Physics - Chapter 5

Chapter 5 of the Pearson Physics textbook by James S. Walker

5.1 §

Newton’s First Law §

Newton’s first law describes how objects are affected by forces but what exactly is a force? force is a push or pull

• An object at rest remains at rest as long as no net force acts on it
• An object moving with constant velocity continues to move with the same speed and in the same direction as long as no net force acts on it

net force - net force exerted is the vector sum of all individual forces that act on it

An object’s motion does not change on its own: If you push a box it doesn’t stop by itself it’s because friction is acting upon it slowing it down

Newton’s first law is known as law of inertia

Law of Inertia §

Inertia = “laziness”

Matter is lazy as it doesn’t change forces unless forced to do so

Inertia - The tendency of an object to resist any change in its motion

Newton’s 2nd Law §

Acceleration is caused by a single force in the form of the equation a = $\frac{F}{m}$

acceleration is proportional to force and inversely proportional to mass in this equation as desired

Newton’s 2nd law (for a single force)

force = mass x acceleration

F = ma

Force is measured with a unit called the newton, 1 newton is defined as the force required to give 1 kg of mass an acceleration of 1 m / s${}^2$

sum of forces = mass x acceleration

The sum of forces (net force) acting on an object is equal to its mass times its acceleration

• $\sum$ F${}_x$ = ma${}_x$
• $\sum$ F${}_y$ = ma${}_y$

Newton’s Third Law §

Newton’s third law describes the forces that objects exert on one another

• For every action force on an object, there is a reaction force acting on a different object
• The action and reaction forces are equal in magnitude and opposite in direction

Exception to newton’s law are for objects that are very small like atoms and molecules

5.2 §

Weight is the measurement of the pull of Earth’s gravity on you

• The weight of an object is equal to its mass times acceleration
• weight = mass x acceleration due to gravity
• SI unit: newton (N)

5.3 §

Friction - force that opposes the motion of one surface over another

Kinetic Friction - Friction encountered when surfaces slide against one another

force of kinetic friction = coefficient of kinetic friction x normal force

f${}_k$ = u${}_k$ N

static friction - Force that opposes the sliding of one nonmoving surface past another

maximum force of Static Friction

maximum force of static friction = coefficient of static friction x normal force

f${}_{s,max}$ = u${}_s$ N

In most cases, u${}_s$ is greater than u${}_k$. Meaning that the force of static friction is usually greater than the force of kinetic function