Investigation 7
Solids and Fluids
1.What weighs more, one kilogram of lead or one kilogram of aluminum?
SAME. THEY HAVE THE SAME MASS.
2.What has more mass, a liter of ice or a liter of water? (1 liter = 1000 cm3)
THE VLOUMES ARE THE SAME, BUT WATER IS DENSER, SO IT HAS MORE MASS.
3.What has a greater density, one cubic centimeter of lead or five cubic centimeters of aluminum?
LEAD HAS A GREATER DENSITY – LOOK ON THE TABLE OF DENSITIES.
4.Two dams are shown below – one with a large lake behind it and one with a small lake behind it.
a.Which dam experiences a greater pressure on it due to the water at the same depth h? Explain.
THE PRESSURE IS THE SAME AT THE SAME DEPTH.
b.Which dam experiences a greater force on it due to the water behind the dam? Explain.
FORCES ARE ALSO THE SAME. FORCE = PRESSURE X AREA, AND AS LONG AS THE AREAS ARE THE SAME, THE FORCES ARE THE SAME.
5.Design a hydraulic lift that lifts 1000 Newtons when a 5 Newton force is applied.
6.A physics student is in a boat that is floating in a lake. Inside the boat with the student is a block of wood and block of steel.
The student throws the block of wood into the lake. She notices that the block of wood floats.
a.What is the density of the wood in relationship to the density of water?
DENSITY OF WOOD IS LESS THAN WATER
b.Will the boat now float higher or lower in the water than before the wood was thrown overboard? Why?
BOATS FLOATS HIGHER BECASUSE IT WEIGHS LESS WHEN THE WOOD IS THROWN OVERBOARD.
c.Will the water level of the lake go up or down? Why?
WATER LEVEL WILL REMAIN THE SAME. THE WOOD DISPLACES THE SAME AMOUNT OF WATER WHETHER IN THE BOAT OR FLOATING IN THE WATER BECAUSE IT IS FLOATING IN BOTH CASES.
The student now throws the block of steel into the lake. She notices that the block of steel sinks.
a.What is the density of the steel in relationship to the density of water?
DENSITY OF STEEL IS GREATER THAN WATER.
b.Will the boat now float higher or lower in the water? Why?
THE BOAT FLOATS HIGHER BECAUSE IT WEIGHS LESS.
c.Will the water level in the lake now go up or down? Why?
WATER LEVEL GOES DOWN BECAUSE STEEL DISPLACES MORE WATER WHEN IT IS IN THE BOAT (FLOATING) THAN WHEN IT SINKS IN THE WATER. When sinking, it only displaces as much water as its own volume.
Try the next two experiments at home and report your results.
7.Place an ice cube in a glass of water. Carefully mark where the water level is on the glass. Cover the glass to prevent any loss of water due to evaporation.
a.Make a prediction as to the water level after the ice melts. Will the water level be higher, lower, or remain the same?
REMAINS THE SAME.
b.Wait until the ice cube is totally melted. Repeat the experiment to ensure that your observations are accurate. Was your prediction correct? Explain why you got the results you did.
BECAUSE THE ICE IS FLOATING, THE WEIGHT OF THE ICE CUBE EQUALS THE BUOYANT FORCE (THE WEIGHT OF THE WATER DISPLACED). THE ICE CUBE IS WATER (WHEN IT MELTS) SO ITS WEIGHT EQUALS THE AMOUNT OF WATER DISPLACED.
Some thought questions: Would the result be the same if there were an air bubble inside the ice cube? How about a hunk of steel embedded in the ice cube?
8.Fill a glass to the top with water. Cover the glass with a card (e.g., a 3”x5” card) so that it completely covers the top of the glass. Hold the card onto the glass and invert the glass. (It might be a good idea to do this over the sink.) Now release your hand. What happened? Why?
THE WATER REMAINS IN THE GLASS. THE FORCE DUE TO AIR PRESSURE PUSHING UPWARD ON THE CARD IS GREATER THAN THE WEIGHT OF THE WATER PUSHING DOWNWARD ON THE CARD.
Another thought question: Will the results be the same if the glass were only partially filled with water?
THE SPACE ABOVE THE WATRER IN THE GLASS HAS AIR IN IT AND EXERTS A PRESSURE ON THE WATER ROUGHLY EQUAL TO ATMOSPHERIC PRESSURE. THIS FORCE DOWNWARD on the water TOGETHER WITH THE WEIGHT OF THE WATER IN THE GLASS WILL EXCEED THE FORCE DUE TO AIR PRESSURE PUSHING UPWARD ON THE CARD. Thus, the card falls.