Newton's Laws of Motion: 20 flashcards
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1. State Newton's first law.
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An object stays at rest or in uniform motion unless acted on by a net external force.
2. What is inertia?
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The tendency of an object to resist changes in its motion.
3. Why does a rolling ball eventually stop on Earth?
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Because friction and air resistance are external forces acting on it.
4. Write Newton's second law as an equation.
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$\vec{F}_{\text{net}} = m\vec{a}$
5. How is acceleration related to net force for a fixed mass?
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Acceleration is directly proportional to net force.
6. How is acceleration related to mass for a fixed net force?
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Acceleration is inversely proportional to mass.
7. In which direction does acceleration point?
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In the direction of the net force.
8. What is net force?
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The vector sum of all forces acting on an object.
9. If net force is zero, what is the acceleration?
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Zero — the object moves at constant velocity.
10. State Newton's third law.
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For every action force there is an equal and opposite reaction force.
11. Why don't action–reaction pairs cancel each other?
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They act on two different objects, so they don't cancel on either one.
12. What does mass measure?
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Inertia — how much an object resists acceleration; measured in kg.
13. What is weight?
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The gravitational force on a mass: $W = mg$, measured in newtons.
14. In $F = ma$, should you use mass or weight?
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Mass (in kg), never weight.
15. What net force is needed for constant velocity?
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Zero net force.
16. Why do seatbelts protect passengers?
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Because of inertia — your body keeps moving forward when the car stops (first law).
17. How does a rocket move forward?
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By expelling gas backward; the gas pushes the rocket forward (third law).
18. What are the steps to solve a mechanics problem?
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Identify the object, list forces on it, find net force, apply $F = ma$.
19. A $10\,\text{kg}$ box is pushed with $30\,\text{N}$ on a frictionless floor. Find its acceleration.
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$a = F/m = 30/10 = 3\,\text{m/s}^2$ in the direction of the push.
20. If friction of $10\,\text{N}$ opposes the $30\,\text{N}$ push on the $10\,\text{kg}$ box, what is the acceleration?
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Net force $= 20\,\text{N}$, so $a = 20/10 = 2\,\text{m/s}^2$.
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