Submerging an object suspended on a spring scale under water: Observational Experiment

Aim

The purpose of this experiment is to investigate how the force that a fluid exerts on an object submerged in it relates to the volume of the submerged part of the object.

Prior Knowledge

1. Some concept of conservation and incompressibility of fluid.
2. Free body diagrams.
3. Newton's laws as applied to equilibrium.

Description of the Experiment

Observe objects of various sizes and masses, suspended from a spring scale. Note the reading of the scale in air for each object. Note the level of the water in the beaker. Observe each object placed in the beaker. Record the new reading on the spring scale and the new level of the water in the beaker. Calculate the weight of water displaced by the object. Devise a rule relating the change in the reading of the scale and the weight of the displaced water.

Addtional Information

Make sure you use consistent units when doing the measurements and calculations. We suggest that you use SI units.

Youtube movies can be stepped frame by frame using the , and . keys on your keyboard. If you want to download the movie to your computer, right-click or control-click HERE.

Questions

1. First draw a free body diagram for any of the objects hanging from the spring scale before being placed in the water.



2. How do the force of the spring on the object and the weight of the object (the force ofthe Earth on the object) compare?



3. What does the spring scale measure?



4. Now consider the same object placed in the water.
   
   
   • What happened to the reading on the spring scale?
   
   
   
   • What happened to the force that the spring exerts on the object?
   
   
   
   • If you agree that the object is in equilibrium, where does the extra force come from to stop it accelerating downwards?
   
   
   
   • List all objects that interact with the submerged object. Draw a free body diagram for the object showing all the forces exerted on it.
   
   
   
   • Can you use the spring scale readings to determine the size of the extra force which appeared when the block was placed in the water?
   
   
   
   • Compare the size of this force with the weight of the displaced water
   
   
   
   • Devise a rule relating the amount of extra force and the weight of the displaced water.
   
   
   
5. Once you understand this, use the readings of all the different blocks to determine what variable the extra force depends on.