Questions: Newton's First Law: The Law of Inertia

Question 1

A hockey puck slides across nearly frictionless ice at constant velocity. What is the net force on it?

• A. A net force in the direction of motion, maintaining its velocity
• B. Zero net force — nothing is changing its velocity
• C. A small net force opposing motion, which will eventually stop it
• D. A net force equal to its weight

Answer: (B)

Newton's First Law: constant velocity means zero net force. The puck continues moving because no net force acts on it — not because some force is 'keeping it going.' Option A is the Aristotelian misconception: that motion requires a continuous cause. The near-frictionless ice eliminates the decelerating force that would otherwise change the velocity.

Question 2

A book rests motionless on a table. Which statement correctly describes the forces on it?

• A. No forces act on the book because it is not moving
• B. Only gravity acts, since the table is rigid and cannot exert force
• C. Gravity pulls down and the normal force pushes up; these cancel to give zero net force
• D. The table pushes up harder than gravity pulls down, keeping the book stationary

Answer: (C)

A stationary object does NOT mean zero force — it means zero NET force. Gravity and the normal force are both real and present; they are equal and opposite, summing to zero. Option A is the most common misconception. Option D is wrong because unequal forces would produce acceleration, not rest.

Question 3

An object moving at constant velocity is expected to have a net force acting on it to maintain that motion.

• T. True
• F. False

Answer: False

This is Aristotle's view, not Newton's. Newton's First Law states that zero net force means constant velocity — including nonzero constant velocity. Force is required only to CHANGE velocity (accelerate or decelerate), not to sustain it. The illusion that motion requires force comes from everyday experience where friction is always present, decelerating moving objects.

Question 4

An inertial reference frame is one in which Newton's laws hold as stated — specifically, a frame that is not itself accelerating.

• T. True
• F. False

Answer: True

Newton's First Law implicitly defines inertial frames as its domain of validity. In a non-inertial frame (like a braking car), objects appear to accelerate without any applied force — a violation of the First Law. Newton's laws apply cleanly only from frames that are not themselves accelerating. This is a foundational constraint for all subsequent mechanics.

Question 5

Why does everyday experience mislead people into thinking that objects require a continuous force to keep moving? What is the Newtonian correction?

Model answer: Everyday objects slow down because friction is always present — it is a real decelerating force. Aristotle observed that moving objects come to rest and concluded that rest is natural and motion requires a continuous cause. Newton recognized that friction is the cause of deceleration, not the absence of a 'driving force.' On a frictionless surface in space, an object would continue at constant velocity indefinitely. The Newtonian correction: force causes change in velocity (acceleration), not velocity itself.

The key reframe is: instead of asking 'what keeps things moving?' ask 'what changes their motion?' Inertia is the tendency to resist changes in motion — which means constant motion (including rest) is the natural state, and force is what departs from it.