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Understanding why objects resist changes in their state of motion.
Why do you keep flying forward when a car suddenly slams on its brakes? It’s not a ghost pushing you—it is a fundamental law of the universe that explains why everything from planets to paperclips is 'lazy.'
Newton’s First Law of Motion states that 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 an unbalanced force. Essentially, objects are 'stubborn'—they want to keep doing exactly what they are already doing. This tendency to resist any change in motion is called Inertia. If no outside force (like friction or a push) acts on an object, its velocity will remain constant: .
Quick Check
If a hockey puck is sliding on perfectly frictionless ice, what will happen to its speed over time if no one touches it?
Answer
The puck will continue to slide at the exact same speed in a straight line forever.
Not all objects have the same amount of inertia. The amount of inertia an object has depends entirely on its mass. Mass is the amount of 'stuff' or matter in an object. The more mass an object has, the more it resists changing its state of motion. For example, it is much harder to start pushing a car than a bicycle. This is because the car has more inertia due to its greater mass.
1. Imagine an empty grocery cart with a mass of . 2. Imagine a full grocery cart with a mass of . 3. When you try to stop both carts suddenly, the cart is much harder to stop because its higher mass gives it more inertia, meaning it wants to keep moving forward more strongly than the empty cart.
Quick Check
Which has more inertia: A lead ball sitting still, or a bowling ball moving at ?
Answer
The lead ball has more inertia because inertia depends only on mass, not speed.
We experience inertia every day. When a bus accelerates quickly, your body feels like it is being thrown backward. In reality, your body was at rest, and it wants to stay at rest while the bus moves forward under you. Similarly, seatbelts are designed specifically to counter inertia. When a car stops abruptly, your body continues moving forward at the car's original speed until the seatbelt applies an unbalanced force to stop you.
1. A magician pulls a tablecloth out from under a set of heavy plates. 2. The plates have significant mass, and therefore high inertia. 3. Because the pull is so fast, the friction (force) applied to the plates is minimal and brief. 4. The plates' inertia causes them to remain at their current position (at rest) while the cloth is zipped away.
1. The Voyager 1 spacecraft is currently over billion kilometers away from Earth. 2. It has been moving at roughly for decades without using its engines for propulsion. 3. Because there is no air resistance (friction) in the vacuum of space, its inertia keeps it moving in a straight line at a constant speed indefinitely, according to Newton's First Law.
What is the primary factor that determines how much inertia an object has?
According to Newton's First Law, if you throw a rock in outer space (away from any gravity or friction), what will happen?
An object at rest has no inertia.
Review Tomorrow
In 24 hours, try to explain to a friend why a headrest in a car protects you from 'whiplash' during a rear-end collision using the word 'inertia.'
Practice Activity
Try the 'Coin Drop' experiment: Place a playing card over a cup, put a coin on the card, and flick the card away quickly. Observe how inertia keeps the coin in place until gravity pulls it down.