A lever scale is balanced with a partially filled glass of water on one side. Will the balance be upset (and if so how?) if you put your finger carefully into the water without touching the glass? What happens if the beaker is filled with air instead of water?
The water exerts a force up on your hand equal to the weight of water displaced. Therefore an equal and opposite force is down on the scale. Ultimately that force comes from your arm pushing the finger down into the water.
In the air there will be no effect because you are not displacing any weight of air. The air you displace simply leaves the glass unlike the water that rises around your finger, thereby raising the pressure on the bottom of the glass.
Hello Damon i have asked my teacher and he said that the air part is not what you said. He gave me one more chance to tell him what is the right answer.
To understand the effects of placing your finger in the water or air-filled beaker on the balance of the lever scale, we need to consider the concept of buoyancy and the principles behind how objects float or sink.
When you put your finger carefully into the water without touching the glass, the balance of the lever scale will be upset. This is because water exerts an upward force known as buoyant force on any object submerged in it, including your finger. By inserting your finger, you are displacing some of the water and creating a change in the distribution of the buoyant force.
When the glass is partially filled with water, the glass initially displaces an amount of water equal to its weight, and the lever scale is balanced. However, as you insert your finger into the water, you are adding more volume (and hence more weight) to one side of the lever, while the other side remains unchanged. This additional weight causes an imbalance, and the lever scale will tilt towards the side with the finger, as the weight on that side increases.
Conversely, if the beaker is filled with air instead of water, the balance of the lever scale will not be upset when you carefully insert your finger into the air-filled beaker. Unlike water, air does not exert a significant buoyant force on your finger, and hence, your finger's presence in the air does not alter the weight distribution on the lever scale. The balance will remain unchanged.
In summary, inserting your finger into a partially filled water beaker will upset the balance of a lever scale due to the change in weight distribution caused by the displacement of water. However, inserting your finger into an air-filled beaker will not affect the balance since air exerts minimal buoyant force.