A moving object does not need a continuing push to keep moving. It needs a net force to change its velocity. Try three experiments, then separate the forces acting on each body.
01Without a net force, velocity stays the same.
In everyday life objects slow down because the floor and air exert forces. Remove those resistive forces. A stationary block stays still; once pushed, it keeps moving at a constant speed in a straight line after the push ends. This is Newton’s first law, also called the law of inertia.
Frictionless horizontal plane · no air resistanceClick the block to push
Drag to orbit · Home restores the view
Horizontal net force0 N
Velocity · right is +0.00 m/s
Acceleration0.00 m/s²
The floor grid stays fixed in the world. The camera follows after the block travels away, so the scrolling floor shows continued motion. The plane is idealized as endless; the block does not wrap around or bounce at a screen edge. Weight and the upward support force cancel vertically.
Velocity includes direction as well as speed. Zero net force preserves both. A body can have forces on it and still obey the first law if those forces add to zero. This statement uses an inertial frame: a reference frame that is not accelerating or rotating.
02Force changes velocity. Mass resists that change.
Acceleration means the change in velocity per second, measured in m/s². A net force produces acceleration in its direction. With the same mass, twice the net force gives twice the acceleration. With the same force, twice the mass gives half the acceleration.
Fnet=∑F=ma,1N=1kgms−2
Fₙₑₜ is the vector sum of external forces on this body; m is its mass; a is its acceleration. The law is supported by experiment. Choosing the SI unit newton makes the proportionality constant 1—we are not discarding an unknown physical constant. For constant mass, zero net force gives zero acceleration, agreeing with the first law.
Equal forces · equal time · two frictionless lanes0.00 s
Drag to orbit · Home restores the view
Gold block · 2 kg
a=0.00m/s2
v=0.00m/s,x=0.00m
Blue block · 8 kg
a=0.00m/s2
v=0.00m/s,x=0.00m
Both pushes last exactly 1.00 s, then both blocks coast for 0.50 s. The whole experiment plays in 3 seconds. Engraved stripes identify the carriers; the mass labels, not their drawn size, specify inertia. The vertical support and weight balance in each lane.
03An interaction always involves two bodies.
When body A exerts a force on B, B simultaneously exerts an equally large, opposite force on A. Neither force waits for the other. They do not cancel in one body’s force balance, because they act on different bodies. Try pulling a heavy load, and identify every source and receiver.
FA→B=−FB→A
Heavy load on a frictionless base · person holds a ropeRight is positive · 0.00 s
Drag to orbit · Home restores the view
Person · 60 kg
Fnet=120.0−109.1=10.9N
a=0.182m/s2,v=0.00m/s
Load · 600 kg
Fnet=T=109.1N
a=0.182m/s2,v=0.00m/s
With grip, the person pushes the ground left; the ground pushes the person right. The taut rope transmits tension T and both bodies accelerate together. The load has no ground friction here, so mass is the reason its response is small. At rest, the arrows show the force arrangement to be applied on release.
f−T=mpa,T=Ma⇒a=mp+Mf
Here f is ground friction on the person, T is rope tension, mₚ is the person’s mass, and M is the load’s mass. Large mass gives small acceleration for the available external force. Starting from rest, this means a small speed after a short time—not a permanent speed limit. On ice, equal forces give very unequal accelerations.
If action and reaction are equal, why can anything accelerate?
Choose one body. Its reaction partner acts on another body and is not included in its net force. For person plus load together, rope forces are internal and cancel in the total; ground friction remains an external force. With no grip, their total momentum stays fixed even though each body accelerates.