The Physics
Hypertextbook
Opus in profectus

# Forces

## Summary

• Newton's first law of motion (also known as the law of inertia) states that…
• an object at rest tends to remain at rest and…
• an object in motion tends to continue moving with constant velocity…
• unless acted upon by a net external force.
• In general, inertia is resistance to change.
In mechanics, inertia is resistance to change in velocity (resitance to acceleration).
• Moving objects do not need to be pushed to continue moving.
• A constant velocity of zero (at rest for an extended period of time) is one type of constant velocity.
• In general, a force is an interaction that causes a change.
In mechanics, a force is an interaction that causes a change in velocity (an interaction that causes acceleration).
• Forces must be external to cause a change. (Objects cannot push themselves.)
• When more than one force is present, it is the net force (combination) that matters.
• Supplemental Materials
Some common forces
name/symbols when/where direction
weight W, Fg due to gravity down
normal N, Fn surfaces in contact normal to surface
friction ƒ, Fƒ surfaces in contact tangent to surface
tension T, Ft strings, ropes, cables, etc along the axis
elasticity Fe, Fs springs, rubber bands, etc. along the axis
buoyancy B, Fb immersed in a fluid up
drag R, D, Fd moving through a fluid opposite velocity of object
lift L, F moving through a fluid perpendicular to flow
thrust T, Ft pushing a fluid opposite velocity of fluid