WORK ENERGY AND POWER Quiz-19
Dear Readers,
Work power energy is the most important chapter when it comes to mechanics, for JEE (Advanced). The chapter is quite tricky and takes a lot of time and devotion on your part to understand and master. But this chapter is a complete gold medal for almost all the questions of the mechanics can be solved by the work power energy approach if you master this topic..
Q1.
Statement 1: The change in kinetic energy of a particle is equal to the work done on it by the net force
Statement 2:Change in kinetic energy of particle is equal to the work done only in case of a system of one particle
Solution
Change in kinetic energy = work done by net force
This relationship is valid for particle as well as system of particles
Q2.
Statement 1:The kinetic energy of a system can be increased without applying any external force on the system
Statement 2: Single external force acting on a particle necessarily changes its kinetic energy
Solution
Work done by internal force=change in kinetic energy
If F ext=0,F(n.C)=0
During motion of circular path work done by the centripetal force is zero
Q3.
Statement 1: Mass and energy are not conserved separately, but are conserved as a single entity called mass-energy J
Statement 2: Mass and energy conservation can be obtained by Einstein equation for energy
Solution
From Einstein equation E=mc^2
Q4.
Statement 1: A car and a heavy lorry moving on a road have same speed. Same braking force is applied and both stop in same distance
Statement 2: Same fore will cause different retardation in two vehicles
Solution
Since mass of carmass of lorry. Retardation on car is more than lorry. And car will stop earlier; at lesser distance
Q5.
Statement 1: When a gas is allowed to expand, work done by gas is positive
Statement 2: Force due to gaseous pressure and displacement (of piston) are in the same direction
Solution
Since the gaseous pressure and the displacement (of piston) are in the same direction. Therefore θ=0°
∴ Work done =Fs cosθ=Fs= Positive
Thus during expansion work done by gas is positive
Q6.
Statement 1: The work done by a conservative force during a round trip is always zero
Statement 2: No force is required to move a body in its round trip
Solution
During a round trip, the body finally comes at its initial position, i.e., the displacement of body is equal to zero. Therefore, work done is equal to zero. So W=F ⃗.S ⃗=0. Statement I is correct and statement II is wrong
Q7.
Statement 1: A heavy weight is suspended from a string. A person raises the weight slowly till the spring becomes slack. The work done by the person inW. The energy stored on the
Statement 2: The work done by the spring force is always negative
Solution
W(mg)+W(spring)+W(Man)=0-∆U+E+W=0
∆U=E+W
Work done by spring force is positive when a compressed spring is released or stretched spring is released
Q8.
Statement 1: A quick collision between two bodies is more violent than slow collision, even when initial and final velocities are identical
Statement 2: The rate of change of momentum determines that force is small or large
Solution
In a quick collision, time t is small. As F×t= constant, therefore, force involved is large, i.e. collision is more violent in comparison to slow collision
Q9. 
Solution
The maximum extension is non-zero, whole the spring never undergoes compression. Hence, statement I is false
Q10. Statement 1: In an elastic collision between two bodies, the relative speed of the bodies after collision is equal to the relative speed before the collision
Statement 2: In an elastic collision, the linear momentum of the system is conserved
Solution
Both the statement 1 & statement 2 is true, statement 2 is not a correct explanation for statement 1. In fact the momentum is conserved both in elastic as well as in inelastic collision. But in elastic collision the total kinetic energy of the system is also conserved
