Lesson 4 of 6 · Newton’s laws
Build a force explanation for one object
8 minute lesson · 3 checks along the way
Your goal for this lessonDistinguish net force on one object from interaction pairs, and calculate acceleration for constant mass.
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Name the object before adding forces
’s laws connect a description of movement to its causes. Start by choosing an object or system: a sample container, an astronaut, or a spacecraft with its cargo. A includes external forces acting on that chosen system. It does not automatically include every nearby force or every force the system exerts on its surroundings. That boundary is part of the reasoning.
For a constant- object in an , the vector sum of external forces equals mass times acceleration: Fnet = ma. One newton is one kilogram meter per second squared. In one dimension, assign opposite signs to opposite directions before adding forces. Two large opposing forces can leave a small net force, so the strongest visible push alone does not determine the acceleration.
Connect all three laws
The first law describes motion when the net external force is zero. The second predicts how a nonzero changes . The third describes an interaction: if object A pushes object B, B simultaneously pushes A with equal magnitude in the opposite direction. Those partner forces act on different objects, which is why they do not cancel within a force diagram for B alone.
Ed White carried a hand-held maneuvering unit outside Gemini 4. Expelling gas provided a way to change motion without pushing against surrounding air. The unit acted on gas, and gas acted back on the unit and astronaut. A tether could supply another external force. Identifying the propulsion interaction does not justify ignoring every other force or treating the photograph as a thrust measurement.
Ed White performs his Gemini 4 spacewalk on June 3, 1965, in NASA photograph S65-34635. The handheld device and connecting lines are visible in this historical color frame. NASA identifies the device as the Hand-Held Self-Maneuvering Unit.
Look closely
For the astronaut-and-device system, where does the expelled gas belong when identifying a propulsion interaction pair?
See what to notice
Gas receives a force from the device, and the device receives an opposite force from gas. Once outside the chosen system, the gas’s force partner is not a second cancelling force on the astronaut. A tether and gravity require separate consideration.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Work through a decision
Correct a force-diagram calculation
A training rig has a 20 kg equipment cart moving on a level track. Vertical forces balance. Two measured horizontal forces act on the cart: 14 N right and 6 N left. Its net horizontal force is 8 N right, so its is 8/20 = 0.4 m/s² right. These are invented laboratory values; the rig is not an actual Gemini maneuvering system.
A student adds the cart’s 14 N leftward push on the operator and concludes that the cart cannot accelerate. Remove that arrow: it belongs on the operator’s force diagram, not the cart’s. The interaction pair is real, but its members cross the chosen boundary. If the cart initially travels left, this rightward acceleration initially reduces its speed rather than instantly making it move right.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Test what follows from equal forces
Two free carts pushing apart exert equal and opposite contact forces on each other. Their accelerations need not have equal magnitudes: with otherwise balanced forces, the less massive cart accelerates more. For masses 20 kg and 40 kg experiencing a 10 N contact force, their acceleration magnitudes are 0.5 and 0.25 m/s². Equal force is not a statement of equal mass or equal motion.
This course uses constant masses and everyday speeds. A firing rocket loses propellant, and a full propulsion calculation must track expelled material and momentum transfer consistently. The basic habit remains useful: state the system, identify external interactions, keep directions, and separate force from velocity. When a calculation contradicts an observation, check those choices before assuming that a physical law has failed.
Check 3 of 3
10 pointsUse the reading above. You can try again without losing points.
Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
Go to the source
Explore the evidence behind this lesson. These optional readings and videos open in a new tab.
- Read: NASA Glenn: Newton’s lawsThe three laws, vector quantities, and the constant-mass form of the second law.
- Read: OpenStax: interaction pairs and system boundariesWhy equal and opposite partners belong to different objects.
- Read: NASA: the first U.S. spacewalkHistorical evidence for Ed White’s maneuvering unit and Gemini 4 photographs.