Physics practice problems Flashcards
If you put 25 J of energy into an engine, and 7 J are expelled as exhaust, how much work did the engine do?
25-7 = 18 J
What is the cause of the pressure exerted by a gas on its container?
bouncing molecules given by P=(NRT)/v
What are the three mechanisms of heat flow?
Convection Conduction Radiation
What are the main differences between solids, liquids, and gases?
The main differences are the separation of the atoms and the strength of the forces between the atoms. The atoms are closest in solids and farthest apart in gases. The forces are strongest in solids and weakest in gases
What is the cause of the pressure exerted by a gas on its container?
Air pressure is due to the average force exerted by the atoms as they collide with the walls of the container. An individual atom exerts a very small force, but there is a very large number of atoms, so the effect is significant.
Why does the earth’s moon have no atmosphere?
The relatively weak gravity of the moon is not sufficient to hold on to any atmospheric particles
. In a class demonstrations a metal can was crushed when the air was pumped out of it. What force caused the can to be crushed?
Before the can was evacuated, there were air molecules hitting both the inside and outside of the can, so the net effect was zero force. When the air molecules on the inside were pumped out, the unbalanced effect of the air molecules hitting the outside of the can crushed it.
How does the pressure on a submerged vessel change with depth?
Pressure increases with depth.
What force suspends the column of mercury in a barometer?
Atmospheric pressure
What principle is illustrated by Pascal’s vases?
Pascal’s vases illustrates the principle that the pressure in a fluid depends only on its depth and not on the shape of the container. Therefore, the fluid level is the same in each of the tubes.
A crate weighing 6400 N has dimensions 1 m by 2 m by 4 m. On what side should this crate be placed on a surface so that it exerts the least pressure on that surface?
The pressure P = F/A, where in this case F is the weight of the crate, W. Thus to minimize the pressure, the crate should be placed on the side with the largest area, which in this case is the 2 m x 4 m side. Then A = 8 m2 and P = 6400 N / 8 m2 = 800 Pa
What is Pascal’s principle?
Pascal’s principle states that a change in pressure in an enclosed fluid at rest is transmitted undiminished to all points in the fluid.
What is the principle underlying the use of a hydraulic jack?
Pascal’s principle is the physics on which the hydraulic jack operates.
When you drink pop using a straw, what principle are you employing?
When you suck on the top of the straw you lower the air pressure at that point. Then the atmospheric pressure on the surface of the pop pushes it up the straw and into your mouth.
Why are dams thicker at their base than at their top?
The pressure of the water is higher at the base of the dam, so the dam must be thicker there to support the water pressure.
Explain the origin of the buoyant force on an object
The pressure in a liquid increases with depth. So any object that is submerged will experience a higher pressure on the bottom side than on the top side. The difference in the pressures on the bottom and top is the origin of the buoyant force.
Why do oil tanker that are filled with crude oil float lower in the ocean than ones that are empty?
A full oil tanker weighs more than an empty one and thus requires a larger buoyant force to remain afloat. The buoyant force is larger when the tanker displaces more water, so when full it sits lower on the surface.
What is the condition necessary for an object to float?
For an object to float, the buoyant force on it must be larger than its weight
Why does a steel bolt float in mercury?
A steel bolt will float in mercury because steel has a lower density (7.86 g/ cm3) than mercury (13.6 g/ cm3).
Why does ice float.
Ice floats because it is less dense than water. (When water freezes, it expands and thus occupies a larger volume.)
Who discovered the laws of fluid dynamics?
Daniel Bernoulli
What is volume flow rate and in what units is it measured?
Volume flow rate is the quantity of fluid by volume that flows per unit time. It is measured in units of m3/s, gallons per minute (gpm), cubic feet per minute, etc.
Describe a method for measuring volume flow rate
To measure the volume flow rate you can time how long it takes to fill a container of a given volume with liquid, then the volume flow rate = QV = V/t, where V is the volume of the container and t is the time it takes to fill it.
Fluid enter the large end of the tube shown below at a speed of 0.5 m/s. If the diameter of the large end of the tube is twice that of the small end, what is the velocity of the fluid as it leaves the small end?
4 times faster (2 m/s)
Why do the roofs of homes often get blown off in a tornado?
The high speed wind blowing over a roof lowers the air pressure. The air under the roof is at normal pressure, and the higher pressure on the inside can lift the roof off of the walls of the house.
How do streamlines help to visualize fluid flow?
The spacing between streamlines is an indication of the speed of the flow. Streamlines that are far apart are associated with low flow speeds, whereas closely spaced streamlines indicate high flows.
What provides the lift force that allows planes to fly?
The wings of a plane are designed to cause the air flow to be higher on the top of the wing. According to Bernoulli, where the speed is higher the pressure is lower. Thus the pressure above the wing is lower than the pressure below it and there is a net upward force (lift) on the wing.
