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In physics, understanding how forces interact is vital, especially when dealing with motion. Net force is essentially the sum of all forces acting on an object. It determines how much push or pull is exerted to overcome opposing forces, like friction or air resistance.
In our exercise, the bicyclist experiences two primary forces: a forward pushing force of 1200 N and an air resistance force of 800 N.
To find the net force, you simply subtract the resistive force from the pushing force:

• Net Force = 1200 N (forward) - 800 N (resistance)
• Net Force = 400 N

This calculation shows that 400 N is the effective force propelling the bicyclist forward. It's important to understand that the greater the net force, the more it influences the object's acceleration. Thus, net force calculation is a cornerstone in predicting how objects move.

Kinetic energy is the energy an object possesses due to its motion. It is a crucial concept in understanding how energy transfers and changes form.
In the problem given, the net work done by the force on the bicyclist results in a change in kinetic energy. Work done is calculated as the net force times the distance over which it acts. Here, this is:

• Work = Net Force * Distance = 400 N * 20 m
• Work = 8000 J (Joules)

According to the work-energy theorem, the work done on an object is equal to its change in kinetic energy. Thus, the bicyclist's kinetic energy increases by 8000 J. Understanding kinetic energy is essential because it helps us comprehend how work translates into motion, and in turn, how speed changes with the forces applied to an object.

Solving physics problems involves breaking them down into smaller, manageable steps. This problem illustrates a typical approach using the work-energy principle to solve it comprehensively.
First, identify all the forces acting on the system. Quantify the net force, then determine how it will do work over the specified distance.
Calculate this work and understand that it translates directly into kinetic energy.

• Use given values and physical equations, like the work-energy relationship, to find unknowns.
• In this example, you rearrange the kinetic energy formula to solve for final speed:
• \[v = \sqrt{\frac{2 \times \text{Kinetic Energy}}{\text{mass}}}\]
• This results in:
• \[v = \sqrt{\frac{2 \times 8000 J}{90 kg}} = 14.1 m/s\]

This structured method of problem-solving helps in systematically tackling complex physics problems, ensuring clarity and accuracy in the results.