Newton's First Law (Cambridge O Level Physics)

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

There are no external forces acting on the car and it is moving at a constant velocity.

State 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