Video Transcript
Fill in the blank. To cut down fuel consumption,
drivers on motorways are advised to drive blank. (A) At high speed, (B) with medium
acceleration, (C) with high acceleration, (D) at medium speed.
This question is asking us to
consider a driver driving down a motorway and figure out what advice to give them if
they want to cut down on their fuel consumption. The fuel consumption of a car
depends on how much work the car’s engine has to do in order to move. The more work done by the car, the
greater its fuel consumption. In order to figure out how to cut
down fuel consumption, we will need to use a property of fluids called
viscosity. Viscosity is defined as the
magnitude of the internal friction of a fluid and will affect how the fluid moves
and how objects move through the fluid. Remember that fluids include both
liquids and gases, so the air outside also has a viscosity associated with it.
When objects move through a fluid,
the viscosity of that fluid causes the object to experience a drag force, 𝐹 sub
𝐷. When an object moves through air,
this drag force is often known as air resistance. Drag forces always oppose the
motion of an object moving through the fluid. So, air resistance will act to slow
a car down. This increases the car’s fuel
consumption because the engine must do work to overcome this force and continue
moving forward.
The magnitude of this drag force
depends on the viscosity of the fluid and the velocity of the object moving through
the fluid. At low speeds, the drag force 𝐹
sub 𝐷 is proportional to velocity. But when objects start moving at
high speeds, 𝐹 sub 𝐷 is proportional to the objects’ velocity squared. When objects are moving through a
fluid at a high velocity, the drag force will be much greater than if it were moving
at a lower velocity. We can see the relationship between
velocity and the drag force in this graph here. Notice that the drag force starts
to increase rapidly after a certain velocity.
Now that we have refreshed our
memories about viscosity and how it affects objects moving through fluids, let’s
take a look at the options we are given to see which one will cut down on fuel
consumption for a driver. Option (A) says we should advise
the driver to drive at high speeds. But we know that the drag force
increases at high speeds, so this will not cut down on fuel consumption. Option (B) advises to drive with
medium acceleration. If the driver is accelerating,
their speed is continually increasing. This means they will eventually
reach high speeds, which will lead to more drag. So this also will not cut fuel
consumption.
Option (C) advises the driver to
drive with high acceleration. Again, this would also end up with
the driver reaching a high speed and hence would not cut down on fuel
consumption. Option (D) advises that the driver
drives with medium speed. If the driver drives with medium
speed, the drag force that the car experiences will be less than if the driver were
to reach a high speed. This leads to lower fuel
consumption for the car. Therefore, option (D), at medium
speed, is the correct answer.
