Forces And Terminal Velocity P
Übersicht
Drop a stone off a cliff and it gets faster the whole way down. Drop a skydiver out of an aircraft and something stranger happens: after roughly half a minute she stops getting faster at all, and spends the rest of the fall at a steady 44 m/s with the ground still a long way below. Nothing has caught her and nothing has switched gravity off. The air itself has grown a force big enough to match her weight, and it did so by the simple trick of her going faster.
This lesson is about that force. You will learn why a fluid pushes back harder the quicker you move through it, why every falling object therefore has a speed limit of its own, and how to read the whole story off a velocity-time graph whose gradient dies away to nothing. Then you will use the idea in both directions: parachutes, which are built to make the speed limit as low as possible, and sharks and racing cars, which are shaped to push it as high as possible.
Ziele
- An object moving through a fluid experiences friction. The faster the object moves, the greater the frictional forces (drag) acting on it.
- An object falling through a fluid will initially accelerate due to the force of gravity and the drag forces increase as the velocity increases. Eventually the resultant force will be zero and the object will move at its terminal velocity.
- Parachutes are designed to increase the drag force on a parachutist so that the terminal velocity is reduced. Students should be able to draw and interpret velocity-time graphs for objects that reach terminal velocity, including a consideration of the forces acting on the object.
- Streamlining reduces the drag force on an object so that its maximum velocity is increased. Students should be able to describe how the streamlining of a shark (an adaptation) or a car (a design feature) reduces the drag force and the object.
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- An object is falling through air at its terminal velocity. What is true about the forces on the object? A. There are no forces acting on it B. Its weight is greater than the drag C. The drag is equal in size to its weight D. The drag is greater than its weight Answer: C
- A submarine increases its speed through the water. What happens to the drag force acting on it? A. It increases B. It decreases C. It stays the same D. It falls to zero Answer: A
- A skydiver of mass 90 kg falls at her terminal velocity. The gravitational field strength is 9.8 N/kg. What is the size of the drag force acting on her? A. 0 N B. 9.2 N C. 90 N D. 882 N Answer: D
- A parachutist opens her parachute while falling at terminal velocity. What happens to her terminal velocity? A. It increases because the drag increases B. It decreases because the drag increases C. It decreases because her weight decreases D. It stays the same because her weight is unchanged Answer: B
- What is the effect of streamlining the body of a car? A. The drag increases and the maximum velocity increases B. The drag increases and the maximum velocity decreases C. The drag decreases and the maximum velocity increases D. The drag decreases and the maximum velocity decreases Answer: C
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Drag Forces
Another interesting force in everyday life is the force of drag on an object when it is moving in a fluid (either a gas or a liquid). You feel the drag force when you move your hand through water. You might also feel it if you move your hand during a strong wind. The faster you move your hand, the h
"College Physics: Drag Forces" by OpenStax, OpenStax, Rice University, licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Source: https://openstax.org/books/college-physics/pages/5-2-drag-forces. Access for free at https://openstax.org/books/college-physics/pages/1-introduction
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