Newtons first law practice
Name: Period: Date:
Thinking About Newton’s Laws (I)
What is the effect of friction on a moving object?
The effect of friction on a moving object is that it opposes the direction of motion, causing the object to slow down or eventually stop if no further force is applied.
If you roll a ball up on a ramp its speed decreases. What if you rolled it on a smooth, flat frictionless surface?
It would continue moving indefinitely at a constant speed because without air resistance there would be no net force to stop it according to Newton’s first law of motion.
Does the law of inertia apply to moving objects or objects at rest? Support your answer with an example.
The law of inertia applies to both moving objects and objects at rest. For an object at rest, like a book on a table, it will remain at rest unless a force (you pushing it) moves it. For a moving object, like a hockey puck sliding on ice, it continues moing at a constant velocity unless an outside force, like friction or a player hitting it, changes its motion.
The law of inertia states that no force is required to maintain motion. Why, then, do you have to keep pedaling your bike to keep moving?
You have to keep pedaling your bike to keep moving because friction and air resistance are forces that oppose your motion. The law of inertia states that no force is required to maintain motion if there is no net force acting on the object. When you stop pedaling, these opposing forces create a net force, causing you to slow down.
If you launched a projectile in space how much force would it require to keep it in motion?
If a projectile were launched in space, it would require no force to keep it in motion once launched. It would continue moving at a constant velocity until acted upon by another force, such as the gravity of a planet or star.
What is inertia? How do we measure inertia?
Inertia is the resistance of an object to changes in its state of motion. It is a measure of an object’s unwillingness to accelerate. We measure inertia by its mass. The more mass an object has, the more inertia it possesses and the harder it is to change its motion.
What is the difference between weight and mass?
The main difference between weight and mass is that mass is a measure of the amount of matter in an object and its inertia, but weight is a measure of the force of gravity acting on that object. Mass is constant regardless of location, while weight changes depending on the gravitational field.
The moon’s gravity is ⅙ that of Earth’s. If you were on the moon explain how it would effect your weight and mass.
Your mass would remain the same, but your weight would be ⅙ of your Earth weight. You would feel lighter, but the amount of matter making up your body has not changed.
Your physics text book is on the dashboard of your car. You take a left and the book slides right. Explain why this happens
This happens because of the book's inertia. The book will continue moving in a straight line, as stated by Newton’s first law. When the car turns left, the car applies a force to turn around the book, but the book tries to maintain its original straight-line motion, making it appear to slide to the right (outward from the turn).
Describe the difference in motion between the two force diagrams below.
The right image has more force towards the right than the image on the left.
Complete the diagram to include a force on the plane as it fly’s at a constant speed.
Is it possible for an object to move if no net force is acting on it? Explain.
Yes it is possible. If the object is already moving, it will continue to move at a constant velocity if the net force is zero. A net force is only required to change motion (accelerate or decelerate).
If an object is at rest, can we conclude that no external forces are acting on it?
No, we can only conclude that the outside force is 0. Multiple forces may be acting on it, but they are balanced.
An object thrown into the air stops at the highest point in its path. Is it in equilibrium at this point? Explain.
No because equilibrium requires a force of 0 and constant velocity (which includes being at rest). At the highest point, the object’s velocity is momentarily 0, but the force of gravity is still acting on it, causing a downward acceleration. It immediately begins to fall back down, showing it is not in a state of equilibrium.
What physical quantity is a measure of the amount of inertia an object has?
The physical quantity that is a measure of the amount of inertia an object has is its mass.
A large crate is placed on the bed of the truck but is not tied down. As the truck accelerates forward, the crate slides across the bed until it hits the tailgate. Explain what causes this.
This happens because the crate resists the change in motion due to its inertia. The truck bed accelerates forward, but the crate tends to remain at rest. The force of friction between the truck bed and the crate is not strong enough to accelerate the crate at the same rate as the truck, so the truck moves out from under the crate, making the crate slide backward.
If the driver slammed on the brakes, what could happen to the crate?
The crate could slide forward because the crate is moving at the truck's speed and wants to continue moving at that speed (due to inertia) when the truck decelerates.
Why is mass a more fundamental property of weight?
Because mass is a measure of matter and inertia and remains the same everywhere in the universe.
If you were floating around in space would you rather get bumped into by an elephant or a mouse? Why?
I would rather get bumped into by a mouse because the elephant has a much larger mass and therefore much more inertia and momentum with the same velocity.
You drop a coin on a train at the station before departure. Where does it land?
What would happen if you drop it while moving at a constant speed? What about if you were accelerating? What accounts for any changes?
The coin lands straight down on the floor.
The coin lands straight down in the same spot because both me and the coin have the same forward velocity, and the coin maintains that velocity as it falls, due to inertia.
The coin would land behind the spot where you dropped it.
As you accelerate, the train speeds up, but the coin tries to maintain the slower velocity it had when you released it.
A pillow drops from the overhead on a jetliner which is flying at 600 km/h. Why does the pillow drop straight down and not on your lap?
Because both the pillow and the airplane are moving at the same constant horizontal velocity. Due to inertia, the pillow maintains this forward velocity as it falls. To an observer inside the plane, the only motion is the vertical fall due to gravity.
If an angry elephant was chasing you how would that elephants mass be to your advantage?
The elephant’s mass would be to my advantage because it is difficult to change the motion of a large mass. Once the elephant starts running, it has a lot of inertia and momentum, making it difficult for it to suddenly stop or change direction to catch you if you make sudden, sharp turns.