How does an object move when no force acts on it(or when the forces acting on it are balanced)? This is one of the most fundamental types of motion in physics.
Basic Knowledge
Uniform linear motion: Motion in a straight line at a constant speed.
Relationship between distance and time: s = vt ( s:distance, v:speed, t:time )
Graphs:
・Horizontal axis time, vertical axis speed: horizontal straight line(constant value)
・Horizontal axis time, vertical axis distance: straight line through the origin(proportional)
Law of inertia(Newton's first law): When no force acts on an object or the forces acting on it are balanced, an object at rest remains at rest, and an object in motion continues uniform linear motion.
Rest and uniform linear motion are treated as equivalent states of motion when no force acts.
Uniform linear motion（Motion in a straight line at a constant speed）
Inertia（Property of an object to maintain its state of motion）
Law of inertia（Newton's first law. Uniform linear motion continues when there is no force）
v-t graph（Graph with time on the horizontal axis and speed on the vertical axis. Uniform linear motion is a horizontal line）
s-t graph（Graph with time on the horizontal axis and distance on the vertical axis. Uniform linear motion is a proportional straight line）
Ideal frictionless state（A hypothetical condition important in academic study）
Further Study (Background・Significance)
Galileo Galilei recognized the law of inertia through experiments with inclined planes, and Newton formulated it as a law. Before this, Aristotle believed that force was necessary to keep an object moving, but this was incorrect. Force is needed not to maintain speed, but to change speed──this is the core of Newtonian mechanics.
Familiar examples of inertia:
・Bus suddenly starts: Your body remains behind(trying to remain at rest)
・Bus suddenly stops: Your body falls forward(trying to continue moving at a constant speed)
・Pulling a tablecloth: Uses the property of dishes to remain at rest
・In space, planets continue moving between planets by inertia even when propulsion is turned off
Inertia is proportional to mass. The heavier an object is, the harder it is to change its state of motion(its inertia is greater). This includes both being difficult to start moving and difficult to stop.
In a v-t graph, the enclosed area represents distance traveled. This useful property holds for all motion, not only uniform linear motion.
- Uniform linear motion: Constant speed, straight line
- Distance = speed × time ( s = vt )
- v-t graph is a horizontal line, and s-t graph is a proportional straight line
- Law of inertia = Newton's first law
- Without force, uniform linear motion continues
- Inertia is proportional to mass(the greater the mass, the greater the inertia)
- Area of a v-t graph = distance traveled
Points to Note (Common Confusions)
① “Force is needed to move” is incorrect. “Force is needed to change speed” is correct. ② Uniform linear motion is an ideal state in which friction and air resistance are ignored. In reality, speed gradually decreases. ③ Inertia is proportional to mass. The heavier an object is, the harder it is to change its state of motion. ④ The slope of a v-t graph is acceleration, and its area is distance traveled. Reading graphs is a frequent topic.
Practice
- What is the motion of an object called when no force acts on it?
- Which scientist proposed the law of inertia?
- Write the formula for finding the distance traveled in uniform linear motion.