All 10 short questions of 9th Class Physics Chapter 4, Turning Effect of Forces, with exam-ready answers in the book's words. Punjab Board new syllabus.
Exam-ready answers. Learn the meaning, then write them in your own words.
Parallel forces that act in the same direction, for example the weights of two children sitting on a see-saw.
Parallel forces that act in opposite directions, for example the two forces of the hands on a steering wheel.
The two components of a vector that are at right angles to each other, usually along the x-axis and y-axis, are its rectangular components. For a force F at angle θ to the x-axis:
The line of action of a force is the straight line along which the force acts, drawn through its point of application. The moment arm of a force is measured from the axis to this line.
The product of the force and its moment arm: τ = F × l. SI unit N m.
Proof: Let F act at a point at distance r from the axis O, at angle θ to r. The moment arm is the perpendicular distance from O to the line of action of F, which is l = r sin θ.
Equally, only the part F sin θ of the force is at right angles to r; the part F cos θ passes through O and has no torque.
Apply two equal and opposite parallel forces F at two different points of a body, so that their lines of action are a distance d apart (a couple).
A paratrooper falling with uniform (terminal) velocity. His weight downward is balanced by the air resistance upward, so the net force is zero and he has no acceleration. He is in dynamic equilibrium. A car moving at a steady speed on a straight level road is another example.
The point of a body where its whole mass can be taken to be concentrated.
The point through which the whole weight of the body appears to act. Near the Earth's surface, where g is uniform, the two points coincide.
Both keep the vertical line through the centre of gravity inside the base even when the body tilts, so it does not topple.
For a body moving in a circle, the velocity at any point is along the tangent to the circle at that point. The centripetal force acts along the radius, towards the centre. A tangent to a circle is always perpendicular to the radius at that point, so the centripetal force is always perpendicular to the velocity.
