Dynamics - Forces Theory

Force

* A pull or push
* Causes a change in the motion of the object, and is a vector
* Denotes as F with a subscript describing the type of force

Inertia

Tendency of objects to resist a change in their motion

* Not a force, just a property of all objects
* Amount of inertia depends on an object’s mass

Newton’s (actually Galileo) First Law of Motion

“An object will tend to remain at rest or in uniform motion unless it is acted on by an external force.”

Kinematics

### Branch of mechanics that examines the motion of objects without acknowleging the forces causing the motion.

Dynamics

Branch of mechanics that examines the motion of objects under the influence of forces.

Mechanics

Branch of applied mathematics that deals with motion of objects and the forces acting upon them.

System diagram

Real-life representation of an event

Free-body diagram

Simplified depiction of objects focusing on forces acting upon object.

Applied force

An applied force is a contact force that is applied to an object by external means.

Net force

Vector sum of forces acting on an object

Force of friction

Force that always opposes the motion of an object, resists the sliding/rolling of an object over another.

Normal force

Force that surfaces exert to prevent a solid object from falling through it; perpendicular to the surface.

Force of tension

Pulling force exerted by a string, rope or cable; direction is always going away from the object.

Force of gravity

Force of attraction to Earth proportional to object’s mass.

An object is in free fall when the only force acting on it is ______________

The force of gravity

Contact force

Any force that occurs as a result of two objects coming in contact (i.e. force of friction, normal force)

Non-contact force

Any force that can act on an object without coming in contact with it (i.e. force of gravity, electromagnetism)

Newton’s Explanation of Gravity

Force of attraction exerted by a mass on another object; commonly experienced by planets.

Einstein’s Explanation of Gravity

Space and time are intertwined like a fabric, and a large mass warps the fabric and creates a curvature of space. Another object would experience Fg inside this gravitational field.

Mass vs Weight

* Mass: amount of matter in an object (kg)
* Weight: force of gravity acting on a mass (N)

Feeling “weightless” examples

Rollercoasters, top of swing, bungee jumping, “vomit comet”

The value g

* g = acceleration due to gravity
* changes depending on distance from Earth’s center
* depends on the planet

Newton’s Law of Universal Gravitation

Any 2 masses exert a force on one another which is:

* Directly proportional to the product of the masses
* Inversely proportional to the square of the distance between them

Two types of frictional forces between two solid objects

* Static frictional force
* Kinetic frictional force

Static frictional force

Friction that resists when you initially move an object from rest

Kinetic frictional force

Friction that exists when the object is moving

What is surface friction dependent on?

* Material of surfaces (jagged surfaces vs smooth surfaces)
* Magnitude of the forces pressing the objects together (force of gravity, normal force, applied force)

What is surface friction not dependent on?

* Velocity
* Surface area
* Volume

What happens on an atomic level as two objects slide against each other?

The atoms interact to form relative strong attactive forces. When the object starts to move, new bonds are continually being formed and broken. The process repeats itself in rapid succession while the object is in motion → responsible for the noise that friction makes.

Static coefficient of friction is always __________ and the kinetic coefficient of friction of the same two object sliding together.

higher

Coefficients of friction are determined through:

experimentation with two surfaces in contact.

Non-inertial frame of reference

When an object is accelerating, Newton’s first law about inertia is no longer valid.

Newton’s 2nd Law

A net or unbalanced force acting on a mass results in acceleration of the mass in the direction of the net force.

Newton’s 3rd Law

For every action force, there is a reaction force equal in magnitude and opposite in direction.

* Interaction between 2 objects/substances

Forces act in paids as action/reaction forces → the forces act on different objects, meaning they don’t cancel each other out.

Examples of Newton’s 3rd Law

* Bullet and rifle: rifle pushes bullet out (AF) and bullet pushes back, causing recoil (RF)
* Trampoline: person jumps down on trampoline (AF) and trampoline bounces person upwards (RF)
* Swimming: person pushes down on water (AF) and water pushes back, propelling person forward (RF)