Physics MCQs: Force

Force and Moment

• Moment of force is not measured in kg m.
• A body is in equilibrium when the resultant of all forces acting on it is zero.
• A couple is not an example of pushing a box.
• Examples of dynamic equilibrium include a rain drop falling from a height and a stone tied to a string and whirled in a circular path.

Torque

• Torque is the tendency of rotation of a body along any axis.
• Torque depends on both force and moment arm.
• If 150 N of force is applied on a spanner of 10 cm to open a nut, the length of the spanner when the force applied on it is 60 N is 25 cm.
• Moment arm is the perpendicular distance from the axis of rotation to the point where the force is applied.

Units and Conversion

• 1 kgf-m is equal to 9.8 N-m.
• 1 gf cm is equal to 980 dyne cm.

Centre of Gravity

• The centre of gravity of a hollow cone lies at a height of h/3 from the base.
• For a stable equilibrium of a body, its centre of gravity must be above the base and near the centre of the body.
• The position of the centre of gravity of a body depends on the distribution of mass.
• The centre of gravity of a tennis ball is at its geometrical centre.
• The point of intersection of diagonals is not the centre of gravity for a quadrilateral.
• The centre of gravity for a ring is not within the material of the body.

Centrifugal and Centripetal Force

• Centrifugal force is a fictitious force.
• Centripetal force acts towards the centre.
• In Uniform Circular Motion, the speed is constant.

Uniform Circular Motion

• In Uniform Circular Motion, the direction of motion is along the tangent to the circle.
• Uniform Circular Motion is also called accelerated motion.
• Clockwise moments are taken to be negative.
• If the sum of clockwise moments acting on a body is equal to the sum of anticlockwise moments acting on it, then the body is balanced.
• The net torque is zero when two children sitting on a see-saw are not swinging.