1. The graph shows the upward speed of a ball thrown up with an initial speed of

Question

1.

The graph shows the upward speed of a ball thrown up with an initial speed of 27.0 m/s from the surface of Saturn.
Select ALL correct answers in alphabetical order, i.e., B, AC, BCD...

A) The acceleration is 0 near t= 3s.
B) The ball's KE increases during the first second.
C) The acceleration is constant.
D) The ball's PE is a constant.
E) The velocity vector and the acceleration vector are in the same direction after t=3s.
F) Maximum height is reached at about t=3s.

Select T-True, F-False, for each statement. If the first is T and the rest F, enter TFFFFF.

2.

A) The kinetic energy of an object is independent of the direction of the motion involved.
B) The work-energy principle is perfectly valid even when there are frictional forces acting.
C) Work can be done in the absence of motion.
D) The work done to raise a box onto a platform depends on how much time it takes to raise it. (Neglect friction, assume it starts on the ground at rest, and ends up resting on the platform)
E) The work done by a constant force acting on an object depends on the orientation of the force relative to the motion of the object
F) The kinetic energy of a moving object is always positive.

Explanation / Answer

At t=0 ,u = 27 m/s

and at t = 3 , v =0 m/s

from kinematics , v = u + at

0 = 27 + a * 3

a= -9 m/s

so, A - FALSE

B) kinetic energy directly proprtional to square of velocity

After first sec , velocity is decreased . hence kinetic energy also decreases

FALSE

C) the plot given is velocity w.r.t to time ..It is linear ..Accleration is derivative of velocity ..derivative of linear funcion is constant ..hence accleration is constant TRUE

D) PE is a function of height ..hence PE is not constant FALSE

E)after 3 sec , velocity is negative ..As calculated accleration is negative so both are in same direction pointing downwards TRUE

F)TRUE the ball ascends until its velocity is positive ..later on it descends

2.

A)kinetic energy only depends on magnitude of velocity of the object TRUE

B)FALSE the work energy principle fails when frictional force exists ..since energy dissipates in the form of heat ..energy is not conserved

C) work = force * displacement ..SO , it's a FALSE

D)FALSE work = force * displacement

E)TRUE , work done is considered as the displacement caused in the direction of force

F)TRUE , SInce kinetic energy directly proprtional to square of velocity and mass can never be negative

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