Dynamics: Forces and Motion: 25 flashcards
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1. What does dynamics study?
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Dynamics studies motion and its causes — specifically force and mass.
2. How does dynamics differ from kinematics?
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Kinematics describes motion (displacement, velocity, acceleration); dynamics explains why motion changes, using force and mass.
3. State Newton's first law of motion.
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An object stays at rest or keeps moving at constant velocity unless a net external force acts on it.
4. Write Newton's second law as an equation.
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$$\vec{F}_{\text{net}} = m\vec{a}$$ — the net force equals mass times acceleration.
5. State Newton's third law of motion.
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For every action force there is an equal-magnitude reaction force in the opposite direction.
6. Why is force called a vector quantity?
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Because it has both magnitude and direction; forces must be added as vectors, not as plain numbers.
7. Define mass.
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Mass is the amount of matter in a body and measures its inertia — its resistance to a change in motion.
8. Define weight and give its formula.
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Weight is the gravitational force on a mass: $$W = mg$$ where $g \approx 9.8\,\text{m/s}^2$ near Earth's surface.
9. How do mass and weight differ on the Moon?
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Mass is unchanged on the Moon, but weight is about one-sixth as much because gravity is weaker there.
10. What is a free-body diagram (FBD)?
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A diagram that isolates one object and shows every force acting on it — weight, normal force, tension, friction, applied force.
11. Which forces belong in a free-body diagram?
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Only the forces acting on the chosen object — never the forces the object exerts on other things.
12. What is static friction?
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The friction that prevents an object from starting to move; it adjusts up to a maximum value.
13. What is kinetic friction?
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The friction that acts once an object is already sliding; it is roughly constant in size.
14. Define momentum and give its formula.
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Momentum is the product of mass and velocity: $$\vec{p} = m\vec{v}$$
15. Write Newton's second law in momentum form.
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$$\vec{F}_{\text{net}} = \frac{\Delta \vec{p}}{\Delta t}$$ — force is the rate of change of momentum.
16. State the impulse–momentum theorem.
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Impulse equals change in momentum: $F\Delta t = \Delta \vec{p}$.
17. Why do airbags reduce injury in a crash?
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They increase the collision time, so the same change in momentum needs a smaller force on the passenger.
18. What is the net force in $F = ma$?
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The vector sum of all forces acting on the object — not any single applied force.
19. If net force doubles while mass stays fixed, what happens to acceleration?
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Acceleration doubles, because $a = F_{\text{net}}/m$.
20. If mass doubles while net force stays fixed, what happens to acceleration?
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Acceleration halves, because acceleration is inversely proportional to mass.
21. Why does the same push send a football flying but barely move a truck?
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The truck has far greater mass, so the same force produces far smaller acceleration.
22. Why do you lurch forward when a bus brakes suddenly?
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Your body keeps its forward velocity (inertia) while the bus decelerates beneath you.
23. Why do heavy vehicles need longer stopping distances?
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Their larger mass means the same braking force produces smaller deceleration, so they take longer to stop.
24. Why do astronauts float in orbit?
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They are in free fall around Earth, not outside gravity — they and the spacecraft fall together.
25. Name three real-world applications of dynamics.
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Vehicle braking and safety design, rocket launch trajectories, and sports science (optimising a sprinter's start or a kick).
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