Newton's First Law (College Board AP® Physics 1: Algebra-Based)
Study Guide
Newton's first law
Newton’s first law of motion states:
If the net force exerted on a system is zero, the velocity of that system will remain constant
A constant velocity could also be a velocity of zero i.e. when the system is stationary
If the net force acting on an object is zero, it is said to be in translational equilibrium
A football obeying Newton's first law
Worked Example
If there are no external forces acting on the car other than friction, and it is moving at a constant velocity, what is the value of the magnitude of the frictional force Ff when the driving force is 6 kN?
A –6 N
B –6 kN
C 6 kN
D 12 kN
The correct answer is C
Answer:
Since the car is moving at a constant velocity, there is no resultant force. This means that the driving force and the frictional forces are balanced
The question asks for the magnitude of the frictional force, therefore, Ff = 6 kN
Examiner Tip
This law may sound counter-intuitive for an object that is moving at constant velocity. How can it be moving if the forces on it are balanced?
This is because a resultant force causes an acceleration. An object moving at constant velocity has no acceleration, so its forces must be balanced, which means the resultant force is zero. The drag forces are invisible to us, which makes this tricky to see.
Inertial reference frame
An inertial reference frame is a non-accelerating reference frame
A reference frame is a coordinate system in which an observer makes position and time measurements of physical events
In an inertial reference frame, an observer can verify Newton’s first law of motion
Newton's first law is also called the law of inertia
Inertia is the tendency of an object to
remain in a constant state of motion
resist a change in motion
For example, a moving vehicle travelling at a constant velocity provides an inertial reference frame for the passengers inside
The passengers and the car are moving together
Inertia and mass
Objects with a greater mass have a greater inertia
Objects with a greater mass have a greater tendency to resist a change in motion
It is more difficult to move a box with a greater mass than it is to move a box with a smaller mass
Moving boxes of different masses
Worked Example
Explain why it is more difficult to stop a fully loaded dump truck than a small passenger car.
Answer:
A fully loaded dump truck is more difficult to stop than a small passenger car because it has a greater mass
Objects with a greater mass have greater inertia
Objects with greater inertia have a greater tendency to resist the change in motion
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