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💬 Think about it
To move a box you must touch it — but a magnet pulls iron from a distance, and the Earth pulls a falling apple without touching it at all. So forces split into two families: those that need contact and those that don't. Let's meet both.
What are the two main types of forces?
- Contact forces — act only when two objects touch.
- Non-contact forces — act even without the objects touching.
- Every force we study fits into one of these two families.
Two Families of Forces
Contact forces
Need touch: muscular force, friction
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Non-contact forces
Act from a distance: magnetic, electrostatic, gravitational
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All forces belong to one of these two groups
5.4.1 Contact forces
What are contact forces?
- Forces that act only when there is physical contact between objects.
- The contact can be direct, like using your hands.
- Or indirect, like using a stick or a rope.
Example:
Pushing a box with your hand or pulling it with a rope are both contact forces.
What is muscular force?
- The force produced by the action of muscles in our body.
- It works when muscles contract and elongate during activity.
- Walking, running, lifting, pushing and jumping all use it.
- Animals, birds, fish and insects use muscular force too.
How did humans use the muscular force of animals?
- Humans have long used animals' muscular force for many tasks.
- Bullocks pull ploughs and carts.
- Horses pull carriages carrying people.
➡ Ever heard of ...
- Muscular force works inside our body too.
- It helps us chew food and push it through the alimentary canal in digestion.
- The heart muscles expand and contract to circulate blood — essential for survival.
What is friction?
- The force that comes into play when an object moves or tries to move over another surface.
- It always acts opposite to the direction of motion.
- It is a contact force, since it arises from two surfaces in contact.
- It is what slows a rolling ball or a coasting bicycle to a stop.
Why does friction arise?
- It arises from tiny irregularities in the two surfaces in contact.
- Even surfaces that look smooth have many minute bumps.
- These irregularities lock into each other and oppose motion.
Does friction depend on the surface?
- Yes — the object stops after different distances on different surfaces.
- So friction depends on the nature of the surfaces in contact.
- Friction is greater on rough surfaces.
Example:
A box slides much farther on smooth glass than on rough sand, because sand gives more friction.
In this Activity, we will push a flat object and see what force brings it to rest.
Materials needed
An object with a flat base (an empty lunch box, geometry box, or a notebook) and a table or floor.
Procedure
1. Place the object on a table or floor.
2. Gently push it and observe. Does it stop after some distance?
3. Repeat by pushing the object in the opposite direction. Does it stop again?
2. Gently push it and observe. Does it stop after some distance?
3. Repeat by pushing the object in the opposite direction. Does it stop again?
Observation
The object stops after sliding a certain distance, in whichever direction you push it.
Explanation
A force acts between the surfaces in contact, opposite to the motion. This force, friction, brings the object to a stop.
◆ Summary
- Pushed object stops
- A force opposes its motion
- That force is friction
In this Activity, we will slide the same object on different surfaces to see how friction changes.
Materials needed
The flat-based object from Activity 5.3 and different surfaces such as glass, cloth, wood, ceramic tile, and sand.
Procedure
1. Try Activity 5.3 again, but place the same object on different surfaces.
2. Does the object stop after travelling the same distance as before?
3. Does it stop at the same distance on all surfaces?
2. Does the object stop after travelling the same distance as before?
3. Does it stop at the same distance on all surfaces?
Observation
The object stops after different distances on different surfaces.
Explanation
Friction depends on the nature of the surfaces in contact, and is greater on rough surfaces.
◆ Summary
- Different surfaces, different distances
- Friction depends on the surface
- Rougher means more friction
➡ A step further
- Friction does not act on solid surfaces alone.
- Air, water and other liquids also exert friction on objects moving through them.
- So aeroplanes, ships, boats and high-speed trains have special shapes to reduce this friction.
✅ Test Yourself
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What is a contact force?
View Answer
A force that acts only when there is physical contact between objects. -
What is muscular force?
View Answer
The force produced by the action of muscles in the body. -
In which direction does friction act?
View Answer
Opposite to the direction in which the object is moving or trying to move. -
Why does friction arise?
View Answer
Because of tiny irregularities in the two surfaces in contact that lock together. -
On which kind of surface is friction greater?
View Answer
On rough surfaces.
Important Definitions
- Contact force — A force that acts only when objects are in physical contact.
- Muscular force — The force produced by the action of muscles in the body.
- Friction — The contact force that opposes the motion of one surface over another.
📋 NCERT Question 7 — Why do we sometimes slip
Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain.
View Answer →
