Frictional forces

 

What are frictional forces?

Moving objects face forces that act against them, except when in a vacuum such as in space. These are frictional forces and act in the opposite direction to the movement. Frictional forces make it harder for objects to move. Frictional forces are greater on rough surfaces than on smoother surfaces.

 

Friction

Friction is a contact force, it acts against a moving object.

Sometimes, friction can be helpful:

 

• Cars need friction between their tyres and the road to stop them from skidding,
• We need friction between our shoes and the floor to keep us from slipping.
• Friction between brakes and wheels help to slow cars and bikes down.

 

But friction can also be unhelpful:

 

• Friction between moving engine parts can cause them to heat up and wear out.

 

Limiting the amount of friction

We can reduce the amount of friction using lubricants such as motor oil. This can stop metal parts from rubbing and wearing out. Modern taps use moving parts made from a low friction plastic called PTFE instead of rubber or brass.

 

Examples of frictional forces

 

• Gripping Objects – You need friction to hold things. Otherwise, they would slip from your hands.
• Erasing – When you use an eraser on paper, it removes the lead particles (from the pencil) that stick to the paper because of friction.
• Smoothing rough surfaces – You can make rough or sharp edges smoother with sandpaper. Rough surfaces have more bumps than smooth surfaces. The sandpaper’s hard surface makes more friction, which scrapes off the bumps on rough surfaces.
• Tug of War – In this game, two teams pull a long rope. The friction between the rope and the players’ hands helps them keep their grip.

 

Air resistance and drag

Drag is the force that slows down an object when it moves through a fluid (like water or air). The object feels more drag when it moves faster. When the fluid is air, we call drag air resistance.

We can make the object more streamlined to reduce air resistance. This means the object has a shape that cuts through the air easily. This lets the object go faster with the same push force. Cyclists and Formula 1 racers try to reduce air resistance. They make their cars, helmets and bikes streamlined so they can go as fast as possible.

 

 

Professional cyclists have specially designed helmets and bikes that are designed to cut through the air easily. This helps the cyclist go as fast as possible.

 

🧪 Knowledge Check: Frictional Forces

Test your understanding of friction, air resistance, and drag based on the lesson content.

1. In which direction does friction always act?

• In the same direction as the motion.
• In the opposite direction to the motion.
• Straight upwards towards the sky.

2. What happens to the frictional force as a surface becomes rougher?

• The frictional force increases.
• The frictional force decreases.
• The frictional force stays exactly the same.

3. Which of these is a common way to reduce friction between moving machine parts?

• Adding more weight to the parts.
• Using a lubricant like oil or grease.
• Making the surfaces as rough as possible.

4. Why are racing cars and planes designed with streamlined shapes?

• To increase the amount of air resistance.
• To reduce drag and allow them to travel faster.
• To make them look more colourful.

5. What is another name for the frictional force experienced by an object moving through air or water?

• Gravity
• Magnetism
• Drag

Click to Reveal Answers

1. In the opposite direction to the motion (Friction opposes the movement of an object).
2. The frictional force increases (Rougher surfaces create more resistance when rubbing together).
3. Using a lubricant like oil or grease (Lubrication smoothes the contact between surfaces).
4. To reduce drag and allow them to travel faster (Streamlining helps air or water flow smoothly around an object).
5. Drag (This term covers both air and water resistance).

 

Summary:

 

• Frictional forces happen in many situations.
• We don’t slip when we walk and a cyclist slows down when they stop pedalling because of frictional forces.
• Friction is a force that touches an object and resists its movement.