Question

Question 38
I
When does an object falling vertically through the air reach terminal velocity?
A when the acceleration of the object becomes negative
when the acceleration of the object is equal to g
C when the air resistance equals the weight of the object
D
when the air resistance is greater than the weight of the object​

Answer 1

Q.1 When does an object falling vertically through the air reach terminal velocity?

Ans. Objects falling through a fluid eventually reach terminal velocity . At terminal velocity, the object moves at a steady speed in a constant direction because the resultant force acting on it is zero.

Q.2 When the acceleration of the object becomes negative?

Ans.According to our principle, when an object is slowing down, the acceleration is in the opposite direction as the velocity. Thus, this object has a negative acceleration. In Example D, the object is moving in the negative direction (i.e., has a negative velocity) and is speeding up.

Q.3 When the acceleration of the object is equal to g

C when the air resistance equals the weight of the object

D?

Ans.The Answer is given on picture||

Q.4 When the air resistance is greater than the weight of the object?

Ans. Yes, but it depends on initial conditions. I almost fell into a trap and said no.

And you also have to be careful what you consider “resistance” to be. Here I will equate it to what in fluid mechanics is defined as the drag force - a combination of viscous effects as the air tries to slide past the object, and a pressure wave as air in the direction of travel struggles to get out of the way of the moving object.

Air resistance always opposes motion so if dropped from rest you will accelerate with increasing air resistance until it is equal and opposite to gravity and you fall at a terminal velocity. But the magnitude of the resistance force is always less than or equal to gravity in this case.

However if you re-enter the atmosphere at speed (like several kilometers per second) then air resistance most definitely can exceed gravity and therefore you slow down (thank goodness for that say the astronauts), until drag again comes into balance at terminal velocity. Hopefully a quite low value aided by parachute before you reach ground level.

Forces due to motion through air can also exceed gravitational forces in two other ways:

The reduction in pressure due to Bernoulli’s principle gives us lift over a wing, and we all know airplanes can rise up in the air against gravity.

The pressure gradient in air due to the buoyancy effect can be used to lift objects less dense than air (like a helium balloon goes up!)

If you look at the accidental destruction of both space shuttles you see the air flow in both cases was a far more destructive force than gravity to the point equivalent g forces due to air flow were so high structures literally tore apart and survival was impossible.

Answer 2

Answer:

The object will reach to terminal velocity C when the air resistance equals the weight of the object.

Explanation:

• Terminal velocity: The maximum velocity attained by an object when it is falling through a fluid.
• When an object falls from a height then its velocity increases and with increasing velocity the drag force (of air) also increase. A moment comes at which the drag force and weight of the object becomes equal, then the object reaches to terminal velocity.
• After reaching terminal velocity, no net force is exerted on the object, so there is no acceleration. Now the object falls with constant speed.

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