CHAPTER 12. FORCE,MOMENTUM AND IMPULSE 12.2
The drag force is very useful for parachutists. They jump from high altitudes and if there was nodrag
force, then they would continue accelerating allthe way to the ground.Parachutes are wide because
the more surface area you have, the greater the drag force and hence theslower you hit the ground.
Systems and External Forces ESBES
The concepts of systemsand forces external to such systems are very important in Physics. A system is
any collection of objects. If one draws an imaginary box around such asystem then an externalforce
is one that is applied byan object or person outside the box. Imagine for example a car pullingtwo
trailers.
B A
If we draw a box aroundthe two trailers they canbe considered a closed system or unit. When welook
at the forces on this closed system the followingforces will apply (we assume drag forces are absent):
- The force of the car pulling the unit (trailer A and B)
- The force of friction between the wheels of thetrailers and the road (opposite to the direction of
motion) - The force of the Earth pulling downwards on the system (gravity)
- The force of the road pushing upwards on the system
These forces are called external forces to the system.
The following forces will not apply:
- The force of A pullingB
- The force of B pullingA
- The force of friction between the wheels of the car and the road (opposite to the direction of
motion)
We can also draw a boxaround trailer A or B, inwhich case the forces will be different.
B A
If we consider trailer A as a system, the followingexternal forces will apply: