Is it Possible for a system to have negative potential energy? Yes. as long as t

Question

Is it Possible for a system to have negative potential energy? Yes. as long as the kinetic energy is positive. Yes, as long as the total energy is positive. Yes, Since the choice of the zero of potential energy is arbitrary. No, because the kinetic energy of a system must equal its potential energy No. because this would have no physical meaning A Stationary shell explodes into two fragments, one fragment 25 times heavier than the other. The fragments travel in opposite directions always, then the momentum change of the lighter fragment is 25 times as great as the momentum change of the heavier fragment. the momentum change of the heavier fragment is 25 times as great as the momentum change of the lighter fragment. the momentum change of the lighter fragment is exactly the same as the momentum change of the heavier fragment. the kinetic energy change of the heavier fragment is 25 times as great as the kinetic energy change of the lighter fragment the kinetic energy change of the lighter fragment is 25 times as great as the kinetic energy change of the heavier fragment A 1000.0 kg car is moving at 15 km/h. If a 2000.0 kg truck has 18 times the kinetic energy of the car, how fast is the truck moving? 75 km/h 63 km/h 45 km/h 36 km/h 54 km/h A baseball is thrown vertically upward and feels no air resistance. As it is rising both its momentum and its mechanical energy are conserved. its momentum is not conserved, but its mechanical energy is conserved. both its momentum and its kinetic energy are conserved. its kinetic energy is conserved, but its momentum is not conserved its gravitational potential energy is not conserved, buts its momentum is conserved A golf ball of mass 0.050 kg is at rest on the tee and has a velocity of 102 m/s immediately after being struck. If the club and boll were m contact for 0 81 ms, what is the average force exerted on the ball? 4.9 kN 7.1 kN 5.5 kN 6 3 kN 2.9 kN

Explanation / Answer

1) yes, potential energy can't be specified in absolute terms,we have to orbitarly define a reference point.

for example in the case of gravitation,any object below the reference point would have negative potential energy.

It not depends on the numbers assigned to the potential energy it depends on the difference between two reference levels. This difference would be the same ,even if you change your reference level.

so answer is option (C)

2) intially the shell at rest so intially momentum is zero then final momemtum also zero

here m2v2 + m1v1 =0 here m2 =25 m1

(25m1)v2 + m1v1 =0

v1 = -25 v2   ----------------------(1)

the momentum change of the two fragments are respectively

m1v1 =-25 m1v2

   m2v2 = 25 m1v2

it is clear that the magnitude of momentum change is same but the signs are opposite.

Now kinetic energy changes are respectively

(1/2)m1v12= (1/2)m1( -25 v2 )2 = 252(m1v22)/2

(1/2)m2v22 =(1/2)(25m1)v22 =25(m1v22)/2

which show that the kinetic energy change of lighter fragment is 25 times the kinetic energy change of heavier fragment

so answer is option (D)

3) mass of the car is =1000 kg and the speed of the car =15 km/h =15 x(5/18) m/s =4.17 m/s

kinetic energy of the car KE(car) =(1/2)(1000)(4.17)2 =8694.45 J

kinetic energy of the truck KE(truck) =18(KE(car)) =18 x8694.45 J =156500.10 J

mass of the truck m =2000.0 kg

  KE(truck) =(1/2)(2000.0) v2

156500.10 =(1/2)(2000.0) v2

v=12.51 m/s =12.51 x(18/5) km/h = 45 km/h

speed of the truck is 45 km/h

so answer is option (C)

4) momentum is conserved in situation where there is no outside force .in this case the gravity acting on the baseball as external force, so the momentum of the baseball is not conserved

but the total energy (mechanical energy) of the baseball is conserved. the total energy is the sum of the kinetic energy and potential energy

when the baseball rises ,it slow down the kinetic energy and increses the potential energy, when it reaches highest point the total kinetic energy is converted into potential energy but over all mechanical energy is constant

so it's momentum is not conserved but it's mechanical energy is conserved

answer is option (B)

5) intial velocity of golf ball is 0 m/s

average force exerted on the ball is obtained from newton's second law of motion

F= ma =m(dv/dt) here a (acceleration) =rate of change in velocity =dv/dt

dv =102 m/s -0 m/s =102 m/s and dt =duration of contact =0.81 ms =0.81 x10-3 s

mass of the ball (m)=0.050 kg

F(average) =(0.050)(102/(0.81 x10-3 ) N =6.296 KN=6.3 KN (approx)

answer is option (D)

  

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