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2.4 Newton's First Law

6 min readdecember 24, 2022

Peter Apps

Peter Apps

Daniella Garcia-Loos

Daniella Garcia-Loos

Peter Apps

Peter Apps

Daniella Garcia-Loos

Daniella Garcia-Loos

Newton's First Law of Inertia

Another term for this law is the Law of Inertia because it explains the concept that objects have the tendency to resist a change in motion. It is also commonly referred to as just a special case of Newton’s Second Law when the net external force is zero.

Key Concept: Frame of Reference: A coordinate system in relation to which judgments can be made, usually from an observer’s point of view, is known as a frame of reference. A frame of reference moving with constant velocity is known as an inertial frame of reference.

Key Concept: - the ability of an object to resist a change in its state of motion. The inertia of an object is measured based on its mass

An object with a small mass will exhibit less inertia and be more affected by other objects, and an object with a large mass will exhibit greater inertia and be less affected by other objects. That’s why it would be a lot harder to move an elephant from rest than an ant! Simply put, is a measure of how difficult it is to change the uniform motion of an object by an external force. (🐜 < 🐘)

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2FScreen%20Shot%202020-04-08%20at%201.26.35%20PM.png?alt=media&token=7319deff-6249-4a53-91e3-9f6f75594b70

Image courtesy of WikiHow.

As shown in the image above, the force of the person, given by mass x acceleration, is equal to Fnet because it is the only unbalanced force. However, if the sum of the forces acting on the object is zero then the system would be in Key Concept: - when the vector sum of the forces acting on an object is equal to zero

Example Problem #1:

A student is sitting in a classroom at rest. The student's desk is pushed by a classmate, causing the student and desk to start moving across the room. The student's teacher then pushes the student and desk back in the opposite direction.

  1. What is the external force acting on the student and desk?
  2. How does the student's inertia affect their motion?
  3. If the student's teacher pushes the student and desk with a greater force, what will happen to the student's motion?
  4. How does the mass of the student and desk affect their motion?
1. The external force acting on the student and desk is the push from the classmate and the push from the teacher.
2. The student's inertia affects their motion by causing them to resist a change in their motion. When the student and desk are at rest, they will remain at rest unless an external force is applied to them. When the student and desk are moving, they will continue to move at a constant velocity unless an external force is applied to them.
3. If the student's teacher pushes the student and desk with a greater force, the student and desk will accelerate in the direction of the force. The greater the force, the greater the acceleration will be.
4. The mass of the student and desk affects their motion because objects with more mass are harder to accelerate than objects with less mass. This is due to the fact that the force required to accelerate an object is proportional to its mass. So, if the student and desk have a greater mass, they will be harder to accelerate, and if they have a smaller mass, they will be easier to accelerate.

Example Problem #2:

Design an experiment to determine the relationship between the net force exerted on a small wooden block, its , and its acceleration.

Identify the variables:

  • Independent variable: net force applied to the object

  • Dependent variables: of the object, acceleration of the object

Determine the experimental setup:

  • Device to apply force to the object: spring scale, force sensor
  • Way to measure the of the object: balance scale
  • Way to measure the acceleration of the object: timer, photogate

Determine the experimental procedure:

  1. Measure the of the object using the balance scale.
  2. Apply a series of increasing forces to the object using the spring scale or force sensor.
  3. Measure the acceleration of the object using the timer or photogate.
  4. Record the values of the force, , and acceleration for each trial.

Analyze the data:

  • Plot the values of the force, , and acceleration on a graph.
  • Examine the relationship between the three variables.

Draw conclusions:

  • The net force applied to an object is directly proportional to its acceleration.
  • The of the object is a measure of its resistance to acceleration.
    • The relationship between the force, , and acceleration follows the equation , where F is the force, m is the , and a is the acceleration.

    Gravitational vs Inertial Mass

    Objects and systems have properties of and that are experimentally verified to be the same and that satisfy conservation principles.

    Key Concept: - determined by the strength of the gravitational force experienced by the body when in the gravitational field g.

    Equation:

    https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2FScreen%20Shot%202020-04-08%20at%201.29.25%20PM.png?alt=media&token=bbbc888c-b4de-4a00-8b3e-8ade9f053fa0

    vs.

    • is measured by comparing the force of gravity of an unknown mass to the force of gravity of a known mass.

    • is found by applying a known force to an unknown mass, measuring the acceleration, and applying , a = F/m.

    Equation:

    https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2FScreen%20Shot%202020-04-08%20at%201.30.19%20PM.png?alt=media&token=82e2ef3d-f22a-448e-adcf-ba87976be3b6

    While these masses are measured in varying applications, they have been experimentally proven to equal the same value; therefore Inertial = Gravitational 

    Key things to remember:

    • is a measure of the amount of matter in an object. It determines how much the object is attracted to other objects with mass due to the force of gravity.

    • is a measure of the resistance of an object to acceleration. It determines how difficult it is to change the velocity of an object, whether by pushing, pulling, or any other force.

    • Both and are properties of matter, and they are usually considered to be the same. In other words, an object with a large will also have a large , and vice versa.

    • However, there are some interesting situations in which and may not be exactly equal. For example, in Einstein's , the two masses can be slightly different due to the effects of gravity on time and space.

    Watch: AP Physics 1 - Unit 2 Streams

    Example Problem:

    You have been asked to design a plan for collecting data to measure both the and the of a golf ball. You will also need to determine whether the golf ball has the same and , or if they are different.

    Part A: Explain the difference between and .

    Part B: Describe how you would design an experiment to measure the of the golf ball. Be sure to include all necessary materials, procedures, and any calculations you would need to make in order to compare the to the .

    Part C: Describe how you would design an experiment to measure the of the golf ball. Be sure to include all necessary materials, procedures, and any calculations you would need to make in order to compare the to the .

    Part D: Explain how you would use the data collected in your experiments to determine whether the golf ball has the same and , or if they are different.

    Part E: Discuss any potential sources of error in your experiments and how you would minimize or correct them.

    Key Terms to Review (6)

    Equilibrium

    : Equilibrium refers to a state of balance or stability in which opposing forces or factors are equal. In physics, it specifically refers to a situation where the net force and net torque acting on an object are both zero.

    F=ma

    : F=ma is Newton's second law of motion, which states that the force acting on an object is equal to the mass of the object multiplied by its acceleration.

    Gravitational Mass

    : Gravitational mass refers to the measure of an object's response to the force of gravity. It determines how strongly an object is attracted towards another object due to gravity.

    Inertial Mass

    : Inertial mass refers to the measure of an object's resistance to changes in its motion. It determines how difficult it is for an external force to accelerate or decelerate an object.

    Newton's Second Law

    : States that when a net external force acts on an object, the object will accelerate in the direction of the force. The acceleration is directly proportional to the net force and inversely proportional to the mass of the object.

    Theory of Relativity

    : The Theory of Relativity, developed by Albert Einstein, explains how space and time are interconnected and affected by gravity. It consists of two parts - Special Relativity deals with objects moving at constant speeds relative to each other, while General Relativity includes accelerated motion and gravity.

    2.4 Newton's First Law

    6 min readdecember 24, 2022

    Peter Apps

    Peter Apps

    Daniella Garcia-Loos

    Daniella Garcia-Loos

    Peter Apps

    Peter Apps

    Daniella Garcia-Loos

    Daniella Garcia-Loos

    Newton's First Law of Inertia

    Another term for this law is the Law of Inertia because it explains the concept that objects have the tendency to resist a change in motion. It is also commonly referred to as just a special case of Newton’s Second Law when the net external force is zero.

    Key Concept: Frame of Reference: A coordinate system in relation to which judgments can be made, usually from an observer’s point of view, is known as a frame of reference. A frame of reference moving with constant velocity is known as an inertial frame of reference.

    Key Concept: - the ability of an object to resist a change in its state of motion. The inertia of an object is measured based on its mass

    An object with a small mass will exhibit less inertia and be more affected by other objects, and an object with a large mass will exhibit greater inertia and be less affected by other objects. That’s why it would be a lot harder to move an elephant from rest than an ant! Simply put, is a measure of how difficult it is to change the uniform motion of an object by an external force. (🐜 < 🐘)

    https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2FScreen%20Shot%202020-04-08%20at%201.26.35%20PM.png?alt=media&token=7319deff-6249-4a53-91e3-9f6f75594b70

    Image courtesy of WikiHow.

    As shown in the image above, the force of the person, given by mass x acceleration, is equal to Fnet because it is the only unbalanced force. However, if the sum of the forces acting on the object is zero then the system would be in Key Concept: - when the vector sum of the forces acting on an object is equal to zero

    Example Problem #1:

    A student is sitting in a classroom at rest. The student's desk is pushed by a classmate, causing the student and desk to start moving across the room. The student's teacher then pushes the student and desk back in the opposite direction.

    1. What is the external force acting on the student and desk?
    2. How does the student's inertia affect their motion?
    3. If the student's teacher pushes the student and desk with a greater force, what will happen to the student's motion?
    4. How does the mass of the student and desk affect their motion?
    1. The external force acting on the student and desk is the push from the classmate and the push from the teacher.
    2. The student's inertia affects their motion by causing them to resist a change in their motion. When the student and desk are at rest, they will remain at rest unless an external force is applied to them. When the student and desk are moving, they will continue to move at a constant velocity unless an external force is applied to them.
    3. If the student's teacher pushes the student and desk with a greater force, the student and desk will accelerate in the direction of the force. The greater the force, the greater the acceleration will be.
    4. The mass of the student and desk affects their motion because objects with more mass are harder to accelerate than objects with less mass. This is due to the fact that the force required to accelerate an object is proportional to its mass. So, if the student and desk have a greater mass, they will be harder to accelerate, and if they have a smaller mass, they will be easier to accelerate.

    Example Problem #2:

    Design an experiment to determine the relationship between the net force exerted on a small wooden block, its , and its acceleration.

    Identify the variables:

    • Independent variable: net force applied to the object

    • Dependent variables: of the object, acceleration of the object

    Determine the experimental setup:

    • Device to apply force to the object: spring scale, force sensor
    • Way to measure the of the object: balance scale
    • Way to measure the acceleration of the object: timer, photogate

    Determine the experimental procedure:

    1. Measure the of the object using the balance scale.
    2. Apply a series of increasing forces to the object using the spring scale or force sensor.
    3. Measure the acceleration of the object using the timer or photogate.
    4. Record the values of the force, , and acceleration for each trial.

    Analyze the data:

    • Plot the values of the force, , and acceleration on a graph.
    • Examine the relationship between the three variables.

    Draw conclusions:

  • The net force applied to an object is directly proportional to its acceleration.
  • The of the object is a measure of its resistance to acceleration.
    • The relationship between the force, , and acceleration follows the equation , where F is the force, m is the , and a is the acceleration.

    Gravitational vs Inertial Mass

    Objects and systems have properties of and that are experimentally verified to be the same and that satisfy conservation principles.

    Key Concept: - determined by the strength of the gravitational force experienced by the body when in the gravitational field g.

    Equation:

    https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2FScreen%20Shot%202020-04-08%20at%201.29.25%20PM.png?alt=media&token=bbbc888c-b4de-4a00-8b3e-8ade9f053fa0

    vs.

    • is measured by comparing the force of gravity of an unknown mass to the force of gravity of a known mass.

    • is found by applying a known force to an unknown mass, measuring the acceleration, and applying , a = F/m.

    Equation:

    https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2FScreen%20Shot%202020-04-08%20at%201.30.19%20PM.png?alt=media&token=82e2ef3d-f22a-448e-adcf-ba87976be3b6

    While these masses are measured in varying applications, they have been experimentally proven to equal the same value; therefore Inertial = Gravitational 

    Key things to remember:

    • is a measure of the amount of matter in an object. It determines how much the object is attracted to other objects with mass due to the force of gravity.

    • is a measure of the resistance of an object to acceleration. It determines how difficult it is to change the velocity of an object, whether by pushing, pulling, or any other force.

    • Both and are properties of matter, and they are usually considered to be the same. In other words, an object with a large will also have a large , and vice versa.

    • However, there are some interesting situations in which and may not be exactly equal. For example, in Einstein's , the two masses can be slightly different due to the effects of gravity on time and space.

    Watch: AP Physics 1 - Unit 2 Streams

    Example Problem:

    You have been asked to design a plan for collecting data to measure both the and the of a golf ball. You will also need to determine whether the golf ball has the same and , or if they are different.

    Part A: Explain the difference between and .

    Part B: Describe how you would design an experiment to measure the of the golf ball. Be sure to include all necessary materials, procedures, and any calculations you would need to make in order to compare the to the .

    Part C: Describe how you would design an experiment to measure the of the golf ball. Be sure to include all necessary materials, procedures, and any calculations you would need to make in order to compare the to the .

    Part D: Explain how you would use the data collected in your experiments to determine whether the golf ball has the same and , or if they are different.

    Part E: Discuss any potential sources of error in your experiments and how you would minimize or correct them.

    Key Terms to Review (6)

    Equilibrium

    : Equilibrium refers to a state of balance or stability in which opposing forces or factors are equal. In physics, it specifically refers to a situation where the net force and net torque acting on an object are both zero.

    F=ma

    : F=ma is Newton's second law of motion, which states that the force acting on an object is equal to the mass of the object multiplied by its acceleration.

    Gravitational Mass

    : Gravitational mass refers to the measure of an object's response to the force of gravity. It determines how strongly an object is attracted towards another object due to gravity.

    Inertial Mass

    : Inertial mass refers to the measure of an object's resistance to changes in its motion. It determines how difficult it is for an external force to accelerate or decelerate an object.

    Newton's Second Law

    : States that when a net external force acts on an object, the object will accelerate in the direction of the force. The acceleration is directly proportional to the net force and inversely proportional to the mass of the object.

    Theory of Relativity

    : The Theory of Relativity, developed by Albert Einstein, explains how space and time are interconnected and affected by gravity. It consists of two parts - Special Relativity deals with objects moving at constant speeds relative to each other, while General Relativity includes accelerated motion and gravity.


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    AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.


    © 2024 Fiveable Inc. All rights reserved.

    AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.