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What is Newton's first law?

Also called the law of inertia, this is the most important thing to realize about motion.

Why do objects slow down?

Before Galileo and Newton, many people thought objects slowed down because they had a natural built in tendency to do so. But those people weren't taking into account the many forces—e.g., friction, gravity, and air resistance—here on Earth that cause objects to change their velocity. If we could observe the motion of an object in deep interstellar space, we would be able to observe the natural tendencies of an object's motion free from any external influences. In deep interstellar space, we would observe that if an object had a velocity, it would continue moving with that velocity until there was some force to cause a change in the motion. Similarly, if an object were at rest in interstellar space, it would remain at rest until there was a force to cause it to change its motion.
In the video below, we can see that objects in the international space station either remain at rest or continue with constant velocity relative to the space station until acted upon by a force.
Khan Academy video wrapper
The idea that objects only change their velocity due to a force is encapsulated in Newton's first law.
Newton's first law: An object at rest remains at rest, or if in motion, remains in motion at a constant velocity unless acted on by a net external force.
Note the repeated use of the verb remains. We can think of this law as preserving the status quo of motion. Newton’s first law of motion states that there must be a cause—which is a net external force—for there to be any change in velocity, either a change in magnitude or direction. An object sliding across a table or floor slows down due to the net force of friction acting on the object. But on an air hockey table, where air keeps the puck from touching the table, the air hockey puck continues moving with a roughly constant velocity until a force acts on it—like when it bumps into the side of the table.

What do force, external force, and net force mean?

A force is a push or a pull exerted on one object by another object. The units of force F are called Newtons or simply start text, N, end text.
An external force is a force originating from outside an object rather than a force internal to an object. For instance, the force of gravity that Earth exerts on the moon is an external force on the moon. However, the force of gravity that the inner core of the moon exerts on the outer crust of the moon is an internal force on the moon. Internal forces within an object can't cause a change in that object's overall motion.
The net force, written as \Sigma, F, on an object is the total force on an object. If many forces act on an object, then the net force is the sum of all the forces. But be careful—since force F is a vector, to find the net force \Sigma, F, the forces must be added up like vectors using vector addition.
In other words, if a box of frozen burritos had a force of magnitude 45 Newtons exerted on it to the right and a force of magnitude 30 Newtons exerted on it to the left, the net force in the horizontal direction would be
\Sigma, F, start subscript, start text, h, o, r, i, z, o, n, t, a, l, end text, end subscript, equals, 45, start text, space, N, end text, minus, 30, start text, space, N, end text
\Sigma, F, start subscript, start text, h, o, r, i, z, o, n, t, a, l, end text, end subscript, equals, 15, start text, space, N, end text
Assuming rightward is the positive direction.
Newton's first law says that if the net force on an object is zero (\Sigma, F, equals, 0), then that object will have zero acceleration. That doesn't necessarily mean the object is at rest, but it means that the velocity is constant. In other words, constant zero velocity—at rest—or constant non-zero velocity—moving with a constant velocity.
For the box of frozen burritos, if the rightward force had a magnitude of 45 Newtons and the leftward force had a magnitude of 45 Newtons, the net force would be zero. The box of burritos would either continue moving with a constant velocity—if it started with a velocity before the forces were applied—or stay at rest—if it was already at rest before the forces were applied.

What does mass mean?

The property of a body to remain at rest or to remain in motion with constant velocity is called inertia. Newton’s first law is often called the law of inertia. As we know from experience, some objects have more inertia than others. It is obviously more difficult to change the motion of a large boulder than that of a basketball, for example.
The inertia of an object is measured by its mass. Mass can be determined by measuring how difficult an object is to accelerate. The more mass an object has, the harder it is to accelerate.
Also, roughly speaking, the more “stuff”—or matter—in something, the more mass it will have, and the harder it will be to change its velocity, i.e., accelerate.

What do solved questions involving Newton's first law look like?

Example 1: space probe drift

A space probe is drifting to the right at a constant velocity in deep interstellar space—far from any influence due to planets and stars—with its rockets off. If two rocket thrusters both turn on simultaneously exerting identical forces leftward and rightward in the directions shown, what would happen to the motion of the rocket?
a. The space probe would continue with constant velocity.
b. The space probe would speed up.
c. The space probe would slow down and eventually stop.
d. The space probe would immediately stop.
The correct answer is a. According to Newton's first law, a non-zero net force is required to change the velocity of an object. The net force on the space probe is zero—since the forces on it cancel—so there is no change in the velocity of the probe.

Example 2: elevator lift

An elevator is being pulled upward at a constant velocity by a cable as seen in the diagram below. While the elevator is moving upward at constant velocity, how does the magnitude of the upward force exerted on the elevator by the cable—start color #e84d39, F, start subscript, c, end subscript, end color #e84d39—compare to the magnitude of the downward force of gravity—start color #1fab54, F, start subscript, g, end subscript, end color #1fab54—on the elevator?
a. start color #e84d39, F, start subscript, c, end subscript, end color #e84d39 is greater than start color #1fab54, F, start subscript, g, end subscript, end color #1fab54.
b. start color #e84d39, F, start subscript, c, end subscript, end color #e84d39 is equal to start color #1fab54, F, start subscript, g, end subscript, end color #1fab54.
c. start color #e84d39, F, start subscript, c, end subscript, end color #e84d39 is smaller than start color #1fab54, F, start subscript, g, end subscript, end color #1fab54.
d. start color #e84d39, F, start subscript, c, end subscript, end color #e84d39 could be larger or smaller than start color #1fab54, F, start subscript, g, end subscript, end color #1fab54 depending on the mass of the elevator.
The correct answer is b. If the elevator is moving with constant velocity, the net force must be zero. In order for the net force on the elevator to be zero, the upward and downward forces must cancel exactly.

Example 3: space probe path

A space probe is drifting to the right with constant velocity in deep interstellar space—far from any influence due to planets and stars. If a rocket thruster turns on and then off for a short burst of force in the direction shown, what would best represent the path traveled by the rocket after the thruster turns off?
a. Path a
b. Path b
c. Path c
d. Path d
The correct answer is c. After the rocket thruster turns off, there will be no net force on the space probe. Once the net force is zero, the velocity—both magnitude and direction—must be constant. Because of Newton's first law, the space probe moves in a straight line at constant speed. The fact that there was a vertical force on the space probe does not affect the horizontal velocity of the space probe, it only changes the vertical velocity. A constant vertical and horizontal velocity yields a diagonal straight line through space.

Want to join the conversation?

  • piceratops seedling style avatar for user Abderrahmane Taouil
    is dark matter considered as an external force??
    (5 votes)
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  • starky sapling style avatar for user Ammar Hummieda
    In free fall, when an object reaches terminal velocity, why does the force of drag (air resistance) become equal to the weight ? Why doesn't it become greater ? practically, this would be a silly question, but i want to know the theory behind it. Thanks a lot, I love your work and would love to one day contribute to this website as one if its own.
    (6 votes)
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    • male robot hal style avatar for user Andrew M
      Drag, like friction, is a reaction force. It can't be greater than the force that it is a reaction to. If it could, objects would reach terminal velocity and then start slowing down. But if they slow down, the drag force would decrease, so then they would speed up, and then slow down, and then speed up....
      (5 votes)
  • aqualine seed style avatar for user sumitdongardiye035
    Sir,What will happen if we are in a lift and the cable of the lift breaks down.
    (3 votes)
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  • blobby green style avatar for user Wejdan Almaoudi
    i didnt understand what is inertia ?
    (3 votes)
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  • duskpin ultimate style avatar for user Adarsh
    3rd Example : I get why the probe follows the straight movement but why it went diagonal is still not clear to me. Can someone elaborate that?

    Thanks for the help.
    (2 votes)
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    • spunky sam blue style avatar for user John
      Because of the thrust to the right, the probe would move to the right, but also forward because it was moving forward. The forces combine, and the probe goes diagonal. Hope this helps.😉
      (9 votes)
  • area 52 blue style avatar for user ❗〽〽ØR৳αL__G@MER
    Is there any time interval between when the force is applied and when an opposite force is given?
    (3 votes)
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    • male robot hal style avatar for user Charles LaCour
      No, there is no lag between a force being applied and the reaction force.

      Let's consider what is happening when you push on an object. As your hand get close to the object the electrons in the outer shells of the atoms and molecules of your hand and the object start to interact. This interaction is the force on the object and your hand, you can't separate the force from the reaction force, they are all part of the same interaction.
      (5 votes)
  • area 52 blue style avatar for user ❗〽〽ØR৳αL__G@MER
    Can anyone give me a better visualization of inertia? I am having a hard time imagining that.
    (3 votes)
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    • male robot hal style avatar for user Charles LaCour
      Don't look at light as photons or waves. Light is a energetic fluctuation in the electromagnetic field. It has properties that are like particles and properties that are like waves.

      You need to understand what a frame of reference is. A frame of reference is what an observer measures things from. From your frame of your velocity is always 0.

      For low velocities translations between reference frames is just Galilean coordination translations.

      When you are dealing with light or even things moving at a large percentage of the speed of light you really need to understand Lorentz transformations from Special relativity.
      (3 votes)
  • blobby green style avatar for user tenzingyaltsen15
    for the third question, while it is moving right and the rocket make short burst of force, it is moving equally in both the horizontal and vertical direction. So if things move equally in both the components, does it mean that we take the resultant force as the final direction?
    (3 votes)
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  • leafers seedling style avatar for user angelfromabove410
    how to the astronauts eat in mid air there is no gravitational force pulling them down doesn't that hurt their bodies when they come down ?
    (2 votes)
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    • blobby green style avatar for user Teacher Mackenzie (UK)
      great question. When they swallow the food, the body pulls it down into the stomach. Its a special movement called 'peristalsis' (looks like a Caterpillar walking :-) and it pushes the food along the digestive system in a wave-like motion.
      I dont know if it hurts them when they land, but I guess they will feel various pains as they come back down to Earth, depending on how long they were up there. It would be an interesting quesiton to ask them.
      (2 votes)
  • blobby green style avatar for user Aman Shrivastava
    in example 2 in this slide ,i think option a is correct becoz , it has to overcome the gravity and air resistance ! correct me if i am wrong !
    (2 votes)
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    • aqualine tree style avatar for user chitraksh
      the air wont affect much to the elevator thats because the force of air won't be much in a "tunnel" in which the elevator is present. we know that he elevator always moves with a constant velocity,so the net force on the elevator must be zero. In order for the net force on the elevator to be zero, the upward and downward forces must cancel exactly.
      (2 votes)