Newton's First Law (Edexcel GCSE Physics: Combined Science)

• 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 

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

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