Newton's First Law (Cambridge O Level Physics)

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Newton's First Law of Motion

  • Newton's first law of motion states:

Objects will remain at rest, or move with a constant velocity unless acted on by a resultant force

  • This means if the resultant force acting on an object is zero:
    • The object will remain stationary if it was stationary before
    • The object will continue to move at the same velocity if it was moving

  • When the resultant force is not zero
    • The speed of the object can change
    • The direction of the object can change

Applying Newton's First Law

  • Newton's first law is used to explain why things move with a constant (or uniform) velocity
    • If the forces acting on an object are balanced, then the resultant force is zero
    • The velocity (i.e. speed and direction) can only change if a resultant force acts on the object

Examples of Objects with Uniform Velocity

Newtons First Law in Action, downloadable IGCSE & GCSE Physics revision notes

Constant velocity can only be achieved when the forces on an object are balanced - in other words, when the resultant force is zero 

Worked example

Lima did some online research and found out that the Moon orbits the Earth at a constant speed of around 2000 mph. She says that this is not an example of Newton's first law of motion.

Is Lima correct? Explain your answer.

Answer:

Step 1: Recall Newton's first law of motion

  • Newton's first law of motion states that objects will remain at rest, or move with a constant velocity, unless acted on by a resultant force

Step 2: Determine if the object in the question is at rest, or if it is moving with a constant velocity

  • The Moon, in this case, is not at rest
  • It is moving at a constant speed
  • But it is not moving in a constant direction - it continually orbits the Earth
  • Hence, it is not moving with a constant velocity, because velocity is a vector quantity

Step 3: State and explain whether Lima is correct

  • Lima is correct
  • The Moon moves with a constant speed, but always changes direction
  • So it is not moving with a constant velocity, and is not an example of Newton's first law of motion

Worked example

There are no external forces acting on the car and it is moving at a constant velocity.

State the value of the frictional force, F.

WE - frictional force on car question image, downloadable AS & A Level Physics revision notes

Answer:

Step 1: Recall Newton's first law of motion

  • Newton's first law of motion states that objects will remain at rest, or move with a constant velocity unless acted on by a resultant force

Step 2: Relate Newton's first law to the scenario

  • Since the car is moving at a constant velocity, there is no resultant force
  • This means the driving and frictional forces are balanced

Step 3: State the value of the frictional force

  • Frictional force, F = driving force = 3 kN

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Leander

Author: Leander

Expertise: Physics

Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.