Balanced and Unbalanced Forces & Friction – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Explain what balanced forces are. Use two daily life examples to show their effect on motion.
Answer:
Balanced forces mean the net force is zero on the object.
The object at rest stays at rest. The object in motion moves with constant velocity.
Example 1: A book on a table. Gravity pulls down. The table pushes up with an equal and opposite force.
So, the forces cancel, and the book does not move.
Example 2: A car at constant speed on a straight road. The engine’s push equals the friction/air resistance.
The forward and backward forces are balanced, so speed and direction stay the same.
No change in motion happens unless the balance changes.
Q2. What are unbalanced forces? Explain how they start motion or change motion with examples.
Answer:
Unbalanced forces mean the net force is not zero.
They cause an object to start moving, speed up, slow down, or change direction.
When you push a box and your push is greater than friction, it accelerates forward.
When you stop pushing, friction is unbalanced against motion, so it slows down.
A ball thrown up slows down going up because gravity is unbalanced.
The same ball speeds up coming down due to gravity, an unbalanced force again.
Unbalanced force is the reason for any change in motion.
Q3. Describe static friction and kinetic friction. How are they different in size and effect?
Answer:
Friction opposes motion between surfaces in contact.
Static friction acts when the object is at rest. It prevents starting motion.
It adjusts up to a maximum value to oppose your push.
Kinetic friction acts when the object is sliding. It resists motion but is usually less than maximum static friction.
So, it is harder to start motion than to keep moving.
Static friction gives grip, like feet on the ground.
Kinetic friction causes sliding and more wear and heat during motion.
Q4. Explain how friction can be both useful and harmful. Give suitable examples.
Answer:
Useful: Friction gives grip for walking. Your shoes push back on the ground and you move forward.
Useful: Car brakes use friction to slow the wheels. This reduces speed safely.
Useful: Friction helps hold nails, screws, and belts in machines.
Harmful: Friction causes heating. Energy is wasted as heat.
Harmful: It causes wear and tear on tyres, shoes, and machine parts.
Harmful: It makes it harder to start moving heavy objects.
We often reduce friction with lubricants when it is harmful, and increase grip when it is useful.
Q5. A car moves at constant speed on a straight road. Identify all main forces and explain why the speed stays constant.
Answer:
Forces: Engine force forward, friction and air resistance backward, gravity down, normal reaction up.
Gravity and normal force are equal and opposite, so no vertical motion.
Engine push equals resistive forces, so net horizontal force = zero.
With balanced forces, the car keeps constant velocity.
If the engine increases push, forces become unbalanced, and the car speeds up.
If the driver reduces push, resistive forces win, and it slows down.
Constant speed needs balance between push and resistance.
High Complexity (Analysis & Scenario-Based)
Q6. In a tug-of-war, both teams pull with equal strength for some time, then Team A suddenly pulls harder. Explain the rope’s motion before and after the change.
Answer:
At first, both pulls are equal and opposite. The net force is zero on the rope.
The rope stays still or moves with constant velocity if it was already moving.
Forces are balanced, so there is no change in motion.
When Team A pulls harder, forces become unbalanced.
The net force points toward Team A, so the rope accelerates that way.
If Team A keeps the extra pull, the rope keeps speeding up toward them.
