1. Rocket engines lift a rocket from the earth surface because of hot gases with high velocity ___________
a) Push against the air
b) Push against the earth
c) React against the rocket and push it up
d) Heat up the air which lifts the rocket
Explanation: Rocket engines live a rocket from the earth surface because hot gases with high velocity react against the rocket and push it up. Hot gases with high velocity react against the rocket and push it up.
2. The motion of a rocket is based on the principle of conservation of ___________
a) Linear momentum
b) Angular momentum
c) Kinetic energy
d) Mass
Explanation: A rocket works on the principle of conservation of linear momentum.
3. A bullet is fired from the rifle. If the rifle recoils freely, then the kinetic energy of the rifle is ___________
a) Less than the kinetic energy of the bullet
b) More than the kinetic energy of the bullet
c) Same as the kinetic energy of the bullet
d) Equal or less than the kinetic energy of the bullet
Explanation: By conservation of momentum,
MV = mv or V = mv/M
Kinetic energy of the rifle is
Kr = 1/2MV2 = 1/2M(m2 v2)/M2
Kr = m/M * Kb
As m is lesser than M, Kr is lesser than Kb
Thus the kinetic energy is less than that of the bullet.
4. A person is standing in an elevator. In which situation he finds his weightless?
a) When the elevator moves upward with constant acceleration
b) When the elevator moves downward with constant acceleration
c) When the elevator moves upward with uniform velocity
d) When the elevator moves downward, with uniform velocity
Explanation: When the elevator moves downward with acceleration a, the apparent weight, W‘=m(g-a) is lesser than W(=mg).
5. A person with a spring balance with a body hanging from it goes up in an aeroplane, then the reading of the weight of the body as indicated by the spring balance will ___________
a) Go on increasing
b) Go on decreasing
c) First increases and then decreases
d) Remains the same
Explanation: As the person goes above the earth’s surface, the value of g decrease. Hence the reading in the spring balance will go on decreasing.
6. Assertion: On a rainy day, it is difficult to drive at high speed.
Reason: The value of the coefficient of friction is lowered due to wetting of the surface.
a) If both assertion and reason are true and the reason is the correct explanation of the assertion
b) If both assertion and reason are true but the reason is not correct explanation of the assertion
c) If assertion is true, but the reason is false
d) If both assertion and reason are false
Explanation: Both assertion and reason are true. Due to wetting, friction decreases between tyres and road. This may lead to skidding of the vehicle.
7. A monkey of mass 20 kg is holding a vertical rope. The rope will not break when a mass of 25 kg is suspended from it but will break if the mass exceeds 25kg. What is the minimum acceleration with which the monkey can climb up along the rope? (g = 10m/s2)
a) 5m/s2
b) 10m/s2
c) 25m/s2
d) 2.5m/s2
Explanation: When the monkey climbs up the rope with acceleration a,
T = m(g+a)
Mg = mg+ma
25×10=20×10+20a
a = 2.5m/s2.
8. A block has been placed on an inclined plane with the slope angle θ, block slides down the plant at a constant speed. The coefficient of kinetic energy is equal to ____________
a) sinθ
b) cosθ
c) g
d) tanθ
Explanation: As the block slides down the inclined plane with a constant speed, the angle of inclination is equal to the angle of response.
μ = tangent of the angle of response
μ = tanθ.
9. A man is standing on a spring platform. Reading of spring balance is 60kg wt. If the man jumps outside from the platform, then the reading of spring balance will ___________
a) Increase
b) Become zero
c) Remains same
d) First increases and then becomes zero
Explanation: To jump outside the platform, the man pushes the platform, so the reading of the spring balance first increases and then becomes zero.
10. The earth moving around the sun in a circular orbit is acted upon by a force and hence work must be done on the earth by the force.
a) True
b) False
Explanation: The gravitational force is a conservative force. So the work done by the gravitational force over every complete orbit of the earth is zero.