If you put your hand out of the window of a car traveling at high speed, the air flow over the vehicle body will create a force on your hand. The direction of this force depends on the relative motion between your hand and the surrounding air.
As the car moves forward, the air flows from the front to the back of the vehicle. This creates an area of higher pressure in front of the car and lower pressure behind it. When you extend your hand outside the car window, the air pressure on your hand will be influenced by these factors:
Air resistance: The air molecules collide with your hand as you move it through the air. This creates a force in the opposite direction of your hand's motion, known as air resistance or drag. The magnitude of this force increases with the speed of the car.
Bernoulli's principle: As the air flows around your hand, it follows the shape of your hand and generates a pressure difference between the upper and lower surfaces. According to Bernoulli's principle, faster-moving air creates lower pressure, while slower-moving air creates higher pressure. This pressure difference contributes to the net force acting on your hand.
Considering these factors, when you place your hand outside the car window:
Forward force: The air resistance or drag force will push your hand backward, opposing its motion. This force increases with the speed of the car.
Upward or downward force: Depending on the angle of your hand relative to the oncoming air, the pressure difference created by Bernoulli's principle can result in an upward or downward force. This effect is similar to the lift generated by an airplane wing. If your hand is tilted upward, it may experience an upward force, and if it's tilted downward, a downward force.
The net effect on your hand will depend on the specific angle, shape, and orientation of your hand as well as the speed of the car. It is essential to note that sticking your hand out of a moving vehicle can be dangerous due to the potential for unexpected forces and loss of control. It is always recommended to keep your hands and body inside the vehicle for safety reasons.