2.4 - Inertia

Inertia and Newton’s First Law of Motion

• Inertia is an object’s tendency to retain its state of rest or straight-line motion at constant velocity.
• Newton’s First Law of Motion: an object remains at rest or moves at uniform velocity unless an external force acts on it.
• When a tablecloth is jerked quickly, objects on it tend to remain at rest. Similarly, a coin tends to remain at rest when the thin cardboard supporting it is jerked horizontally.

Mass and the Inertial Balance

• Greater mass means greater inertia: a bowling ball is harder to set in motion or stop than a lighter football.
• The horizontal oscillation of an inertial balance is not affected by gravitational force; its period depends on the mass attached.
• An inertial balance can determine astronauts’ body mass from their oscillation periods in a spaceship.

Experiment 2.2: Relationship between Inertia and Mass

• Inference: inertia depends on mass.
• Hypothesis: the greater an object’s mass, the greater its inertia.
• Aim: investigate the relationship between inertia and mass.
• Manipulated variable: mass of plasticine, $m$.
• Responding variable: period of oscillation, $T$.
• Constant variable: distance between the G-clamp and plasticine.
• Apparatus: stopwatch, G-clamp, ruler and jigsaw blade.
• Materials: plasticine of masses 20.0, 30.0, 40.0, 50.0 and 60.0 g.

1. Clamp the jigsaw blade to a table leg and attach 20.0 g of plasticine to its free end. Keep the clamp-to-plasticine distance at 20 cm.
2. Displace the free end horizontally and release it to oscillate.
3. Record time $t_1$ for 10 complete oscillations.
4. Repeat the timing to obtain $t_2$.
5. Repeat with the other four masses, keeping the clamp-to-plasticine distance fixed.
6. Calculate the mean time and period; analyse a graph of $T^2$ against $m$:

• Record each mass, the two times, mean time and period.
• Conclusion: inertia increases with mass; the balance’s horizontal oscillation depends on the attached mass.

Effects in Daily Life

Situation	Explanation or protective measure
Wet umbrella	When rotation stops suddenly, moving water droplets continue moving and leave the umbrella.
Bus passengers	A sudden start throws passengers backwards as their bodies tend to remain at rest; a sudden stop throws them forwards as they tend to keep moving.
Sauce bottle	Sauce continues downwards and leaves the bottle when the downward-moving bottle stops suddenly.
Oil tanker	Separate tank compartments reduce the impact of the petrol’s inertia on the tank walls during sudden stopping.
Roller coaster	Restraints keep riders seated when speed and direction change suddenly.
Car occupants	Seat belts prevent occupants from being thrown forwards and injured when a car stops abruptly.