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Newton's First Law (CIE IGCSE Physics)
Revision Note
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.
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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