When something is in equilibrium, the vector sum of all forces acting on it = 0.
It moves with constant velocity.
Unit for force
kg m / s2
When things are at translational equilibrium, the vector sum of all forces = 0.
Translational Equilibrium for an incline
When things are at rotational equilibrium, the sum of all torques = 0.
Positive and Negative torques
Conventionally, positive torques act counterclockwise, negative torques act clockwise.
Newton's First Law on Equilibrium
The significance of Newton's first law on equilibrium is: things in equilibrium will remain in equilibrium unless acted on by an external force.
Newton's First Law on Momentum
The significance of Newton's first law on momentum is: things resist change in momentum because of inertia (try stopping a truck. It's not easy because it resists changes to its huge momentum).
Torque is the angular equivalent of force - it makes things rotate, have angular acceleration, change angular velocity and direction.
The lever arm consists of a lever (rigid rod) and a fulcrum (where the center of rotation occurs): F1L1 = F2L2
If you apply a force at a long distance from the fulcrum....
you exert a greater force on a position closer to the fulcrum
when there is no net gravitational force acting on you. Either you are so far out in space that there's no objects around you for light-years away, or you are between two objects with equal gravitational forces that cancel each other out.
this is what we "weightlessness" really means when we see astronauts orbiting in space. The astronauts are falling toward the earth due to gravitational forces (weight), but they are falling at the same rate as their shuttle, so it appears that they are "weightless" inside the shuttle.
Momentum = mv, where m is mass, v is velocity and the symbol for momentum is p.
Impulse = Ft, where F is force and t is the time interval that the force acts.
Impulse = change in momentum
Conservation of Linear Momentum
Total momentum before = total momentum after.
Perfectly elastic collisions: conservation of both momentum and kinetic energy.
Conservation of momentum only.
Kinetic energy is lost during an inelastic collision.