Terminal velocity
When an object is falling through a fluid (or, less commonly, accelerating across a solid surface with friction), it will eventually reach a point where the resistive forces (drag or friction) are equal in magnitude to the driving force (usually gravity), and the object will stop accelerating and continue to move at a constant velocity.
This velocity is called the terminal velocity of the object.
we’ll focus on terminal velocity in the context of an object falling through a fluid under the force of gravity. The same principles apply to an object accelerating across a solid surface with friction.
What happens when an object is falling?
When any object falls through any fluid, it speeds up because of the force of gravity causing it to accelerate downwards. The velocity increases.
However, as velocity increases, the drag force (or frictional force) also increases. It increases at a faster rate than the velocity, so the drag force will eventually increase so much to the point where it becomes equal in magnitude to the gravity.
Basically, when an object gets fast enough, the air resistance becomes equal to the weight of the object.
What happens at terminal velocity?
When the drag force becomes equal in magnitude to the weight of the object, the object will stop accelerating and will continue to fall at a constant velocity - the terminal velocity.
So it stops accelerating but it doesn’t stop moving - it continues to fall, but at a constant velocity (doesn’t speed up anymore).
What affects terminal velocity?
- The object’s mass:
- The surface area of the object (and how streamlined it is):
- The density of the fluid:
flashcards
| Question | Answer |
|---|---|
| When an object is falling through a fluid, what eventually happens to the forces acting on it? | The resistive forces (drag or friction) become equal in magnitude to the driving force (usually gravity), so the object stops accelerating and continues to move at a constant velocity. |
| What is terminal velocity? | The constant velocity an object reaches when the resistive forces (drag or friction) are equal in magnitude to the driving force (usually gravity), causing acceleration to stop. |
| What causes an object to speed up when it first starts falling through a fluid? | The force of gravity causing it to accelerate downwards. |
| How does drag force change as an object’s velocity increases while falling? | The drag force increases at a faster rate than the velocity. |
| At terminal velocity, what is the relationship between drag force and weight? | The drag force becomes equal in magnitude to the weight of the object. |
| What happens to an object’s motion at terminal velocity? | It stops accelerating but continues to move (fall) at a constant velocity. |
| How does an object’s mass affect its terminal velocity? | A heavier object has a greater weight, so it needs a greater drag force to balance the weight, so it must fall faster to generate that drag, resulting in a higher terminal velocity. |
| How does surface area (and streamlining) affect terminal velocity? | A larger surface area increases the drag force for a given velocity, so it can balance the weight at a lower velocity, resulting in a lower terminal velocity. |
| How does the density of the fluid affect terminal velocity? | A denser fluid increases the drag force for a given velocity, so it can balance the weight at a lower velocity, resulting in a lower terminal velocity. |