# Newton's First Law of Motion: Inertia

This chapter discusses Newton's first law of motion which states that an object at rest will remain at rest unless acted upon by an external force and an object in motion will continue in motion with the same velocity unless acted upon by an external force. This law overturned Aristotle's view of motion in the 4th century BC.

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1. Newton’s First Law of Motion—Inertia Newton’s First Law of Motion—Inertia Chapter 4 Chapter 4

2. Aristotle (4 th Century B.C.) Aristotle (4 th Century B.C.)  Believed in two different types of motion: natural motion & violent motion  Believed in two different types of motion: natural motion & violent motion  Natural Motion – either straight up or straight down, objects would seek out their natural resting places (it was “natural” for heavy things to fall and light things to rise)  Natural Motion – either straight up or straight down, objects would seek out their natural resting places (it was “natural” for heavy things to fall and light things to rise)  Violent Motion – imposed motion, result of forces that pushed or pulled  Violent Motion – imposed motion, result of forces that pushed or pulled  The proper state of objects was that of rest.  The proper state of objects was that of rest.

3. Aristotle Aristotle

4. Copernicus (1473-1543 A.D.) Copernicus (1473-1543 A.D.)  Because Earth was already in its natural resting place, it could not move (there was no force big enough to move Earth)  Because Earth was already in its natural resting place, it could not move (there was no force big enough to move Earth)  Copernicus concluded that the only way to make sense of the way the planets move is to assume that Earth and the other planets move around the sun  Copernicus concluded that the only way to make sense of the way the planets move is to assume that Earth and the other planets move around the sun  He worked in secret to avoid being persecuted by the church  He worked in secret to avoid being persecuted by the church

5. Copernicus Copernicus

6. Galileo Galileo  The foremost scientist of the late- Renaissance in Italy  The foremost scientist of the late- Renaissance in Italy  Outspoken in his support of Copernicus, later put on trial and confined to house arrest  Outspoken in his support of Copernicus, later put on trial and confined to house arrest  A force is any push or pull  A force is any push or pull  Friction – the force that acts between materials that touch as they move past each other  Friction – the force that acts between materials that touch as they move past each other  If friction were absent, an object would need no force to remain in motion  If friction were absent, an object would need no force to remain in motion

7. Galileo Galileo

8. Galileo’s Inclined Planes Galileo’s Inclined Planes  Noted that a ball that rolls down an inclined plane picks up speed  Noted that a ball that rolls down an inclined plane picks up speed  When the same ball rolls up the inclined plane, it slows down  When the same ball rolls up the inclined plane, it slows down  When a ball rolls on a flat horizontal surface, it rolls at near constant velocity  When a ball rolls on a flat horizontal surface, it rolls at near constant velocity  Stated that with the absence of friction, the ball would roll forever  Stated that with the absence of friction, the ball would roll forever

9. Galileo’s Inclined Planes Cont. Galileo’s Inclined Planes Cont.  If you have two inclined planes face each other, a ball rolled down one plane will reach nearly the same height as it rolls up the other plane  If you have two inclined planes face each other, a ball rolled down one plane will reach nearly the same height as it rolls up the other plane  He noticed that the ball ended up at the same height, even is the plane was elongated or at a different angle  He noticed that the ball ended up at the same height, even is the plane was elongated or at a different angle  Inertia – the property of a body to resist change (the tendency of a moving body to keep moving and every material object resists change to its state of motion)  Inertia – the property of a body to resist change (the tendency of a moving body to keep moving and every material object resists change to its state of motion)

10. Inclined Planes Inclined Planes

11. Newton’s First Law Newton’s First Law  Every object continues in a state of rest, or of motion in a straight line at constant speed, unless it is compelled to change that state by forces exerted upon it.  Every object continues in a state of rest, or of motion in a straight line at constant speed, unless it is compelled to change that state by forces exerted upon it.  An object will keep doing what it’s already doing  An object will keep doing what it’s already doing  Toss an object out the door of the International Space Station, and it will keep moving at the speed you threw it at forever (no friction in space)  Toss an object out the door of the International Space Station, and it will keep moving at the speed you threw it at forever (no friction in space)

12. Newton’s 1 st Law Newton’s 1 st Law

13. The Law of Inertia The Law of Inertia

14. The Law of Inertia The Law of Inertia

15. Pioneer and Voyager Pioneer and Voyager  After they initially were sent into space, these two spacecraft utilize Newton’s Law of Inertia to move through space  After they initially were sent into space, these two spacecraft utilize Newton’s Law of Inertia to move through space  They are now cruising outside of our solar system  They are now cruising outside of our solar system

16. Mass – A Measure of Inertia Mass – A Measure of Inertia  The amount of inertia an object has depends on its mass—which is roughly the amount of material present in the object  The amount of inertia an object has depends on its mass—which is roughly the amount of material present in the object  Mass is NOT volume, the measure of space that an object takes up  Mass is NOT volume, the measure of space that an object takes up  Mass is NOT weight, the force of gravity on an object  Mass is NOT weight, the force of gravity on an object  Mass is a measure of the inertia that an object exhibits in response to any effort made to start it, stop it, or otherwise change its state of motion  Mass is a measure of the inertia that an object exhibits in response to any effort made to start it, stop it, or otherwise change its state of motion  Mass and weight may not be the same, but they are proportional to each other  Mass and weight may not be the same, but they are proportional to each other Weight = mass x acceleration due to gravity Weight = mass x acceleration due to gravity Measured in Newtons (N) Measured in Newtons (N) A 1-kg bag of nails weighs 9.8 N on the surface of Earth (2.2 lbs.) A 1-kg bag of nails weighs 9.8 N on the surface of Earth (2.2 lbs.)

17. Net Force Net Force  In the absence of a net force , objects do not change their state of motion  In the absence of a net force , objects do not change their state of motion  If you push with equal and opposite forces on opposite sides of an object at rest, it will remain at rest  If you push with equal and opposite forces on opposite sides of an object at rest, it will remain at rest  Net Force – the combination of all forces acting on an object  Net Force – the combination of all forces acting on an object  We use force diagrams to figure out the net force  We use force diagrams to figure out the net force

18. Force Diagrams Force Diagrams

19. Equilibrium Equilibrium  If only the force of gravity was acting on an object at rest, the object would constantly be in free fall  If only the force of gravity was acting on an object at rest, the object would constantly be in free fall  The fact that the object is at rest means that another force must be acting upon it  The fact that the object is at rest means that another force must be acting upon it  The other force exactly balances out the weight of the object and produces a net force of zero  The other force exactly balances out the weight of the object and produces a net force of zero  The other force is called the support force , or normal force  The other force is called the support force , or normal force  Equilibrium – the net force on an object is zero  Equilibrium – the net force on an object is zero

20. Equilibrium Equilibrium

21. Vector Addition of Forces Vector Addition of Forces  We can use the same vector addition techniques on forces as we did on velocities!  We can use the same vector addition techniques on forces as we did on velocities!  Force, like velocity has a magnitude and a direction  Force, like velocity has a magnitude and a direction  For any pair of scales, ropes, or wires supporting a load, the greater the angle from the vertical, the larger the tension force in them  For any pair of scales, ropes, or wires supporting a load, the greater the angle from the vertical, the larger the tension force in them  The resultant of the tension forces in a rope must be equal and opposite to the load being supported  The resultant of the tension forces in a rope must be equal and opposite to the load being supported