Lesson 3 of 6 · Inertia
Explain motion after the push ends
8 minute lesson · 3 checks along the way
Your goal for this lessonApply the first law in an appropriate frame without confusing coasting, force balance, and orbital free fall.
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Separate maintaining motion from changing it
Everyday experience can suggest that movement needs continuous pushing. A shopping cart slows after you let go, and a sliding book eventually stops. Those observations include interactions with wheels, surfaces, and air. They do not show that motion naturally runs out. To investigate the underlying rule, first account for the forces acting after the original push has ended.
In an , an object with zero net external force keeps the same : both its speed and direction remain unchanged. An object already at rest remains at rest in that frame. This is Newton’s first law. Inertia names resistance to a change of velocity; it is not an additional forward force stored inside the object by the person who pushed it.
Choose a frame where the rule applies
An inertial frame is one in which a force-free object travels uniformly along a straight line. A turning or accelerating cabin is not such a frame. If a spacecraft changes course, a loose object can move relative to its walls even without a new push from those walls. Treating the cabin as perfectly inertial would hide the cause of that relative movement.
Near Earth, astronauts and spacecraft experience gravity. Bruce McCandless II’s untethered flight during STS-41-B did not remove Earth’s pull; his path remained orbital. For brief movements over small distances, a freely falling local frame can be a useful approximation if tidal differences and frame rotation are negligible. That approximation must not become a claim that an entire orbit is straight-line force-free motion.
Bruce McCandless II uses the Manned Maneuvering Unit during STS-41-B in February 1984 (S84-27017). This archived color photograph shows untethered flight above Earth. Untethered describes the missing physical connection to the Shuttle; it does not mean gravity or orbital motion disappeared.
Look closely
Which visible hardware suggests a way to change relative motion, and which force remains even without a tether?
See what to notice
The maneuvering backpack provides propulsion for changing motion. Earth’s gravitational attraction remains. A still image cannot tell us which thrusters were firing or the astronaut’s instantaneous acceleration.
Check 1 of 3
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Work through a decision
Predict a cart after release
Imagine a horizontal laboratory air track whose vertical support balances gravity and whose horizontal resistance is negligible for this exercise. A cart leaves your hand traveling right at 0.4 m/s. The hand then stops touching it. Predict its horizontal during the next 5 s: with zero net horizontal force, its velocity stays 0.4 m/s and it travels 2 m right.
Now compare a real measurement: the cart travels right but slows steadily. The measured velocity change means the zero-net-horizontal-force model is incomplete. A slight track slope, contact friction, or air resistance could explain it; additional checks are needed to distinguish them. “Its faded” supplies neither a measurable interaction nor a prediction for a revised experiment.
Check 2 of 3
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Interpret coasting without erasing forces
The McCandless photograph shows his maneuvering backpack and Earth below. Thrusters could change his motion relative to Challenger; their absence from a particular frame would not prove zero . Nor does a nearly fixed camera imply zero velocity relative to Earth. The image invites a model of controlled relative motion, while the timing and forces require information beyond the photograph.
Zero can also mean several forces balance, not that no individual forces exist. A box resting on a level table has gravity downward and a upward. A cart moving steadily can have balanced horizontal pushes. For every proposed inertia explanation, name the object, frame, initial velocity, and relevant external forces. Then ask what would change if one of those assumptions failed.
Check 3 of 3
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Put it together
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