Equilibrium and Newton's First Law
A body with no resultant force stays still or keeps a constant velocity. Setting the components in two perpendicular directions to zero turns that into equations that find unknown forces.
What a learner can do afterwards
- Show three given forces are in equilibrium by resolving in two directions
- Find the tension in each of two strings holding a hanging mass
- Name every force on a book resting on a table and say what exerts it
1 · Read
A body with no resultant force keeps doing what it is doing: rest stays rest and steady motion stays steady. Equilibrium means the vector sum of the forces is zero. Test it by resolving in two perpendicular directions: both totals must be zero, so 5 N east cancels 5 N west and leaves any northward push unbalanced.
A 6 kg mass hangs at rest from two vertical strings sharing the load equally, with g = 10 N/kg. Its weight is 6 times 10 = 60 N downward. The two tensions add to 60 N upward, so each string pulls 30 N.
Name every force and what exerts it: on a book resting on a table, gravity pulls down from Earth and a normal force pushes up from the table. A puck gliding in a straight line at steady speed has balanced horizontal forces, since steady velocity means zero net force by the first law.
Two forces combine tip to tail: 6 N east and 8 N north give a 10 N resultant, since root (36 + 64) = 10. The balancing third force is 10 N pointing exactly opposite. Equal size, opposite direction, total zero.
Zero resultant means rest or steady motion, so resolve both directions and balance every push.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.