Newton Flashcards
What are the four fundamental forces of nature, and where does each dominate?
Gravitational Force: Attracts objects with mass over long distances. Weakest force but acts over vast scales (e.g., galaxies, planets, weight on Earth).
Strong Force: Holds atomic nuclei (protons and neutrons) together. Strongest force, but only over nuclear distances (e.g., heat in stars).
Electromagnetic Force: Acts between charged particles. Dominates in everyday interactions (e.g., friction, tension, normal forces).
Weak Force: Involved in nuclear processes like radioactive decay. Acts over subatomic distances (e.g., particle interactions in stars).
Which fundamental force is responsible for your weight on Earth?
Gravitational force. It pulls objects with mass toward each other, causing weight (w = m * g, where g = 9.8 m/s² on Earth).
State Newton’s First Law and give its formula
Definition: If the net external force on a body is zero, it moves with constant velocity (including zero) and zero acceleration.
Formula: ΣF = 0 (vector sum of forces equals zero).
Example: A book at rest on a table stays at rest because the net force (gravity + normal force) is zero.
State Newton’s Second Law and its formula.
Definition: If a net external force acts on a body, it accelerates. The net force equals mass times acceleration.
Formula: ΣF = m * a (vector equation).
Component form: ΣF_x = m * a_x, ΣF_y = m * a_y.
Example: A 2 kg object with a 10 N force applied has a = 10 / 2 = 5 m/s².
Why is Newton’s First Law a special case of Newton’s Second Law?
If ΣF = 0, then m * a = 0. Since mass (m) is not zero, acceleration (a) must be zero, meaning constant velocity (including zero), which is Newton’s First Law.
State Newton’s Third Law and explain its key feature.
Definition: If body A exerts a force on body B, body B exerts an equal and opposite force on body A.
Formula: F_A on B = -F_B on A (equal magnitude, opposite direction).
Key Feature: These forces act on different bodies, not canceling each other out.
Example: You push a wall (F on wall), wall pushes back (F on you).
When do Newton’s laws hold, and when do they not?
Valid: In inertial reference frames (non-accelerating, e.g., Earth’s surface approximated as inertial).
Not Valid: In non-inertial frames (accelerating, e.g., a braking car) or at speeds near light (requires Special Relativity).
Example: In a turning car, objects slide due to inertia, not a force, in a non-inertial frame.
Define weight and give its formula
Definition: The gravitational pull on a body’s mass.
Formula: w = m * g (g = 9.8 m/s² on Earth).
Direction: Downward toward Earth’s center.
Example: A 5 kg object has w = 5 * 9.8 = 49 N.
Define normal force and its direction
Definition: Force perpendicular to a surface, opposing the force pushing a body into it.
Direction: Perpendicular to the contact surface.
Example: A book on a table has a normal force upward equal to its weight (49 N) if at rest.
Define tension and its direction.
Definition: Pulling force exerted by a rope or cable.
Direction: Parallel to the rope, away from the object.
Example: A 10 kg mass hanging from a rope has T = m * g = 10 * 9.8 = 98 N upward.
Define kinetic friction and give its formula
Definition: Force opposing motion when a body slides over a surface.
Formula: f_k = μ_k * n (μ_k = coefficient of kinetic friction, n = normal force).
Example: A 10 kg box (n = 98 N) with μ_k = 0.2 has f_k = 0.2 * 98 = 19.6 N.
Define static friction and give its formula
Definition: Force preventing motion up to a maximum value when a body is at rest.
Formula: f_s ≤ μ_s * n (μ_s = coefficient of static friction, f_s varies from 0 to max).
Example: A 10 kg box (n = 98 N) with μ_s = 0.4 has max f_s = 0.4 * 98 = 39.2 N.
A 50 kg crate on a truck (μ_s = 0.4, μ_k = 0.3) accelerates at 2 m/s² without slipping. What is the friction force?
Static friction applies (no slipping).
F_net = m * a = 50 * 2 = 100 N.
Friction provides this force: f_s = 100 N (≤ μ_s * n = 0.4 * 50 * 9.8 = 196 N, so it holds).
Define centripetal acceleration and give its formulas
Definition: Acceleration toward the center of a circular path, keeping an object in uniform circular motion.
Formulas: a_c = v² / R = ω² * R (v = tangential speed, R = radius, ω = angular velocity).
Example: v = 5 m/s, R = 2 m, a_c = 5² / 2 = 12.5 m/s².
What provides the centripetal force in circular motion?
Ordinary forces (e.g., tension, friction, gravity, normal force) provide F_c = m * a_c. No special “centripetal force” exists—it’s the net force toward the center.
How do you write Newton’s Second Law in vector components?
ΣF = m * a.
Choose axes (e.g., x horizontal, y vertical).
ΣF_x = m * a_x, ΣF_y = m * a_y.
Example: F_hand - f = m * a_x.
You walk on a floor without slipping. What friction force is in your free-body diagram?
Static friction from floor on shoes (f_s), forward, prevents slipping. Not kinetic (no sliding).
A crate on an accelerating truck doesn’t slip. What’s the friction direction?
Forward. Static friction (f_s) accelerates the crate with the truck (Newton’s Second Law).
Why do you feel thrown outward on a merry-go-round?
Inertia (Newton’s First Law). Your body wants to move straight, but the centripetal force (e.g., hands on bars) pulls you inward. “Centrifugal force” is not real—it’s the sensation of inertia.
Why don’t action-reaction pairs cancel each other?
They act on different bodies. E.g., you push a box (F on box), box pushes you (F on you)—separate effects.
What’s an inertial reference frame?
A frame where Newton’s laws hold—non-accelerating (e.g., Earth’s surface, approximately). Accelerating frames (e.g., turning car) appear to violate Newton’s laws.
