Slide 1
Constant straight-line motion needs no net force
An object moving in a straight line at constant speed needs no net force to keep that motion. This is Newton’s first law.
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Everyday objects often slow because friction or air resistance acts on them. Removing those influences changes the situation: slowing is an effect to explain, rather than motion’s inevitable natural ending.
Slide 2
Opposing forces can balance while an object moves
Net force is the combined effect of forces acting on one object. Opposing forces can balance even while the object moves.
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A car cruising steadily on level ground can have engine thrust balanced by resistance. Its engine still transfers energy, but its velocity stays constant because the horizontal forces sum to zero.
Slide 3
Net force changes velocity through acceleration
An unbalanced force changes velocity. With the same net force, a larger mass has a smaller acceleration than a smaller mass.
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For a constant mass, acceleration equals net force divided by mass. An ideal 2-kilogram cart subject to 6 newtons accelerates at 3 metres per second squared, provided all relevant forces are included.
Slide 4
Newton’s first law uses a nonaccelerating reference frame
Rest and motion are described relative to a reference frame. Newton’s first law takes its simple form in a nonaccelerating frame.
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A passenger on a smoothly moving train can treat the carriage as nearly stationary for local experiments. When the train brakes, objects seem to surge forward because the carriage’s frame is accelerating.
Slide 5
Forces explain changes in motion
Forces explain changes of motion, not the mere presence of motion. The useful question is which interactions are acting now.
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Action and reaction forces act on different objects. The floor pushes a walking person forward while the person pushes the floor backward; those forces do not cancel within a force diagram of the person alone.
Sources
- Newton’s Laws of Motion
NASA
- Momentum and Its Conservation
The Physics Classroom