1st Law of Motion
The Law of Inertia
Dynamics is the study of the relationship between force and motion. Open the full Dynamics notes →
Once the magnet is set in motion, why does it stay in motion?
Listen to an AI discussion of this lesson
Video Instruction
showmethephysics.com
1st Law — Law of Inertia
When Fnet = 0…
An object at rest…
…tends to stay at rest.
An object in motion…
…tends to stay in motion.
Balanced Forces
Unbalanced Forces
Identify all the unbalanced forces below
Explain This Using Newton's 1st Law
Why doesn't the ball roll backward after the cart stops and starts up again?
Inspired by Dr. Richard Feynman
Notice that at the exact moment the ball reaches the front of the cart, the cart instantly moves ahead at the same velocity it had previously. Therefore the ball never actually comes to rest — it maintains its "moving inertia." If there had been even the smallest time interval between when the ball reached the front of the wagon and when the wagon began moving again, we would have had different results: the ball would have bounced off the front of the wagon, and when the wagon continued forward, the ball would then have rolled to the back of the wagon at an even faster rate.
Watch the animation again, but this time follow the movement of the ball with a ruler — you'll see that once the ball starts moving, it never really stops moving.
Sonic Wind I — John Paul Stapp, 1954
Stapp decelerating in the G-Whiz sled. This five-frame "movie" shows pretty well the stress he experienced. Courtesy EAFB History Office.
John Stapp stops short after traveling 421 mph. He suffered repeated and various injuries including broken limbs, ribs, detached retina, and miscellaneous traumas that eventually resulted in lifelong vision problems caused by permanently burst blood vessels in his eyes. In one of his final rocket-propelled rides, Stapp was subjected to 46.2 times the force of gravity.
Physics Application of the 1st Law
If an Object Changes Direction, It Must Have a Force on It
“The Moon must have a force on it.” — Isaac Newton
Mass and Inertia
What makes the world go 'round?
NYSSLS Reading Passage · Dynamics
The Shoulders of Giants: Newton, Hooke, and the Rivalry That Forged Modern Physics
Open Reading Passage → Regents Practice Version →
Check Your Understanding
5 quick questions — pick an answer to see if you’re right.
1. According to Newton's 1st Law, an object in motion with no unbalanced force on it will…
2. A hockey puck slides on perfectly frictionless ice. The net force on it is zero. What happens to its velocity?
3. An unbalanced force acting on an object can change…
4. The Moon constantly changes direction as it orbits Earth. What does Newton's 1st Law tell us about the Moon?
5. A car stops suddenly and an unbelted passenger lurches forward. Why?
Crash-Test Sled
Engineers test seat designs using a crash-test sled. A dummy sits on the sled, which travels at 15 m/s and then stops suddenly against a barrier. In Trial 1 the dummy wears no seat belt; in Trial 2 it is belted in.
In Trial 1, the unbelted dummy flies forward off the seat when the sled stops. Which explanation is correct?
▶ Want a challenge? Open 6 more Regents-style questions (harder · mixed formats)
Challenge score: 0 / 6
C1. Place a check mark in three boxes to indicate the objects that have zero net force acting on them.
C2. A ball rolls around the inside of a circular track. At one point, the track ends. Which path does the ball follow after leaving the track (top view)?
C3. A student makes the following claim.
Place a check mark in either the Support or Refute box. Justify your response.
C4. In Trial 2, the sled moves to the right toward the barrier. The seat belt brings the 70.-kg dummy to rest with an average acceleration of magnitude 150 m/s². Choose the direction of the average force the belt exerts on the dummy, and determine its magnitude.
●
C5. A car at rest is hit from behind. Why do engineers put headrests on car seats?
C6. Engineers are choosing between a stiff seat belt that barely stretches and a belt that stretches slightly during a crash. Using Newton’s first law, explain why a seat belt is needed, and identify one benefit of the stretchy belt.