<< Chapter < Page Chapter >> Page >

Newton’s second law of motion is more than a definition; it is a relationship among acceleration, force, and mass. It can help us make predictions. Each of those physical quantities can be defined independently, so the second law tells us something basic and universal about nature. The next section introduces the third and final law of motion.

Section summary

  • Acceleration, a size 12{ { {a}} sup { ' }>a} {} , is defined as a change in velocity, meaning a change in its magnitude or direction, or both.
  • An external force is one acting on a system from outside the system, as opposed to internal forces, which act between components within the system.
  • Newton’s second law of motion states that the acceleration of a system is directly proportional to and in the same direction as the net external force acting on the system, and inversely proportional to its mass.
  • In equation form, Newton’s second law of motion is a = F net m size 12{a= { {F rSub { size 8{"net"} } } over {m} } } {} .
  • This is often written in the more familiar form: F net = m a size 12{F rSub { size 8{"net"} } =ma} {} .
  • The weight w size 12{w} {} of an object is defined as the force of gravity acting on an object of mass m size 12{m} {} . The object experiences an acceleration due to gravity g size 12{g} {} :

    w = m g size 12{w=mg} {} .

  • If the only force acting on an object is due to gravity, the object is in free fall.
  • Friction is a force that opposes the motion past each other of objects that are touching.

Conceptual questions

Which statement is correct? (a) Net force causes motion. (b) Net force causes change in motion. Explain your answer and give an example.

Got questions? Get instant answers now!

Why can we neglect forces such as those holding a body together when we apply Newton’s second law of motion?

Got questions? Get instant answers now!

Explain how the choice of the “system of interest” affects which forces must be considered when applying Newton’s second law of motion.

Got questions? Get instant answers now!

Describe a situation in which the net external force on a system is not zero, yet its speed remains constant.

Got questions? Get instant answers now!

A system can have a nonzero velocity while the net external force on it is zero. Describe such a situation.

Got questions? Get instant answers now!

A rock is thrown straight up. What is the net external force acting on the rock when it is at the top of its trajectory?

Got questions? Get instant answers now!

(a) Give an example of different net external forces acting on the same system to produce different accelerations. (b) Give an example of the same net external force acting on systems of different masses, producing different accelerations. (c) What law accurately describes both effects? State it in words and as an equation.

Got questions? Get instant answers now!

If the acceleration of a system is zero, are no external forces acting on it? What about internal forces? Explain your answers.

Got questions? Get instant answers now!

If a constant, nonzero force is applied to an object, what can you say about the velocity and acceleration of the object?

Got questions? Get instant answers now!

The gravitational force on the basketball in [link] is ignored. When gravity is taken into account, what is the direction of the net external force on the basketball—above horizontal, below horizontal, or still horizontal?

Got questions? Get instant answers now!

Problem exercises

You may assume data taken from illustrations is accurate to three digits.

A 63.0-kg sprinter starts a race with an acceleration of 4 . 20 m /s 2 size 12{4 "." "20"" m/s" rSup { size 8{2} } } {} . What is the net external force on him?

265 N

Got questions? Get instant answers now!

If the sprinter from the previous problem accelerates at that rate for 20 m, and then maintains that velocity for the remainder of the 100-m dash, what will be his time for the race?

Got questions? Get instant answers now!

Questions & Answers

the diagram of the digestive system
Assiatu Reply
How does twins formed
William Reply
They formed in two ways first when one sperm and one egg are splited by mitosis or two sperm and two eggs join together
Oluwatobi
what is genetics
Josephine Reply
Genetics is the study of heredity
Misack
how does twins formed?
Misack
What is manual
Hassan Reply
discuss biological phenomenon and provide pieces of evidence to show that it was responsible for the formation of eukaryotic organelles
Joseph Reply
what is biology
Yousuf Reply
the study of living organisms and their interactions with one another and their environments
AI-Robot
the study of living organisms and their interactions with one another and their environment.
Wine
discuss the biological phenomenon and provide pieces of evidence to show that it was responsible for the formation of eukaryotic organelles in an essay form
Joseph Reply
what is the blood cells
Shaker Reply
list any five characteristics of the blood cells
Shaker
lack electricity and its more savely than electronic microscope because its naturally by using of light
Abdullahi Reply
advantage of electronic microscope is easily and clearly while disadvantage is dangerous because its electronic. advantage of light microscope is savely and naturally by sun while disadvantage is not easily,means its not sharp and not clear
Abdullahi
cell theory state that every organisms composed of one or more cell,cell is the basic unit of life
Abdullahi
is like gone fail us
DENG
cells is the basic structure and functions of all living things
Ramadan
What is classification
ISCONT Reply
is organisms that are similar into groups called tara
Yamosa
in what situation (s) would be the use of a scanning electron microscope be ideal and why?
Kenna Reply
A scanning electron microscope (SEM) is ideal for situations requiring high-resolution imaging of surfaces. It is commonly used in materials science, biology, and geology to examine the topography and composition of samples at a nanoscale level. SEM is particularly useful for studying fine details,
Hilary
cell is the building block of life.
Condoleezza Reply
what is cell divisoin?
Aron Reply
Diversity of living thing
ISCONT
what is cell division
Aron Reply
Cell division is the process by which a single cell divides into two or more daughter cells. It is a fundamental process in all living organisms and is essential for growth, development, and reproduction. Cell division can occur through either mitosis or meiosis.
AI-Robot
What is life?
Allison Reply
life is defined as any system capable of performing functions such as eating, metabolizing,excreting,breathing,moving,Growing,reproducing,and responding to external stimuli.
Mohamed
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply
Practice Key Terms 7

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'College physics' conversation and receive update notifications?

Ask