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.

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.

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.

 θ=400rad
ω_0=0, t = 10s, α = ?
θ=ω₀ t+1/2 αt²
400 = 0+1/2 α10²
α = 8 rad/s².

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.