Forces
Different Forces Covered
A contact force is any force that requires contact to occur. A non-contact force is a force is any force that does not requires contact to occur. In this unit, we will mainly focus on the following forces.
- Applied Force and Tension
- Weight
- Normal Force
- Frictional force
- Spring Force
- Drag Force
- Buoyancy
In the unit of mechanics, it is important to know different forces and understand how to apply Newton's laws.
https://www.pinterest.com/pin/427912402092848772/
Weight
- The weight of an object is defined as the force of gravity on the object and may be calculated as the mass times the acceleration of gravity, W = mg. Weight results from the gravitational attraction between a body of mass and a planet.
- The weight of a body is given by the formula of W=mg, where m is mass and g is gravitational acceleration.
- Since the weight is a force, its SI unit is the newton. In some textbook, weight W might be represented by a capital letter of G, standing for gravity. However, weight and gravity are still are different quantities.
- Weight is non-contact force because it does not require physical contact. In another word, both people standing on the street and birds flying in the sky also receive a force of gravity by earth.
https://www.tes.com/en-nz/teaching-resource/physics-equation-posters-aqa-triple-synergy-and-trilogy-11736677
Extension
- In the formula of W=mg, gravitational g is assumed as a constant variable. However, the value of gravitational acceleration decreases as the altitude increases, so the formula is no longer accurate. Instead of using the formula W=mg, scientist uses the universal gravitational constant to calculate the force of gravity.
- For more detail about gravitation, please go to the chapter of gravitation.
A Fun Investigation
- Gravitational acceleration decreases as the altitude increases because gravity decreases. Design an investigation that explores the relationship between gravitational acceleration and altitude or an experiment that allows you to measure the value of gravitational acceleration by earth.
Normal Force
The normal force is a contact force exert that prevent solid objects from passing through each other, and the direction of the force is alway perpendicular to their contacting surfaces. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. If two surfaces are not in contact, they can't exert a normal force on each other. On occasions, a normal force is exerted horizontally between two objects that are in contact with each other. For instance, if a person leans against a wall, the wall pushes horizontally on the person.
Figure 1: Normal force on a horizontal surface
Figure 2: Normal force in an inclined place
Credit for the two images: https://energyeducation.ca/encyclopedia/Normal_force
Friction
Friction is a force that resists the sliding or rolling of one solid object over another. They are two types of friction: kinetic and static friction (also known as dynamic friction). In most cases, the friction force opposes the motion of an object, and it is always prependicular to the direction of friction.
Challenging Questions
- The relationship between static friction coefficient and kinetic friction coefficient?
- How could you design an experiment to testify your hypothesis?
- Why is static friction coefficient always larger than kinetic coefficient coefficient?
- How could you explain this?
Further Study
Friction at Nanoscale: Lecture Form MIT (Highly Recommended)
New nanogenerator harvests energy from friction
Fun Experiment at Home
Objects in An Inclined Plane
Practice Problems Involving Friction
Practice Problems From IB QuestionBank
Spring Force
Spring force is proportional to its extension, calculated by the formula of F=-kx, where k is the spring constant and x is change in position. This relationship between spring force and extension is also known as the Hooke's law. The spring constant describes stiffness. In another word, how hard it is to pull a spring when you apply a force.
The investigation task above explores trajectory of a spring's periodic or harmonic motion. As can be seen, harmonic motion can be modeled by a cosine or sine trigonometric function, and this function can be explained in terms of energy, forces, and motion.
Drag Force
- Drag is a force acting opposite to the relative motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers (or surfaces) or a fluid and a solid surface
- There are two types of drag, which are form drag and skin drag
- Form drag is caused by the resistance of fluids (liquids or gases) to being pushed out of the way by an object in motion through the fluid
- Terminal velocity is reached when the resultant force of a moving object is zero.
Quadratic Drag
- In fluid dynamics, the drag equation is a formula used to calculate the force of drag experienced by an object due to movement through a fully enclosing fluid. The drag equation with a constant drag coefficient gives the drag force experienced by an object moving through a fluid at a relatively high velocity. For instance, high Reynolds number, Re > ~1000. This drag equation is called the quadratic drag, contributing to Lord Rayleigh, who originally used L2 in place of A.
- In the equation ρ( Rho) represents density of the fluid the object is moving through, C represents drag coefficient, v represents velocity of the moving object, and A represents cross-sectional area.
Stroke Drag
By <a href="//commons.wikimedia.org/wiki/User:AllenMcC." title="User:AllenMcC.">AllenMcC.</a> - <span class="int-own-work" lang="en">Own work</span>, CC BY-SA 3.0, Link
Drag fore, especially stroke drag will be discussed in more detail in the topic of fluid dynamics (Optional topic B3, engineering physics of IB students) when introducing concepts of turbulent, laminar, and ideal flow.