Science · Class 9 · Chapter 6

How Forces Affect Motion

• Force — A push or pull, measured in newtons (N).
• Balanced / Unbalanced forces — Cancel out / leave a non-zero net force.
• Friction — A force opposing relative motion between surfaces.
• Newton's First Law — No net force means no change in motion (inertia).
• Newton's Second Law — F = ma — force, mass, and acceleration linked.
• Weight — The gravitational force on an object, F = mg.
• Newton's Third Law — Every force has an equal, opposite reaction force.
• Tension — The internal pulling force transmitted through a string.

Chapter 6 · Concept 1 of 11

The Concept of Force

• A force can set an object moving, change its speed or direction, or change its shape.
• Direction always matters — Force is measured in newtons (N) and always needs a direction along with its magnitude — like friction opposing motion, or gravity pulling downward.

Chapter 6 · Concept 2 of 11

Balanced and Unbalanced Forces

• Balanced forces — Equal in magnitude, opposite in direction — they cancel out, so the object doesn't move (or its motion doesn't change).
• Unbalanced forces — A non-zero net force results, and the object moves (or accelerates) in the direction of the larger or combined force.

Chapter 6 · Concept 3 of 11

Finding the Net Force

• Two forces on a block | Net force
• 10 N and 6 N, same direction — 10 + 6 = 16 N, in that direction
• 10 N right and 6 N left — 10 − 6 = 4 N, towards the right
• 6 N right and 10 N left — 10 − 6 = 4 N, towards the left

Chapter 6 · Concept 4 of 11

The Force of Friction

• Push a box and it may not move — friction between the box and the floor opposes your applied force.
• Motion needs a net force — The box starts moving only once your push exceeds friction. Once moving, if you stop pushing, friction alone brings it to rest — this is why a rolling ball or a coasting bicycle eventually stops.

Chapter 6 · Concept 5 of 11

Newton's First Law of Motion

• The law — An object at rest remains at rest, and an object in motion continues to move with constant velocity, unless a net force acts upon it.
• Named for a tendency to resist change — Newton called this tendency inertia — first argued by Galileo, who reasoned a moving object needs no force to keep moving forever, if nothing opposes it.

Chapter 6 · Concept 6 of 11

Newton's Second Law of Motion

• The law — A net force accelerates an object in its own direction. Acceleration is proportional to the net force, and inversely proportional to the object's mass.
• The equation — a = F ÷ m, or F = ma 1 newton is the force that gives a 1 kg object an acceleration of 1 m/s².

Chapter 6 · Concept 7 of 11

Weight: The Force of Gravity

• Falling objects accelerate under gravity at g ≈ 9.8 m/s² (often rounded to 10 m/s² for quick estimates).
• F = mg — Example 6.4: A 30 kg barbell needs an upward force of F = mg = 30 kg × 9.8 m/s² = 294 N from the weightlifter, just to hold it steady.

Chapter 6 · Concept 8 of 11

Slowing Down Gently Reduces Force

• A longer stop, a smaller force — A fielder pulls their hands back while catching a fast ball. Stretching out the time it takes the ball's velocity to reach zero reduces the acceleration — and so the force needed.
• Airbags — and coconuts — A car airbag cushions a passenger, increasing stopping time to lower the force of impact. A coconut cracked on hard ground stops almost instantly — needing a very large force, which breaks its shell.

Chapter 6 · Concept 9 of 11

Newton's Third Law of Motion

• The law — Whenever one object exerts a force on a second object, the second object simultaneously exerts an equal and opposite force on the first.
• Two forces, two different objects — These paired forces act on different objects — so unlike balanced forces on one object, they never cancel each other out.

Chapter 6 · Concept 10 of 11

Action and Reaction, Side by Side

• Paddle pushes water backwards
• Water pushes paddle (and canoe) forward

Chapter 6 · Concept 11 of 11

Forces on a System of Objects

• Two boxes connected by a string, pulled by force F: treat internal tension as invisible, and the whole system as one object.
• a = F ÷ (m₁ + m₂) — The system accelerates just like one object of combined mass.
• Box 2 → Box 1

Quick Recap

Check Your Understanding

1. 1Using a horizontal force F, a table is moved across the floor at a constant velocity. How much is the frictional force exerted by the floor on the table?
2. 2100 oarsmen row a snake boat, each applying a horizontal force of 200 N. 95 row backwards (to propel the boat forward) but 5 mistakenly row in the opposite direction. What is the net force on the boat?
3. 3A sailor jumps out from a small boat forward, onto the shore. Will the boat move? If yes, in which direction, and why?