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Class 9 How Forces Affect Motion: cheat sheet
CBSE Science. Newton’s three laws of motion, inertia, force and acceleration, momentum, and why momentum is conserved in collisions and recoil.
Key ideas
- First law: an object stays at rest or keeps moving steadily unless a net force acts. This tendency is inertia, and more mass means more inertia.
- Second law: the net force equals the rate of change of momentum, F = ma.
- Third law: every action has an equal and opposite reaction, acting on a different body.
- Momentum p = mv has a direction. In a collision or explosion with no outside force, total momentum stays the same.
- A gun recoils because the bullet and gun get equal and opposite momentum.
- Increasing the time of impact (sand pit, catching a ball by pulling the hands back) reduces the force.
Remember
| Second law | F = ma; 1 N = 1 kg m/s² |
| Momentum | p = mv (kg m/s) |
| Impulse | F × t = mv − mu |
| Conservation | m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂ |
Watch out
- Thinking action and reaction cancel (they act on different bodies).
- Using grams instead of kilograms in F = ma or p = mv.
- Ignoring the direction (sign) of velocity in momentum problems.
Worked example
A rifle of mass 5 kg fires a bullet of mass 35 g at 600 m/s. Find the recoil velocity of the rifle. (Give the answer to 2 decimal places where needed.)
Working: total momentum stays zero: 5 × v = 0.035 × 600
Answer: 4.2 m/s
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