Forces and Interactions
Core ideas
A force is a push or a pull described by both strength (measured in newtons) and direction. Forces may be contact forces (friction, air resistance, normal force, an applied push) or non-contact forces (gravity, magnetism, electric force). Gravity pulls any two objects with mass toward each other; near Earth, the gravitational force on an object is its weight. Mass (in kilograms) is the amount of matter and does not change with location; weight is a force and does change.
Most questions turn on net force, the single combined effect of all forces acting on one object. Add forces pointing the same way, subtract forces pointing opposite ways, and give the answer the direction of the larger force. Balanced forces (net force = 0) mean motion does not change: the object stays at rest or keeps moving at constant speed in a straight line. Unbalanced forces always change motion: speeding up, slowing down, or turning.
Newton's third law says forces come in pairs: if A pushes on B, B pushes back on A with equal strength in the opposite direction. Paired forces act on different objects, so they never cancel for a single object.
Traps to avoid
- Thinking zero net force means no motion; balanced forces allow steady, constant-velocity motion.
- Confusing mass and weight, or treating kilograms as a force.
- Assuming a heavier object always falls faster. Greater mass brings greater gravitational force but also greater resistance to a change in motion, so free-fall acceleration is the same.
- Canceling an action-reaction pair as if both acted on one object.
Rule of thumb and worked example
Give directions signs: right/up positive, left/down negative. A sled is pushed forward with 50 N while friction is 20 N backward and air resistance is 5 N backward. Net force = 50 - 20 - 5 = 25 N forward, so the sled accelerates forward.