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.
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?
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
