introduction to forces assignment quizlet edgenuity

Want to create or adapt books like this? Learn more about how Pressbooks supports open publishing practices.

5 Chapter 5: Introduction to Forces

Textbook Chapter 5: Newton’s Laws of Motion

Section 5.1: Newton’s First Law

Textbook Section 5.2: Newton’s First Law

Textbook Section 5.4: Mass and Weight

A force is defined as a push or a pull on an object. This might be due to a direct interaction between two objects ( contact force ) or due to a long-range force such as gravity. The contact forces we will consider include the normal, friction, and tension forces.

Forces are vector quantities with both magnitude and direction. Forces are added as vectors.

Newton’s First Law: the law of inertia. An object in motion (has a constant or zero velocity) will remain in motion unless acted upon by an outside force.

Newton’s first law tells us that any change in velocity (magnitude, direction, or both) must be due to a force acting on the object. What forces might act on a book pushed across a table to bring it to a stop? These forces can include friction with both the table and the air (we will address frictional forces in Chapter 6). Our own experience tends to disagree with Newton’s first law because we are always in the presence of frictional forces on Earth. But if you threw a book into the emptiness of space, it would continue to move at the velocity you threw it until it ran into something. There’s nothing to slow it down.

Newton’s first law is only valid in inertial reference frames (where the object is not accelerating – moving at constant velocity or not moving at all). We will only consider these cases.

Section 5.2: Newton’s Second Law

Textbook Section 5.3: Newton’s Second Law

Section 5.3: Newton’s Third Law

Textbook Section 5.5: Newton’s Third Law

Newton’s Third Law: if object A exerts a force on object B, object B exerts an equal and opposite force on object A.

This law refers to the interaction between two objects. If one person pushes something, the object pushes back. You feel this every time you interact with something — heavy objects put up a lot of resistance to motion, and lighter objects put up less resistance to motion, but everything pushes back at least a little bit.

A common mistake people make when interpreting the third law is to think that since the forces are equal and in opposite directions, the reactions of the two objects will be equal. This isn’t the case! If their masses are different, the objects will both have different accelerations. The more massive object has a small acceleration, and the less massive object has a much larger acceleration.

Another common mistake is to think that because two forces are of equal value and in opposite directions, they must be third-law pairs. This isn’t necessarily the case. Third law pairs are between two objects only. If there are more than two objects between these forces, it’s not a third law pair.

Section 5.4: Free Body Diagrams (FBDs)

Textbook Section 5.7: Drawing Free Body Diagrams

For any object, you can draw a free body diagram (FBD). The free body diagram represents the object as a point, and you include all the individual forces acting on the object. You do NOT include the total force acting on the object (which is instead the vector sum of all individual forces) and you do not include forces the object is asserting on other objects. Only forces directly acting on the object at hand are included.

Consider a can of soda resting on your desk. What forces are present? Gravity is pulling down on the soda can, and the desk is pushing up on the soda can. The sum of these two forces must be equal to zero if the soda can is not accelerating (and it’s not, it’s sitting still). Thus, these forces are of equal and opposite magnitudes. Are they third law pairs? No! One force is between the soda can and the Earth, and the other force is between the soda can and the desk. So where are our third-law pairs? You will never have a third law pair on a FBD. Since the FBD only includes forces acting on the object, it will not include any forces of the object acting on something else (which must be the case for the third law pairs). The third law pairs to the forces above would be the soda can pulling up on the Earth and the soda can pushing down on the desk. Your resulting FBD for the soda can is given below.

Because the upwards force of the desk on the soda is equal to the downwards force of the Earth on the soda, these two vectors should be given the same magnitude (length) in your drawing. If one force is bigger than another, make sure to indicate that in your FBD by the length of the vectors. Always label your vectors according to which force is acting on the object.

Section 5.5: Normal Forces

Textbook Section 5.6: Common Forces

What do we mean by a normal force? In this case, normal means perpendicular . Whenever you come into contact with an object (pushing on a door to open it, standing on a table or the ground, hitting a baseball with a baseball bat) you exert a normal force on that object (a force perpendicular to the surface of the object).

Think of the surface of any object (door, Earth, table, ball, etc.) like the rubber sheet of a trampoline. When you put an object on a trampoline, the surface deforms. But the trampoline has a restoring force associated with it, which acts like a spring when compressed. When you remove the object from the trampoline surface, the surface goes back to it’s normal state, a flat sheet. A heavier object deforms the surface further, while a light object only deforms the surface a little bit.

This restoring force is our normal force. Any force applied to an object, no matter how hard the surface of the object appears, deforms the surface in some way. This deformation leads to a restoring force that attempts to bring the surface back to where it was. When you push down on the ground, a restoring force called the normal force pushes back up.

There are limits to the normal force. You can only push down so far on some things before the normal force can no longer counter the applied force, and this is when objects break (like a table with an elephant on top).

A normal force always acts perpendicular to the surface which supplies the normal force. This does  not mean the normal force is always perpendicular to the applied force. Consider a box sitting on a ramp. The force of gravity applies straight down, but the normal force from the ramp applies perpendicular to the ramp, not opposite the force of gravity. As a result, the gravitational and normal forces do not cancel out in this example. There is a net force in the direction down the ramp, and the box will experience an acceleration in that direction, which is why it will slide down the ramp.

Section 5.6: Tension Forces

Tension forces refer to forces applied to or by a string, chain, wire, etc. If you are hanging from a rope ladder above the ground, there is a tension force in the rope ladder equal to the force of gravity (assuming you haven’t fallen yet). But put too much force on the ladder (keep adding more weight) and it will eventually break. Everything that can give a tension force has a limit to how much it can take. That’s where the expression “you’re only as strong as your weakest link” comes from — the tension a chain of links can take is limited to the tension that can be applied to the weakest link of the chain. If one breaks, the tension is gone. In all of the problems we do, assume that the tension is equal along the entire rope/chain/etc.

Section 5.7: Solving Force Problems

Draw a picture.

Draw and label all forces acting on the object.

Draw a FBD for your object.

Solve for whatever variable you are looking for using the equations you wrote in the previous step. Keep in mind you might need to use the kinematics equations as well.

In Class Group Problem 5.1:

A person pushes a box across a frictionless surface with a force of 12 N. If the box starts from rest and has reached a velocity of 3.5 m/s after 6.0 s, what is the mass of the box?

Draw a free body diagram (FBD) for the box and label all the forces acting on the box. Is the net force on the box equal to zero? In which direction does the net force point?

In Class Group Problem 5.2:

What is Bill’s mass in kg?

Why don’t we see the Earth moving up to meet us halfway as we fall towards it?

In Class Group Problem 5.3:

How does the force exerted by the mosquito on the car compare to the force exerted by the car on the mosquito?

Consider your answer to the previous question. Think about what happens to the mosquito. Explain why your answer makes sense in terms of Newton’s second law.

What is the change in acceleration of the car as a result?

In Class Group Problem 5.4:

A box is sitting at rest on a frictionless incline as shown below. A rope attached to the box (arrow) keeps it stationary. If the box has a mass of 35 kg, what is the tension in the rope (in other words, what force is it exerting to keep the box from moving)?

In Class Group Problem 5.5:

A 42 kg gymnast is dangling on a massless rope. Draw a FBD for the person and find the tension in the rope.

In Class Group Problem 5.6:

In Class Group Problem 5.7:

In Class Group Problem 5.8:

In Class Group Problem 5.9:

In class group problem 5.10:, in class group problem 5.11:.

While driving in the mountains, you notice that when the freeway goes steeply downhill, there are emergency exits every few miles. These emergency exits are straight dirt ramps which leave the freeway and are sloped uphill. They are designed to stop trucks and cars that lose their breaks on the down hill stretches of the freeway even if the road is covered in ice. You wonder at what angle from the horizontal an emergency exit should rise to stop a 50 ton truck going 70 mph up a ramp 300 ft long, even if the frictional force of the road surface is negligible.

A push or pull on an object.

Objects that come into direct surface-to-surface contact to transmit a force.

Objects do not need to come into direct surface-to-surface contact to transmit these forces. They are instead transmitted by fields, which we'll get into in PHYS 202.

The amount of stuff you have that makes up an object. Mass is measured in kg, and doesn't change when you move an object to another planet or put it in space.

Weight is the amount Earth (or another body) pulls on an object. This is NOT the same thing as mass, since it changes depending on the position of the object (on the Earth vs. the Moon vs. floating in space). Weight is mass x gravity, and is a force.

The gravitational force is the force between two objects that both have mass.

Forces that are equal in magnitude, opposite in direction, and are the opposites of each other - object A acts on object B, so object B acts back on object A.

A diagram that represents an object as a dot, and all the external forces acting on that object as vectors originating from the dot.

A normal force acts perpendicular to the surface causing it.

A force that tries to put something back where it was.

Introductory Physics Resources Copyright © by Adria C Updike is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

Share This Book

Section Learning Objectives

By the end of this section, you will be able to do the following:

• Differentiate between force, net force, and dynamics
• Draw a free-body diagram

Teacher Support

The learning objectives in this section will help your students master the following standards:

• (C) analyze and describe accelerated motion in two dimensions using equations, including projectile and circular examples;
• (E) develop and interpret free-body diagrams.

[BL] [OL] Point out that objects at rest tend to stay at rest. A ball, for example, moves only when pushed or pulled. The action of pushing or pulling is the application of force. Force applied to an object changes its motion.

[AL] Start a discussion about force and motion. Ask students what would happen if more than one force is applied to an object. Take a heavy object such as a desk for demonstration. Ask one student to push it from one side. Explain how force and motion work. Now ask a second student to push it in the opposite direction. Ask students why no motion occurs, even though the first student applies the same amount of force. Introduce the concept of adding forces.

Section Key Terms

Defining Force and Dynamics

[OL] Explain that the word dynamics comes from a Greek word meaning power . Also point out that the word dynamics is singular, like the word physics .

[BL] [OL] You may want to introduce the terms system, external force , and internal force .

[AL] Explain that both magnitude and direction must be considered when talking about forces.

Teacher Demonstration

By using physical objects, demonstrate how different forces acting together can be additive if they act in the same direction or cancel one another if they act in opposite directions. Explain the terms acting on and being acted on .

Force is the cause of motion, and motion draws our attention. Motion itself can be beautiful, such as a dolphin jumping out of the water, the flight of a bird, or the orbit of a satellite. The study of motion is called kinematics , but kinematics describes only the way objects move—their velocity and their acceleration . Dynamics considers the forces that affect the motion of moving objects and systems . Newton’s laws of motion are the foundation of dynamics. These laws describe the way objects speed up, slow down, stay in motion, and interact with other objects. They are also universal laws : they apply everywhere on Earth as well as in space.

A force pushes or pulls an object. The object being moved by a force could be an inanimate object, a table, or an animate object, a person. The pushing or pulling may be done by a person, or even the gravitational pull of Earth. Forces have different magnitudes and directions; this means that some forces are stronger than others and can act in different directions. For example, a cannon exerts a strong force on the cannonball that is launched into the air. In contrast, a mosquito landing on your arm exerts only a small force on your arm.

When multiple forces act on an object, the forces combine. Adding together all of the forces acting on an object gives the total force, or net force . An external force is a force that acts on an object within the system from outside the system. This type of force is different than an internal force, which acts between two objects that are both within the system. The net external force combines these two definitions; it is the total combined external force. We discuss further details about net force, external force, and net external force in the coming sections.

Like displacements, velocities, and accelerations, forces are vectors that have magnitude and direction. We may represent a force as the sum of two vectors at right angles. These are its one-dimensional components, which we can represent by a signed scalar quantity. When we do so, we must choose a coordinate system and the direction along each axis that will be the positive direction. This choice must be the same for all vectors in the problem—forces, accelerations, etc.

Commonly, in a problem set in a vertical plane, horizontal and vertical axes are chosen as the two primary directions. The usual convention is to make the positive directions right and up. Sometimes a problem concerns a sloping plane, and it may be more convenient to choose axes parallel to the plane and normal to it. In this case, it is usual to take positive in each axis as the directions that tend upward.

Thus, a horizontal force of –3 N means pushing with 3 N to the left.

Unknown values are taken to be in the positive direction, so if your calculation of a vertical force comes out as +5.2 N, then you know it is an upward force. A negative result means the force is downward.

Free-Body Diagrams and Examples of Forces

[BL] Review vectors and how they are represented. Review vector addition.

[AL] Ask students to give everyday examples of situations where multiple forces act together. Draw free-body diagrams for some of these situations.

For our first example of force, consider an object hanging from a rope. This example gives us the opportunity to introduce a useful tool known as a free-body diagram . A free-body diagram represents the object being acted upon—that is, the free body—as a single point. Only the forces acting on the body (that is, external forces) are shown and are represented by vectors (which are drawn as arrows). These forces are the only ones shown because only external forces acting on the body affect its motion. We can ignore any internal forces within the body because they cancel each other out, as explained in the section on Newton’s third law of motion. Free-body diagrams are very useful for analyzing forces acting on an object.

Figure 4.2 shows the force of tension in the rope acting in the upward direction, opposite the force of gravity. The forces are indicated in the free-body diagram by an arrow pointing up, representing tension, and another arrow pointing down, representing gravity. In a free-body diagram, the lengths of the arrows show the relative magnitude (or strength) of the forces. Because forces are vectors, they add just like other vectors. Notice that the two arrows have equal lengths in Figure 4.2 , which means that the forces of tension and weight are of equal magnitude. Because these forces of equal magnitude act in opposite directions, they are perfectly balanced, so they add together to give a net force of zero.

Not all forces are as noticeable as when you push or pull on an object. Some forces act without physical contact, such as the pull of a magnet (in the case of magnetic force) or the gravitational pull of Earth (in the case of gravitational force).

In the next three sections discussing Newton’s laws of motion, we will learn about three specific types of forces: friction, the normal force , and the gravitational force . To analyze situations involving forces, we will create free-body diagrams to organize the framework of the mathematics for each individual situation.

Tips For Success

Correctly drawing and labeling a free-body diagram is an important first step for solving a problem. It will help you visualize the problem and correctly apply the mathematics to solve the problem.

Check Your Understanding

Use the questions in Check Your Understanding to assess whether students have mastered the learning objectives of this section. If students are struggling with a specific objective, the Check Your Understanding assessment will help identify which objective is causing the problem and direct students to the relevant content.

• Kinematics is the study of motion.
• Kinematics is the study of the cause of motion.
• Kinematics is the study of dimensions.
• Kinematics is the study of atomic structures.

Do two bodies have to be in physical contact to exert a force upon one another?

• No, the gravitational force is a field force and does not require physical contact to exert a force.
• No, the gravitational force is a contact force and does not require physical contact to exert a force.
• Yes, the gravitational force is a field force and requires physical contact to exert a force.
• Yes, the gravitational force is a contact force and requires physical contact to exert a force.
• Force is a scalar quantity.
• Force is a vector quantity.
• Force is both a vector quantity and a scalar quantity.
• Force is neither a vector nor a scalar quantity.

Which forces can be represented in a free-body diagram?

• Internal forces
• External forces
• Both internal and external forces
• A body that is not influenced by any force

This book may not be used in the training of large language models or otherwise be ingested into large language models or generative AI offerings without OpenStax's permission.

Want to cite, share, or modify this book? This book uses the Creative Commons Attribution License and you must attribute Texas Education Agency (TEA). The original material is available at: https://www.texasgateway.org/book/tea-physics . Changes were made to the original material, including updates to art, structure, and other content updates.

Access for free at https://openstax.org/books/physics/pages/1-introduction

• Authors: Paul Peter Urone, Roger Hinrichs
• Publisher/website: OpenStax
• Book title: Physics
• Publication date: Mar 26, 2020
• Location: Houston, Texas
• Book URL: https://openstax.org/books/physics/pages/1-introduction
• Section URL: https://openstax.org/books/physics/pages/4-1-force

© Jun 7, 2024 Texas Education Agency (TEA). The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.