Study guides › C/P › Physics
Forces & Newton's Laws
Forces explain why motion changes. You will draw free-body diagrams, apply Newton's three laws, handle friction, inclines, and ropes, and connect circular motion and gravity to the same rule: net force equals mass times acceleration.
Study this chapter in PrepMed (free)
What this chapter covers
- Newton's Laws & Free-Body Diagrams. Every force problem starts with one clean free-body diagram; get that right and the algebra is easy.
- Friction. Friction is the force that makes real problems messy; know when to use the inequality and when to use the equality.
- Inclined Planes. Tilt the axes to run along and perpendicular to the slope; every incline problem reduces to two short equations.
- Tension & Connected Objects. Ropes and pulleys simply transmit force; treat the whole system first, then isolate one piece to find the tension.
- Circular Motion & Gravitation. Circular motion needs a net inward force; gravity supplies it for planets, and the inverse-square law drives the numbers.
Key points to know
- Zero net force means constant velocity (or rest), not zero motion
- ΣF = ma, solved along each axis separately
- Static friction is variable: f_s ≤ μₛN, equal to the push while at rest
- Kinetic friction is constant: f_k = μₖN
- Along the slope: mg sinθ; perpendicular: mg cosθ
- N = mg cosθ on an incline, which is less than mg
- Ideal rope and pulley: one tension throughout
- Hanging mass at rest or constant velocity: T = mg
Try a practice question
An object moves at a constant velocity of 5 m/s along a straight line. What is the net force on it?
- Equal to its weight
- Zero
- Equal to 5 N
- In the direction of motion, and growing
Show the answer
B. Constant velocity means zero acceleration, so by the second law the net force is zero. The idea that motion requires a net force is the classic misconception the first law corrects.
Common questions
What does this chapter cover?
Forces & Newton's Laws covers Newton's Laws & Free-Body Diagrams, Friction, Inclined Planes, Tension & Connected Objects, Circular Motion & Gravitation.
What should I know first?
Zero net force means constant velocity (or rest), not zero motion ΣF = ma, solved along each axis separately Static friction is variable: f_s ≤ μₛN, equal to the push while at rest
24 original practice questions accompany this chapter in the app. Spaced review brings missed ideas back at the right time; it helps memory but cannot promise you will not miss an idea again.