Newton's First Law (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

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Leander

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Leander

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Newton's first law

  • 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

  • Newton's first law means if the resultant force acting on an object is zero:
    • A stationary object will remain stationary 
    • An object moving in a straight line at a constant speed will continue to move in a straight line at a constant speed
  • When the resultant force is not zero
    • The speed of the object can change
    • The direction of the object can change

  • A simpler way to think about Newton's first law is for an object to change its motion, it requires a resultant force to act upon it

Examples of Newton's first law

  • A mug on a table will remain stationary on the table unless acted upon by a resultant force
    • For example, if someone picks up the mug, or knocks into the table
  • A piece of space debris will continue moving in a straight line at a constant speed unless acted upon by a resultant force
    • For example, if it enters the gravitational field of a planet or collides with an asteroid
  • A car travelling in a straight line at a constant speed will continue to travel in a straight line at a constant speed unless acted upon my a resultant force
    • For example, if the driver brakes or accelerates

Worked example

A student did some online research and found out that the Moon orbits the Earth at a constant speed of around 2000 mph.

The student says that this is not an example of Newton's first law of motion. Is the student 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 the student is correct

  • The student 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

A car moves at a constant velocity. The driving force from the engine is 3 kN.

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

Determine the frictional force acting on the car.

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.