Force and motion -- A water drop on the windscreen of a car

In summary, the conversation discusses the relationship between the horizontal force exerted by the wind and the change in normal force, as well as the relationship between friction and normal force. It is mentioned that the drop stays on the windscreen due to surface tension, and that the magnitude of static friction is only as large as needed to prevent sliding.
  • #1
shk
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Homework Statement
I am helping someone with her homework and need help with part C.
Relevant Equations
I have attached all my working.
F=ma
Hi
Please help me with part C.
I have answered the question and have attached it for you to see.
 

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  • #2
Yes, the wind will exert a horizontal force, and R may also change.
You will need to make some assumption about the nature of F to decide whether that will change,
You also know that the drop stays on the windscreen, which gives you information relating the acceleration of the car to the acceleration of the drop.
 
  • #3
haruspex said:
Yes, the wind will exert a horizontal force, and R may also change.
You will need to make some assumption about the nature of F to decide whether that will change,
You also know that the drop stays on the windscreen, which gives you information relating the acceleration of the car to the acceleration of the drop.
F is friction. When car accelerates, the drop goes to the left with the same acceleration so R has to increase since it needs a bigger component to the left . F is always directly proportionality to R so F increases as well.
haruspex said:
Yes, the wind will exert a horizontal force, and R may also change.
You will need to make some assumption about the nature of F to decide whether that will change,
You also know that the drop stays on the windscreen, which gives you information relating the acceleration of the car to the acceleration of the drop.
Thanks for the comment. F is friction. When car accelerates, the drop goes to the left with the same acceleration so R has to increase since it needs a bigger component to the left . F is always directly proportionality to R so F increases as well. Is this correct ?
 
  • #4
shk said:
F is friction... F is always directly proportionality to R so F increases as well.
Water is not solid. When it flows over a surface, it does not all need to move at once.
I would think a raindrop can stay where it is on a sloping surface because of surface tension. That does not increase when the normal force increases.
Anyway, it is not true that static frictional force is proportional to normal force. It is only ever as large as it needs to be to prevent sliding. ##\mu_sN## is only the upper limit of the magnitude of the frictional force.
 

Related to Force and motion -- A water drop on the windscreen of a car

1. What causes the water drop to move on the windscreen of a car?

The force of the wind created by the car's movement causes the water drop to move on the windscreen. This force is known as air resistance.

2. Why does the water drop eventually fall off the windscreen?

The force of gravity pulls the water drop downwards, causing it to eventually fall off the windscreen. This force is constantly acting on all objects on Earth.

3. How does the size of the water drop affect its motion on the windscreen?

The larger the water drop, the more surface area it has to interact with the wind resistance, causing it to move faster and potentially stay on the windscreen longer. Smaller water drops have less surface area and are more easily affected by gravity, causing them to fall off the windscreen faster.

4. Can the shape of the water drop affect its motion on the windscreen?

Yes, the shape of the water drop can affect its motion on the windscreen. A more streamlined shape will experience less air resistance and therefore move faster, while a rounder shape will experience more air resistance and move slower.

5. How does the speed of the car impact the motion of the water drop on the windscreen?

The faster the car is moving, the stronger the force of the wind will be on the water drop, causing it to move faster and potentially stay on the windscreen longer. Slower speeds will result in less wind resistance and a slower movement of the water drop.

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