Newton's First Law (Cambridge (CIE) IGCSE Physics)
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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
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
Objects with Zero Resultant Force
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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