I need to answer this question for a physics lab. I think the question up there is pretty straightforward ^^. Thank you!
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Why does it take longer to bring an object to rest when it falls in water rather than when it falls in sand?
When an object falls on a surface, the surface exerts an upward force on the object. This force decreases the downward acceleration of the object. As the moves into the substance, the upward force may increase, decrease, or stay the same.
When a rock falls on a surface of water, you can see the dent in the water. As the rock continues to move downward, the buoyant force of the water is constant. The net force causing the rock to accelerate downward = Weight – Buoyant force. If the rock is more dense than water, the rock will continue accelerate until it reaches the bottom.
When a rock falls on a surface of sand, you can see the dent in the sand. The rock will continue accelerating downward until the upward force exerted by the sand is greater than the weight of the rock. So far, the 2 stories sound very similar. However, the similarity ends right here.
As the rock exerts a downward force on the sand, the sand is compressed. So, the upward force exerted by the sand increases as the rock move down. Very soon, the upward force exerted by the sand has caused the velocity of the rock to decrease to 0 m/s.
As the rock exerts a downward force on the water, the water moves out of the way. The water is “fluid”. As the surface of the rock collides with the water molecules, the water molecules rebound in the opposite direction. So, the rock can continue moving downward, until it collides with sand or whatever is below the water.
The dry sand does not have the ability to quickly “flow” away from the rock.
If you go to the website below, you will discover how sand can become “fluid”!
http://science.howstuffworks.com/environ…
But the rock will still move slower in the quicksand than in water.
When an object falls on a surface, the surface exerts an upward force on the object. This force decreases the downward acceleration of the object. As the moves into the substance, the upward force may increase, decrease, or stay the same.
When a rock falls on a surface of water, you can see the dent in the water. As the rock continues to move downward, the buoyant force of the water is constant. The net force causing the rock to accelerate downward = Weight – Buoyant force. If the rock is more dense than water, the rock will continue accelerate until it reaches the bottom.
When a rock falls on a surface of sand, you can see the dent in the sand. The rock will continue accelerating downward until the upward force exerted by the sand is greater than the weight of the rock. So far, the 2 stories sound very similar. However, the similarity ends right here.
As the rock exerts a downward force on the sand, the sand is compressed. So, the upward force exerted by the sand increases as the rock move down. Very soon, the upward force exerted by the sand has caused the velocity of the rock to decrease to 0 m/s.
As the rock exerts a downward force on the water, the water moves out of the way. The water is “fluid”. As the surface of the rock collides with the water molecules, the water molecules rebound in the opposite direction. So, the rock can continue moving downward, until it collides with sand or whatever is below the water.
The dry sand does not have the ability to quickly “flow” away from the rock.
If you go to the website below, you will discover how sand can become “fluid”!
http://science.howstuffworks.com/environ…
But the rock will still move slower in the quicksand than in water.
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