing wear and tear.
   • Example: Machines use lubricants to avoid overheating and reduce energy loss.
5. Surface Area (in Some Cases)
   In theory, friction doesn’t depend directly on surface area if the normal force remains constant.
   • However, in real-world cases, a larger surface area might:
     • Increase friction due to more contact points.
     • Decrease pressure per unit area, which might reduce wear.
   • Example: Snowshoes spread weight over a larger area, reducing sinking while keeping friction balanced.
6. Temperature
   Heat can change the texture, softness, or chemical properties of materials.
   • Higher temperatures may soften surfaces, increasing contact and friction.
   • In some materials, heat reduces viscosity (as in lubricants), thus reducing friction.
   • Example: Brakes get hot after use because friction converts motion into heat.

Friction-Evil or friend?

Friction is often called a “necessary evil” because It is necessary for many everyday activities to function but it can also be undesirable or harmful in certain situations.

Why Friction is a friend:

1. Friction between our feet (or tires) and the ground prevents slipping while walking or driving.
2. Friction between fingers and an object helps in holding objects.
3. Pens and pencils need friction to leave marks on paper helps in writing
4. Friction is essential for slowing down or stopping a moving vehicle.
5. Friction allows belts and gears to grip and move properly.

Why Friction is evil:

1. Wears Out Materials: Friction causes wear and tear in shoes, tires, and machine parts.
2. Wastes Energy: It turns useful energy into heat (like in engines), reducing efficiency.
3. Requires More Force: Extra effort is needed to overcome friction in moving objects.
4. Heat Production: Unwanted heat from friction can damage machinery and cause overheating.

Increasing or Reducing Friction

Depending on the situation, friction can be either increased (to gain better grip or control) or reduced (to improve speed or efficiency).

 Methods to increase Friction (When More Grip is Needed):

1. Making Surfaces Rougher, Example: Adding treads to tires or roughening shoe soles.
2. Using High-Friction Materials, Example: Rubber on tires or grips on tools.
3. Increasing the Normal Force, more weight = more friction (e.g., pressing harder on brakes).
4. Using Textures or Grooves Example: Basketballs and car tires have patterns for better grip.
5. Removing Lubricants, Clean surfaces to increase dry contact.

Ways to Reduce Friction (When Smooth Movement is Needed):

1. Using Lubricants such as oils, grease, or wax reduce friction between surfaces (e.g., bike chains).

2. Making Surfaces Smoother by polishing or sanding surfaces decreases rough contact.
3. Using Rollers or Ball Bearings which converts sliding friction into rolling friction (used in machinery, drawers).
4. Using Streamlined Shapes reduces air or fluid resistance (a type of friction), helpful in vehicles.
5. Using Anti-friction Coatings like teflon (used in non-stick pans) reduces surface friction.

Purpose	Method	Example
Increase Friction	Rough surfaces	Shoe soles, sandpaper
	More pressure/weight	Pressing harder on brakes
	High-friction materials	Car tires, rubber grips
Reduce Friction	Lubrication	Engine oil, bike chain lube
	Rollers or ball bearings	Conveyor belts, fidget spinners
	Smooth/streamlined surfaces	Airplanes, racing cars

Question 1. Fill in the blanks.

Answers:

1. Friction opposes the relative motion between the surfaces in contact with each other.
2. Friction depends on the nature of surfaces.
3. Friction produces heat.
4. The sprinkling of powder on the carom board reduces
5. Sliding friction is less than static friction.

Question 2.

Four children were asked to arrange forces due to rolling, static, and sliding frictions indecreasing order. Their arrangements are given below.

Choose the correct arrangement.

 Answer: (c) Static, sliding, rolling

Question 3.

Alida runs her toy car on the dry marble floor, wet marble floor, newspaper, and towel spread onthe floor. The force of friction acting on the car on different surfaces in increasing order will be

Answer:

(a) Wet marble floor, dry marble floor, newspaper, and towel.

Question 4. Suppose your writing desk is tilted a little. A book kept on it starts sliding down.Show the direction of frictional force acting on it.

Answer: Fig. Frictional force is acting opposite to the movement of book i.e. upwards.

Question 5. You spill a bucket of soapy water on a marble floor accidentally. Would it make iteasier or more difficult for you to walk on the floor? Why?

Answer:

Soapy floor reduces the frictional force applied by the floor to the maximum. That is why it is difficult to walk on a soapy floor.

Question 6. Explain why sportsmen use shoes with spikes.

Answer: Sportsmen use shoes with a spike to increase the friction so, that their shoes do not slip while they run or play.

Question 7. Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?

Answer: Seema will experience more frictional force because the heavy object will be pressed hard against the opposite surface and produces more friction.

Question 8. Explain why sliding friction is less than static friction.

Answer: Suppose a box kept on the surface has to be pushed. If the box is slid then, when the box starts sliding, the contact points on its surface do not get enough time to lock into the contact points on the floor. So the sliding friction is slightly less than the static friction and we find it easier to move the box already in motion than to get it started (i.e., static friction.)

Question 9.Give examples to show that friction is both a friend and a foe.

Answer: Friction is a friend because:

• It allows us to walk comfortably on the ground.
• It allows us to grip and catch different objects.
• Things don’t move from their places because of friction.
• Speeds of moving objects can be minimized or stopped by friction only.

Friction is a foe because:

• It wears out materials, whether they are screws, ball- bearings, or soles of shoes.
• It also produces heat, which increases the wear and tear of machine parts.
• It also causes much wastage of energy, because this heat is not utilized.

Question 10. Explain why objects moving in fluids must have special shapes.

Answer: When objects move through fluids, they have to overcome friction acting on them. In this process. They lose energy. To minimize friction, objects are given special shapes.