Newton’s First Law (OCR GCSE Physics A (Gateway))
Revision Note
Written by: Ashika
Reviewed by: Caroline Carroll
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
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
A few examples with uniform velocity are shown below:
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
If there are no external forces acting on the car and it is moving at a constant velocity, what is the value of the frictional force, F?
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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