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Newton's First Law

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Newton's First Law

Newton's first law of motion states that a body will stay at rest or keep moving at a constant velocity unless acted upon by an unbalanced force. The first part seems to make sense since objects at rest typically do not just start moving. However, the second part appears to contradict our everyday experiences.

When a ball is thrown, it does not keep moving forever. It eventually stops. For a car to move, for the ball to stop, or for a plane to stay in the air, a constant force must be applied. To understand Newton's first law of motion, you need to understand the forces that act on a body in motion.

Forces, Newton's first law, StudySmarterDemonstration of Newton's first law

Newton's first law in everyday life

In everyday life, we (and other objects) experience resistive forces, whether they be gravity, friction, etc. These are what slow us down, requiring the application of constant force to keep moving.

Think of something like an ice skating rink, which is a surface with very little friction. If you push a hockey puck, it will glide much farther than it would on a table (where there is more friction). However, there still is some friction present in its interaction with the ice, so it eventually stops. Newton's first law proposes that if you pushed this same hockey puck on a surface completely free of fiction, it would retain the exact velocity it had when it left the stick, and it would keep gliding forever unless some other forces were to act on it.

Inertia

Inertia is a quantity that relates to the willingness of objects to change the state of their motion.

Inertia is the natural tendency of an object to remain at rest or in motion at a constant velocity along a straight line.

Newton's First Law, Inertia, StudySmarterInertia

An object will have inertia proportional to its mass, measured in kilograms. This means the more the mass of an object, the greater its inertia. In this way, mass is the quantitative measurement of an object's inertia, whether at rest or in motion.

So Newton's first law can be summarised as the law of inertia. In the absence of a net force, an object will remain in its state of motion, whether at rest or moving with constant velocity. The sum of multiple forces acting on an object is called the net force. If the net force is zero, there will be no acceleration. This can be modelled mathematically as F = 0.

We are only going to deal with coordinate systems that do not accelerate. They are called inertial reference frames. They are either at rest or moving with constant velocity (accelerating reference frames). A good example of inertia at work is when passengers fall forward when a fast-moving bus stops suddenly.

What is the importance of Newton's first law?

As discussed earlier, Newton's first law can be considered as the law of inertia. This is very significant – it is what holds the universe together. If inertia were to be turned off, everything would fall apart.

Remember that net force is the addition of all the forces that act on an object. All the forces considered here exist on Earth. It is quite different in space. Since there is almost no other force acting on bodies in space, they keep moving and could be moving forever. So Newton's first law gives us an understanding of the movements of bodies in space.

This same law makes it possible for us to develop cars by knowing how much force it will take to get the car to move and how much it will need to stop it. The heavier the car, the more force it will require. This isn't just the case with cars but ships, bicycles, machines, rockets, etc.

Newton's First Law - Key takeaways

  • Newton's first law of motion states that a body will stay at a constant velocity unless acted upon by an external force.
  • Inertia is the natural tendency of an object to remain at rest or in motion at a constant velocity along a straight line.
  • An object is compelled to stay in motion till it encounters a net force.
  • Newton's first law can be summarised as the law of inertia.
  • There will be no acceleration if the net force is 0.
  • An object's inertia is proportional to its mass.

Frequently Asked Questions about Newton's First Law

Newton's first law of motion states that a body will stay at a constant velocity unless acted upon by an external force.

Driving a car and then braking suddenly will stop the car, but the people inside will keep moving until they are pulled back by their seatbelts.

Newton's second law of motion states that the time rate of change of the momentum of a body is equal in both magnitude and direction to the force imposed on it. 

Newton's third law of motion states that when two bodies interact, they apply forces to one another that are equal in magnitude and opposite in direction.

A rocket will remain motionless on a launchpad till a force is exerted for it to move upward.

Final Newton's First Law Quiz

Question

What is Newton's first law?

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Answer

Newton's first law of motion states that a body will stay at a constant velocity unless acted upon by an external force.

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Question

Which of these is an example of the presence of the first law of motion?

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Answer

A ball at rest will continue to be motionless until someone kicks it, or some other force acts on it.

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Question

Which law is known as the law of inertia?


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Answer

Newton's first law of motion.

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Question

Inertia is the natural tendency of an object to remain at rest or in motion at a constant velocity along a straight line. 


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Answer

True

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Question

When does net force = 0?


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Answer

When a body is in equilibrium.

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Question

What is net force?


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Answer

Net force is the vector sum of all the forces that act on an object.

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Question

Can net force be equal to zero?


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Answer

Yes, when it is in equilibrium

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Question

A vector sum of all the forces that act on the object is what ...?


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Answer

Net force

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Question

When an object's acceleration is zero and velocity is at rest, it ....?


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Answer

Stays at rest

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Question

People experience a jerk when a moving bus stops due to what law of motion?


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Answer

Newton's first law of motion

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Question

What causes the motion of a body which was initially at rest?


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Answer

Force

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Question

Which of these does Newton's first law best describe?


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Answer

Inertia


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Question

In which of these cases is net force not equal to zero?


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Answer

A free-falling ball


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