# Class 9 Physics Turning Effect of Forces EXAMPLE 4.1Find the resultant of three forces 12 \mathrm{~N} along x-axis 8 \mathrm{~N} making an angle of 45^{\circ} with x-axis and 8 \mathrm{~N} along y -axis.

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##### EXAMPLE 4.1Find the resultant of three forces 12 \mathrm{~N} along x-axis 8 \mathrm{~N} making an angle of 45^{\circ} with x-axis and 8 \mathrm{~N} along y -axis.

4.12 Why a body cannot be in equilibrium due to single force acting on it?

4.6 The steering of a car has a radius 16 \mathrm{~cm} . Find the torque produced by a couple of 50 \mathrm{~N} . (16 \mathrm{Nm})

EXAMPLE 4.6 A uniform rod of length 1.5 \mathrm{~m} is placed over a wedge at 0.5 \mathrm{~m} from its one end. A force of 100 \mathrm{~N} is applied at one of its ends near the wedge to keep it horizontal. Find the weight of the rod and the reaction of the wedge.

4.3 Differentiate the following:(ii) torque and couple

4.8 Two blocks of masses 5 \mathrm{~kg} and 3 kg are suspended by the two strings as shown. Find the tension in each

4.2 Define the following:(i) resultant vector(ii) torque(iii) centre of mass(iv) centre of gravity

ii. The number of forces that can be added by head to tail rule are:(a) 2(b) 3(c) 4(d) any number

4.13 Why the height of vehicles is kept as low as possible?

v. A couple is formed by(a) two forces perpendicular to each other(b) two like parallel forces(c) two equal and opposite forces in the same line(d) two equal and opposite forces not in the same line

iv. A force of 10 \mathrm{~N} is making an angle of 30^{\circ} with the horizontal. Its horizontal component will be:(a) 4 \mathrm{~N} (b) 5 \mathrm{~N} (c) 7 \mathrm{~N} (d) 8.7 \mathrm{~N}

4.7 A picture frame is hanging by two vertical strings. The tensions in the strings are 3.8 \mathrm{~N} and 4.4 \mathrm{~N} . Find the weight of the picture frame.(8.2N)

4.3 Find the magnitude and direction of a force if its x -component is 12 \mathrm{~N} and \mathrm{y} -component is \mathbf{5 N} .(13 \mathrm{N} making 22.6^{\circ} with \mathrm{x} -axis)

4.4 A force of 100 \mathrm{~N} is applied perpendicularly on a spanner at a distance of 10 \mathrm{~cm} from a nut. Find the torque produced by the force.(10 Nm)

4.14 Explain what is meant by stable unstable and neutral equilibrium. Give one example in each case.

4.3 Differentiate the following:(i) like and unlike forces

4.1 Find the resultant of the following forces:(i) 10 \mathrm{~N} along x-axis(ii) 6 \mathrm{~N} along \mathrm{y} -axis and(iii) 4 \mathrm{~N} along negative \mathrm{x} -axis.(8.5 \mathrm{N} making 45^{\circ} with x-axis)

iii. The number of perpendicular components of a force are:(a) 1(b) 2(c) 3(d) 4

4.5 How can a force be resolved into its perpendicular components?

4.5 A force is acting on a body making an angle of 30^{\circ} with the horizontal. The horizontal component of the force is 20 \mathrm{~N} . Find the force. (23.1 \mathrm{~N})

EXAMPLE 4.3 A mechanic tightens the nut of a bicycle using a 15 \mathrm{~cm} long spanner by exerting a force of 200 \mathrm{~N} . Find the torque that has tightened it.

vii. A body is in neutral equilibrium when its centre of gravity:(a) is at its highest position(b) is at the lowest position(c) keeps its height if displaced(d) is situated at its bottom

EXAMPLE 4.5 A block of weight 10 \mathrm{~N} is hanging through a cord as shown in figure 4.29. Find the tension in the cord.Figure 4.29

4.10 A block of mass 10 \mathrm{~kg} is suspended at a distance of 20 \mathrm{~cm} from the centre of a uniform bar 1 \mathrm{~m} long. What force is required to balance it at its centre of gravity by applying the force at the other end of the bar? (40 \mathrm{~N})

4.9 A nut has been tightened by a force of 200 \mathrm{~N} using 10 \mathrm{~cm} Iong spanner. What length of a spanner is required to loosen the same nut 150 \mathrm{~N} force?

4.2 Find the perpendicular components of a force of 50 \mathrm{~N} making an angle of 30^{\circ} with x axis. (43.3 \mathrm{~N} 25 \mathrm{~N})