CIE A Level Physics復(fù)習(xí)筆記3.1.6 Drag Force & Air Resistance

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年8月16日上午11:30

Drag Forces

Drag forces are forces acting the?opposite?direction to an object moving through a fluid (either gas or liquid)
Examples of drag forces are?friction?and?air resistance
A key component of drag forces is it increases with the speed of the object. This is shown in the diagram below:

3.1.2.1-Drag-force-increases-with-speed

Frictional forces on a car increase with its speed

Worked Example

A car of mass 800 kg has a horizontal driving force of 3 kN acting on it.Its acceleration is 2.0 m s^-2.What is the frictional force acting on the car? WE-frictional-force-on-car-question-image_1

WE-frictional-force-on-car-answer-image

Exam Tip

Remember to consider drag forces in your calculation for the resultant force. More details of this are in the notes “Force and acceleration”.

Air Resistance

Air resistance is an example of a drag force which objects experience when moving through the air
At a walking pace, a person rarely experiences the effects of air resistance
However, a person swimming at the same pace uses up much more energy - this is because air is 800 times less dense than water
Air resistance depends on the?shape?of the body (object) and the?speed?it’s travelling
Since drag force increases with speed, air resistance becomes important when objects move faster

3.1.2.2-Racing-cyclist

A racing cyclist adopts a more streamline posture to reduce the effects of air resistance.?The cycle, clothing and helmet are designed to allow them to go as fast as possible

Exam Tip

If a question considers air resistance to be?‘negligible’ this means in that question, air resistance is taken to be so small it will not make a difference to the motion of the body. You can take this to mean there are no drag forces acting on the body.

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