FORCES

Unit Introduction

This unit covers the various types of forces: normal forces, tension forces, friction, and gravitational forces. You will use force diagrams to determine net force and calculate gravitational forces from mass.

What is a Force?

A force is a push or pull on an object as a result of two objects interacting.

Force is a vector quantity because it has magnitude and direction. The SI unit of force is a Newton. One Newton of net force will cause a one-kilogram object to experience an acceleration of 1m/s². A Newton can also be expressed as a kg*m/s². The formula to calculate force is as follows:

Types of Forces

Normal Force

A normal force exists between two solid objects when their surfaces are pressed together due to other forces acting on one or both objects. The normal force is exerted by a surface to keep solid objects from passing through each other. It is perpendicular to the surfaces in contact. For example, when you fall onto the floor and feel pain, that is the result of the floor’s normal force. Another example of the normal force is when a book is sitting on top of a table. The table exerts a force on the book when the objects are in contact with one another. If two solid objects remain separate from one another, there is no normal force. The normal force is exerted as a result of contact.

Learn more about the normal force by reading the first two sections of this article.

Tension Force

Another contact force between objects is called the tension force. Tension force is transmitted when a cable, wire, string, or rope is pulled tight from forces on each end. An example of the tension force is a game of tug of war. The rope is pulled taut by people on both ends. A tension force only pulls; it cannot push since the rope, wire, etc., would go slack.

Read the first section of the following article to learn more about tension.

Gravitational Force

Gravitational force is the attraction force between all objects in the universe. Objects that are closer to one another have stronger gravitational force. The greater the mass of the object, the more gravitational force it has. The Sun’s gravitational force keeps the planets in our solar system in orbit. As objects gain distance between them, the gravitational force is less. Watch the following video clip to learn more about gravitational force.

Gravitational Force (03:56)

Gravitational force relates to Newton’s second law of motion. The law states that how much an object accelerates depends on the object’s mass and how much force is applied to it. The formula is f = ma. Related to gravity, you can substitute the “a” for acceleration with a “g” for acceleration due to gravity. Therefore, you can use the formula f = mg when solving problems dealing with the force of gravity.

Watch this video clip on Newton’s second law of motion:

To see what the F = ma formula looks like in action, complete the following PhET Simulation activity. Download the printable activity and answer the questions as you interact with the sim at the following website. Submit your work as question #15 in the assessment portion of the unit.

Printable: PHET SIMULATION DOCUMENT

The weight of an object is a force that can be calculated by multiplying the object’s mass by the force of gravity exerted on the object. We often think of measuring our “weight” when we are really measuring mass—how much matter is in our bodies. Weight is a vector quantity that measures the downward force we exert due to the Earth’s gravitational force. Watch the following video clip which further explains the differences between mass and weight.

Now, use the F = mg formula to solve the following problems.

1. What is the weight or force exerted by an object on Earth’s surface whose mass is 75kg?

2. What is the force on a 1 kg ball that is falling freely due to the pull of gravity?

3. What is the mass of a person who weighs 300N on Earth?

Solve the problems and submit your work as question #16 in the assessment portion of the unit.

Printable: F MG STUDENT DOCUMENT

Read the following NASA article about weight as it applies to an airplane.

https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/what-is-weight/

Friction Force

Friction is a force that opposes sliding motion between two objects. There are two types of friction known as static friction and kinetic friction. Static friction keeps an object at rest from moving. You must overcome the force of static friction to get an object in motion. Kinetic friction is an oppositional force between two moving objects. As the objects’ surfaces slide past one another, kinetic friction slows down the movement. The amount of friction depends on the materials of the surfaces in contact.

Learn more about friction by reading the first section of the following article.

Force Diagrams

Force diagrams are drawn to depict all the forces acting on an object, the force’s magnitude and direction. The net force is the sum of all the forces acting on the object. Balanced forces on an object produce no motion or no change in motion. Unbalanced forces on an object produce a change in motion (starting or stopping).

In order to draw a force diagram, you must understand the forces and their direction.

Select an object for which you want to draw a force diagram. Ask yourself: What are all the forces acting on this object? Draw the object, or simply draw a dot or box to represent the object. Then draw a vector arrow to represent each force. Make sure you label which force is represented by each arrow. If you know the values of the forces, include them. If you don’t know the values, write a positive (+) or negative (-) sign to indicate relative values of the forces.

The way the arrows are depicted in the examples above is how they are drawn in force diagrams.

PhET Simulation: Forces and Motion – Basics

You will revisit the PhET simulation called “Forces and Motion – Basics.” This time, click on “Net Force.”

Interact with the following simulation to learn more about net forces. Add people to each side of the tug-of-war rope and click the checkboxes for “sum of forces” and “values.” Experiment with how the number of people of various sizes affects the total force of the team, and the net force overall.

Once you are finished with the “Net Forces” section, examine the remaining sections of this simulation—motion and friction. Notice how in each section you have different colored vector arrows—these are your force diagrams in each example. You are able to see the object and the vector arrows, along with the magnitude and direction of the forces.

Reading and Activity: Force Diagrams

Read the following website and answer the questions. When you are finished, check your work against the solutions provided on the website.

http://www.physicsclassroom.com/class/newtlaws/Lesson-2/Drawing-Free-Body-Diagrams

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