1. A hammer is resting on a plane surface. If force P is applied on the hammer along with a line not passing through its centre of mass, them the hammer _____________
a) Moves along a straight line path
b) Stays still
c) Is thrown away
d) Rotates about its centre of mass
Explanation: If single force acts on a body and the line of action of force passes through the centre of mass, the body will have only linear acceleration. When the force is applied along with a line not passing through its centre of mass, then the hammer rotates about its centre of mass.
2. The point where whole weight may be assumed to act is called ___________
a) Centre of mass
b) Centre of weight
c) Centre of gravity
d) Centre of acceleration
Explanation: The centre of mass of body is point where whole mass of the body may be assumed to be concentrated for describing its translator motion. On the other hand, the centre of gravity is a point at which the resultant of gravitational forces on all particles of the body acts.
3. The Sun’s attraction of both earth and moon are external forces acting on the centre of mass of the earth-moon system.
a) True
b) False
Explanation: The moon moves around the earth in a circular orbit and the earth moves around the sun in an elliptical orbit. Here the mutual force of gravitation between the earth and the moon are internal forces while the Sun’s attraction of both earth and moon are the external forces acting on the centre of mass of the earth-moon system.
4. Which of the following retain their relative positions even when they move under the action of an external force?
a) Rotation body
b) Stationary body
c) Rigid body
d) A body in equilibrium
Explanation: A body is said to be rigid if it does not undergo any change in its size and shape, however large the external force may be acting on it. A rigid body is one whose constituent particles retain their relative positions even when they move under the action of an external force.
5. On the application of a constant torque, a wheel is turned from rest through 400 radians in 10s. Find the angular acceleration.
a) 6 rad/s2
b) 4 rad/s2
c) 2 rad/s2
d) 8 rad/s2
Explanation: θ=400rad
ω_0=0, t = 10s, α = ?
θ=ω0 t+1/2 αt2
400 = 0+1/2 α102
α = 8 rad/s2.
6. A ball of mass 2kg and another of mass 4kg are dropped together from a 60feet tall building. After a fall of 30 feet each towards earth, their respective kinetic energies will be in the ration of ___________
a) √2:1
b) 1:4
c) 1:2
d) 1:√2
Explanation: Both balls attain same velocity v after falling through 30 feet height.
E1/E2 = (1/2 m1 (v1)2)/(1/2 m2 (v2)2) = (2v2)/(4v2)=1/2=1:2.
7. How much water a pump of 2kW can raise in one minute to a height of 10m? (g=10m/s2)
a) 1000litres
b) 1200litres
c) 10litres
d) 2000litres
Explanation: P = W/t=mgh/t
m = Pt/gh=(2000×60)/(10×10)=1200kg.
8. A body of mass 2kg is placed on a rough horizontal plane. The coefficient of friction between body and the plane is 0.2. Then ___________
a) Body will move in forward direction if F=3N
b) Body will move in backward direction with acceleration of 0.5m/s2, if force F=3N
c) If F=3N, then body will be in rest condition
d) The body will move both in forward during a particular period and then remain at rest
Explanation: Limiting friction, f = μR = μmg = 0.2×2×9.8=3.92N
When F>3.92N the body will move in a forward direction. When F=3N, the body will move to remain at rest.
9. Statement 1 – The length of the day is slowly increasing.
Statement 2 – The dominant effect causing a slow down in the rotation of the earth is the gravitational pull of other planets in the solar system.
a) Statement 1 and statement 2 are correct and statement 2 is the correct explanation of statement 1
b) Statement 1 and statement 2 are correct but statement 2 is not the correct explanation of statement 1
c) Statement 1 is true but statement 2 is false
d) Both statement 1 and 2 are false
Explanation: Both the statements are false. The length of the day depends on the speed of rotation of the earth about its axis. As it remains constant, the length of the day is not changing. Further, the earth is not slowing down due to the gravitational pull of other planets in the solar system.
10. If the earth stops rotating, the value of g at the equator ___________
a) Increases
b) Decreases
c) No effect
d) First increases and then decreases
Explanation: At the equator, ge=g-Rω2
When ω=0, ge=g
The value of g increases if the earth stops rotating.