We know that we do not need to wear a helmet when we go outside in the rain. But why? Let's think about it.
Terminal speed
As a raindrop falls through the air, it experiences a number of forces. The force of gravity pulls the raindrop down, and the drag force opposes the motion of the raindrop through the air. The drag force is caused by the friction between the raindrop and the air.
When a raindrop falls from a cloud, it initially accelerates due to the force of gravity, F=mg.
- F is the net force acting on the object in newtons (N).
- m is the mass of the object in kilograms (kg).
- g is the gravitational acceleration in meters per second squared (m/s²).
The drag force is proportional to the square of the velocity of the raindrop. This means that as the raindrop falls faster, the drag force increases. The drag force eventually becomes equal to the force of gravity, and the raindrop reaches terminal velocity.
Here is the equation we can use to calculate the drag force.
When the drag force equals gravity, the raindrop reaches terminal velocity. We can express terminal velocity as the following:
From this equation, we can tell:
- The velocity of the object is inversely proportional to the square root of the density of the fluid. This means that the velocity of the object will be higher in a less dense fluid, such as air, than in a more dense fluid, such as water.
- The velocity of the object is inversely proportional to the reference area.
- This means that the velocity of the object will be higher for objects with a small reference area, such as a needle, than for objects with a large reference area, such as a plate.
- Final velocity of a heavy object falling is faster than that of a lighter object.
Kinetic Energy
If raindrops diameter to 2mm, and assume its density to water, we can get its final speed 4 meters per second. Let’s calculate its kinetic energy with the equation:
Where:
- E is kinetic energy in Joules
- m is the mass of the object in kilograms
- v is the velocity of the object in meters per second
As you can see, the kinetic energy of an object increases as its mass and velocity increase. A heavier object has more mass, so it has more kinetic energy. A faster moving object has more velocity, so it also has more kinetic energy .
We can compare the kinetic energy of a typical raindrop to the kinetic energy of a typical bullet(mass = 7.5g, v= 380m/S).
 |
| Kinetic energy comparison |
The table tells us that the bullet has a much higher kinetic energy than the raindrop. This is because the bullet has a much greater mass and velocity. The difference in kinetic energy is so great that the bullet can cause serious injury or death, while the raindrop is only capable of causing a slight sting.
It is important to note that the kinetic energy of an object can vary depending on the mass, velocity, and shape of the object. However, the table above can be used to get a general idea of the difference in kinetic energy between a bullet and a raindrop.
It is important to note that the kinetic energy of an object can vary depending on the mass, velocity, and shape of the object. However, the table above can be used to get a general idea of the difference in kinetic energy between a bullet and a raindrop.
Size of raindrops
As a raindrop falls, it can collide with other raindrops. When two raindrops collide, they can merge together to form a larger raindrop. This process is called aggregation.
Raindrops can also break apart into smaller raindrops. This process is called dispersion. Dispersion can be caused by the drag force, the wind, or collisions with other raindrops.
The process of aggregation and dispersion is continuous as a raindrop falls. This process determines the final size of the raindrop when it reaches the ground.
 |
Softly falling feathers with a negligible amount of kinetic energy |
Reference
Drag force
Tags:
General science