Newton’s First Law of Motion, also known as the Law of Inertia, is the foundation of classical mechanics and a crucial concept in AP® Physics 1. It states that an object will remain at rest or move at a constant velocity unless acted upon by a net external force. This principle explains why objects resist changes in their motion and why forces are required to accelerate them. Understanding inertia helps explain real-world phenomena, such as why passengers lurch forward when a car suddenly stops or why a hockey puck glides across the ice with little resistance. Mastering this law provides a strong foundation for Newton’s other laws and makes solving force-related physics problems more intuitive and manageable.
What We Review
What is Newton’s First Law of Motion?
Newton’s First Law states:
“An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by a net external force.”
In simpler terms, objects don’t change what they’re doing unless something else forces them to.
For example, a book resting on a table experiences gravity pulling downward and a normal force pushing upward—since these forces cancel out, the book stays still. Similarly, imagine a hockey puck sliding on ice. If there’s no friction or obstacles, the puck will keep moving indefinitely because there is no net force acting on it to change its motion. Understanding these principles is essential for analyzing force interactions and solving equilibrium problems in AP® Physics.
Analyzing Forces
Forces are responsible for changing an object’s speed, direction, or shape, making them a fundamental concept in Newtonian mechanics. However, not all forces result in motion—how an object responds depends on whether the forces acting upon it are balanced or unbalanced.
- Balanced Forces: When forces acting on an object are equal in magnitude but opposite in direction, they cancel each other out, resulting in a net force of zero. In this case, the object remains at rest or continues moving at a constant velocity according to Newton’s First Law of Motion.
- Unbalanced Forces: When the net force acting on an object is not zero, it causes a change in motion. This means the object will accelerate, which could involve speeding up, slowing down, or changing direction. In some cases, unbalanced forces can also deform an object, such as compressing a spring or stretching a rubber band.
Real-Life Examples:
- In a tug-of-war game, if both teams pull with equal force, the rope doesn’t move. This is balanced force.
- A car accelerating down the street is an example of unbalanced force—engine power overcomes resistance.
Example: A Broken String
Problem:
A ball is being swung in a circle while attached to a string. Suddenly, the string snaps. In which direction will the ball move immediately after the string breaks?
Solution:
- Identify the Ball’s Motion Before the String Breaks:
- While the string is attached, the ball is constantly changing direction as it moves.
- The string pulls on the ball, keeping it moving along a curved path.
- What Happens When the String Breaks?
- The string is the only force keeping the ball in its circular path.
- When the string snaps, that force disappears instantly.
- Without this force, there is nothing causing the ball to continue turning.
- Determine the Ball’s New Path:
- At the moment the string breaks, the ball keeps moving in the direction it was headed at that exact instant.
- Since the ball had been following a curved path, the direction it moves after the string snaps is a straight line that points away from the curve.
Final Answer:
The ball will move in a straight line, continuing in the direction it was traveling at the moment the string broke. It will not fly outward from the center but will simply move forward from its last position in a straight path. In the image above, this is path B.
Inertial Reference Frames
An inertial reference frame is an ideal place where Newton’s First Law holds true. Objects in an inertial frame keep a constant velocity unless acted upon by a net force.
Why It Matters:
Understanding inertial and non-inertial frames is essential in confirming that objects behave according to Newton’s Laws of Motion. In an inertial frame of reference, an object will remain at rest or move at a constant velocity unless acted upon by an external force, following Newton’s First Law. However, in a non-inertial frame of reference, acceleration occurs, and it may seem as though forces are acting on objects without a clear physical cause. These apparent forces, known as fictitious forces, arise because the observer is in an accelerating frame rather than a truly stationary or uniformly moving one.
Examples of Inertial and Non-Inertial Frames
- Inertial Frame: A train moving at a constant speed on a straight track is an inertial frame because passengers inside feel no additional forces unless the train accelerates or decelerates. Objects at rest inside the train stay at rest, and moving objects continue in motion without deviation.
- Non-Inertial Frame: A car that is slowing down, speeding up, or turning is a non-inertial frame because its velocity is changing over time. Passengers inside feel apparent forces—like being pushed backward when the car accelerates or forward when it brakes—due to the car’s changing motion. These effects occur because the observer inside the car is in an accelerating reference frame, making the analysis of forces more complex.
Recognizing the difference between these frames is crucial in physics problems, especially when analyzing motion in rotating systems, accelerating vehicles, or free-fall scenarios, where additional forces may seem to appear.
Practical Example of Newton’s First Law
Let’s examine a detailed example involving a book sliding across a table.
Problem:
A book slides on a frictionless surface. What happens to the book if no outside forces are applied?
Solution:
- Identify the object: The book.
- Determine if forces are acting on it: On a frictionless surface, no horizontal forces act on the book.
- Analyze using Newton’s First Law: The book continues sliding with unchanged speed and direction.
- Conclusion: The book moves at constant velocity since no net force acts upon it.
Application in Real Life
Newton’s First Law can be seen in countless real-world scenarios. For example, in a moving vehicle, both the car and its passengers are in motion. According to Newton’s First Law, an object in motion remains in motion unless acted upon by an external force. If a car suddenly stops due to a crash, the passengers continue moving forward at the same speed until something stops them.
- Seatbelts and Airbags: Seatbelts apply an external force to the passenger, stopping their motion gradually and reducing the impact force on the body. Airbags further slow down deceleration, preventing serious injuries by spreading the force over a larger area.
- Crumple Zones: Modern cars are designed with crumple zones that absorb energy by deforming upon impact, reducing the force transferred to passengers and allowing a gradual deceleration rather than a sudden stop.
By applying Newton’s First Law, automotive engineers can design vehicles that mitigate inertia-related injuries, significantly improving passenger safety.
Conclusion: Newton’s First Law of Motion
Embracing Newton’s First Law is crucial for mastering AP® Physics 1. It forms the basis for understanding more complex topics. Regular practice with diverse problems solidifies this knowledge. Remember, understanding the basics of motion will set the stage for success in your physics journey. Keep practicing, and watch as these concepts become second nature!
| Term | Definition |
| Newton’s First Law | An object at rest stays at rest, and an object in motion stays in motion unless acted on by a net external force. |
| Net Force | The combined total of all forces acting on an object. |
| Translational Equilibrium | A state where the net force on an object is zero, resulting in no change in motion. |
| Inertial Reference Frame | A frame in which Newton’s First Law holds true, typically one with constant velocity. |
Sharpen Your Skills for AP® Physics 1
Are you preparing for the AP® Physics 1 test? We’ve got you covered! Try our review articles designed to help you confidently tackle real-world physics problems. You’ll find everything you need to succeed, from quick tips to detailed strategies. Start exploring now!
Need help preparing for your AP® Physics 1 exam?
Albert has hundreds of AP® Physics 1 practice questions, free response, and full-length practice tests to try out.
