Hard ground can bring an object from high speed to rest in a very short period of time, while a soft object needs a long time. Why is this so? and is it true that when an object hits the ground, the action force is exerted by the ground on the object, while the force exerted by the object is the reaction force? When you are falling in the air in the case of free fall, is it true that you experience no force exerted upwards on you? Then where is the air resistance? When you are falling with parachute and are falling in terminal velocity, if you exert a force on the air, will you float upwards in the air? Will there be any parachute whose surface area is so large that when you open it when falling, you won't fall but will float upwards in the air? If body A exerts a force on body B, but the force is the maximum force body B can take, what will happen to B? In this case will B still exert an equal amount of force on A? Why some of the insects can float on water? Is it because they are too light and water can exert a force equals its weight, i.e, their weight is less than the maximum force water can exert at the surface, so they can float.

Let's break down your questions step by step, as they touch on several fundamental concepts in physics, particularly related to forces, motion, and fluid dynamics.

The Impact of Hard and Soft Surfaces

When an object hits a hard surface, like concrete, it comes to a stop almost instantaneously. This is because the hard surface exerts a large force over a very short time period, resulting in a rapid deceleration. In contrast, a soft surface, such as sand, allows the object to sink in slightly, which increases the time it takes to come to a stop. The longer the time over which the deceleration occurs, the smaller the average force exerted on the object, according to Newton's second law of motion (F = ma).

Action and Reaction Forces

You're correct in stating that when an object hits the ground, the ground exerts an action force on the object. This is a classic example of Newton's third law, which states that for every action, there is an equal and opposite reaction. So, when the ground pushes up on the object, the object also exerts a force downward on the ground. These forces are equal in magnitude but opposite in direction.

Free Fall and Forces

During free fall, you are indeed accelerating downwards due to gravity, and you do not feel any upward force acting on you. This is because you are in a state of free fall, where the only force acting on you is gravity. However, air resistance does exist; it acts in the opposite direction to your motion. The faster you fall, the greater the air resistance becomes until it balances the gravitational force, leading to terminal velocity.

Parachutes and Terminal Velocity

When you deploy a parachute, it increases your surface area significantly, which increases air resistance. At terminal velocity, the force of air resistance equals the gravitational force, resulting in no net acceleration. If you exert a force on the air while falling with a parachute, you may experience a slight upward movement, but it won't be enough to float upwards unless the parachute is designed to create a significant lift. Theoretically, a parachute could be large enough to create enough lift to counteract your weight, but in practice, this is limited by the physics of air resistance and the design of the parachute.

Forces Between Bodies

If body A exerts a force on body B that equals the maximum force body B can withstand, body B will not be able to sustain that force and may break or deform. However, according to Newton's third law, body B will still exert an equal and opposite force on body A, regardless of whether it can withstand the force. This is a crucial aspect of understanding interactions between objects.

Insects Floating on Water

Insects like water striders can indeed float on water due to surface tension. Their weight is less than the maximum force that the surface tension of the water can exert. This allows them to remain on the surface without sinking. The combination of their lightweight bodies and the cohesive forces between water molecules enables them to "walk" on water.

In summary, the principles of force, motion, and fluid dynamics are interconnected in fascinating ways. Each scenario you presented illustrates different aspects of these principles, showcasing the complexity and beauty of physics in our everyday lives.