Question

1. Explain what is work (physics term)? What is kinetic energy? The relationship between the work

done by the net force on the object and kinetic energy of object.

2. How do you know the work-energy theorem is true? (hint: describe clearly how it works in yourexperiment or in real life)

Answer 1

In physics, a force is said to do work if, when acting on a body, there is a displacement of the point of application in the direction of the force.
For example, when a ball is held above the ground and then dropped, the work done on the ball as it falls is equal to the weight of the ball (a force)
multiplied by the distance to the ground (a displacement).
The work done on a body that caused the body to be set in motion with some speed v can be expressed as function of the body's final speed v and mass m,
independent of type of force that acted on the body. We call this function the body's Kinetic Energy.
Kinetic energy is the energy of motion. An object that has motion - whether it is vertical or horizontal motion - has kinetic energy
KE = 1/2*m*V^2

The energy a body possesses by virtue of its motion -- its Kinetic Energy -- is not dependent upon how the object reached its state of motion, only upon its current state of motion.

Proof :
From Newton’s Second Law of motion, we know that F = ma, and because of the definition of acceleration we can say that
F=m dv/dt

If we multiply both sides by the same thing, we haven’t changed anything, so we multiply by v:

Fv=mv dv/dt

But remember that v = dx/dt:

F dx/dt=mv dv/dt

We rearrange and integrate:

F dx = mv dv

integration

Fx = m(½v2) = ½mv2 = Ek

But Fx = Work; therefore Work = ΔEk.