A force is a push or a pull that can change the state of motion or the shape of an object. This chapter covers balanced and unbalanced forces, Newton's three laws of motion, inertia, momentum and the law of conservation of momentum.
A force of 10 N acts on an object of mass 5 kg. Find the acceleration produced and name the law used.
Given: F = 10 N and m = 5 kg.
Newton's second law of motion states that F = ma.
Rearranging for acceleration:
a = F/m = 10/5 = 2 m/s²
The acceleration acts in the same direction as the applied force.
Formula: F = ma (1 N = 1 kg·m/s²)
Final Answer: a = 2 m/s², obtained from Newton's second law (F = ma).
Common Mistake: Multiplying force by mass instead of dividing force by mass.
Exam Tip: Write the formula, then substitute, then answer with the unit — each step earns marks.
Q2. Question 2 — Inertia of Motion
Explain why passengers tend to fall forward when a moving bus stops suddenly.
While the bus is moving, the whole body of a passenger is moving forward with the bus.
When the brakes are applied, the feet of the passenger — being in contact with the floor — come to rest along with the bus.
However, the upper part of the body continues to move forward because of its inertia of motion, that is, its tendency to keep doing what it was already doing.
As a result the passenger leans or falls forward.
Final Answer: The lower body stops with the bus while the upper body keeps moving forward due to inertia of motion.
Common Mistake: Saying a force pushes the passenger forward — no forward force acts; it is inertia.
Exam Tip: Mention that this follows from Newton's first law (the law of inertia).
Q3. Question 3 — Conservation of Momentum
State the law of conservation of momentum and use it to explain why a rifle recoils when a bullet is fired.
Law: in the absence of an external unbalanced force, the total momentum of a system remains constant before and after an interaction.
Before firing, both the rifle and the bullet are at rest, so the total momentum of the system is zero.
After firing, the bullet moves forward with momentum m₁v₁.
Since the total momentum must still be zero, the rifle must gain an equal momentum in the opposite direction: m₁v₁ + m₂v₂ = 0.
The rifle therefore moves backwards — this backward motion is the recoil.
Because the rifle's mass is much larger than the bullet's, its recoil velocity is comparatively small.
Formula: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
Final Answer: Total momentum stays zero, so the rifle gains backward momentum equal and opposite to the bullet's forward momentum.
Common Mistake: Forgetting that the initial total momentum is zero when both objects start at rest.
Exam Tip: State the law first in one line, then apply it — statement and application are marked separately.
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