physics test chapter 6

Acceleration

A net (unbalanced) force acting on an object causes it to accelerate.

Balanced Forces

The object's motion doesn't change (it stays at rest or moves at constant velocity).

Unbalanced Forces

The object accelerates in the direction of the net force.

Newton's Second Law of Motion

The acceleration of an object is directly proportional to the net force and inversely proportional to the mass. Formula: F = m × a

Force and Acceleration Relationship

More force → more acceleration (if mass is constant); More mass → less acceleration (if force is constant).

Friction and Applied Force

Friction opposes applied force. If applied force is greater, the object moves; if equal, it moves at constant speed.

Causes of Friction

Microscopic roughness and molecular attraction between surfaces.

Inertia

The resistance of an object to a change in motion. Greater mass = greater inertia.

Inertia and Newton's Second Law

Objects with more mass (more inertia) need more force to accelerate.

Calculating Force

F = m × a

Calculating Acceleration

a = F/m

Vector

A quantity with both magnitude and direction (e.g., force, velocity, acceleration).

Finding Net Force (Straight Line)

Add if they're in the same direction; subtract if they're in opposite directions.

Finding Net Force (Right Angles)

Use the Pythagorean theorem: F_net = √(F_x² + F_y²)

Free Fall

Motion under the influence of gravity only (no air resistance). Acceleration = g = 9.8 m/s²

Object Falling with Air Resistance

Air resistance acts upward, reducing acceleration until terminal velocity is reached.

Weight

The force of gravity on an object. Formula: W = m × g

Difference Between Mass and Weight

Mass: amount of matter (kg), same everywhere; Weight: force of gravity (N), changes with location.

Mass and Rate of Fall

All objects fall at the same rate regardless of mass.

Frontal Area and Air Resistance

Larger frontal area → greater air resistance → slower acceleration.

Air Resistance Effect on Velocity and Acceleration

Increases with speed; reduces acceleration until velocity becomes constant.

Terminal Velocity

The constant speed of a falling object when air resistance equals its weight. At that point, acceleration = 0.

Pressure

The force per unit area applied on a surface. Formula: P = F/A

Units of Pressure

Pascal (Pa) = 1 N/m²

Force and Area Effect on Pressure

Increase force → increase pressure; Increase area → decrease pressure.

Example of Pressure in Action

A sharp knife cuts better because smaller area increases pressure for the same force.

Meaning of 'Inversely'

Two values change in opposite directions (one increases, the other decreases).

Free-Body Diagram

A diagram showing all the forces acting on an object with arrows representing vectors.

Air Resistance

A type of friction (drag) that opposes motion through air.

Acceleration When Air Resistance Equals Gravity

Acceleration becomes zero; the object falls at terminal velocity.

Acceleration Formula

𝑎 = 𝐹𝑛𝑒𝑡 / 𝑚

Example Problem (Acceleration)

A 20 kg cart is pushed with a net force of 60 N. Find its acceleration. 𝑎 = 60 / 20 = 3 m/s². ✅ Answer: 3 m/s²

Force Formula

𝐹 = 𝑚 × 𝑎

Example Problem (Force)

A 1,200 kg car accelerates at 2 m/s². Find the net force on the car. 𝐹 = 1200 × 2 = 2400 N. ✅ Answer: 2400 N

Friction & Applied Force

Friction opposes motion. Net force = applied force - friction.

Friction Formula

𝐹𝑛𝑒𝑡 = 𝐹𝑎𝑝𝑝𝑙𝑖𝑒𝑑 − 𝐹𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛

Example Problem (Friction) A 10 kg box is pushed with 80 N, and friction is 20 N. Find the acceleration. 𝐹𝑛𝑒𝑡 = 80 − 20 = 60 N; a = 60 / 10 = 6 m/s².

Answer: 6 m/s²

Example Problem (Friction Surface) If a wooden box slides on carpet and on ice with the same applied force, which surface has greater friction?

Answer: Carpet (rougher surface → greater frictional force)

Example Problem (Newton's Second Law) A 50 N force is applied to a 25 kg object. Find acceleration. 𝑎 = 50 / 25 = 2 m/s².

Answer: 2 m/s²

Example Problem (Inertia) Which requires more force to accelerate to 2 m/s²: a 500 kg car or a 1500 kg truck? 𝐹𝑐𝑎𝑟 = 500 × 2 = 1000 N; 𝐹𝑡𝑟𝑢𝑐𝑘 = 1500 × 2 = 3000 N.

Answer: Truck (more mass → more inertia → more force needed)

Example Problem (Force Calculation) A 2 kg skateboard accelerates at 4 m/s². Find the net force. 𝐹 = 2 × 4 = 8 N.

Answer: 8 N

Example Problem (Vectors) One person pulls a sled with 40 N east, another pulls 10 N west. Find net force and direction. 𝐹𝑛𝑒𝑡 = 40 − 10 = 30 N east.

Answer: 30 N east

Example Problem: Air Resistance Comparison

A feather falls slower than a rock due to greater air resistance relative to its weight.

Example Problem: Terminal Velocity

When a falling object's weight is 200 N and air resistance is also 200 N, the net force is 0 N and acceleration is 0 m/s².

acceleration

a=mF​

force

F=m×a

mass

m=aF​

pressure

P=AF​