Newton's First Law of Motion
- 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
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
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
Exam Tip
Higher-tier students must be able to state Newton's first law from memory. Other students just need to be able to understand it and identify examples of it in action.
