Sec 3 Physics Turning Effects of Forces topical questions

Calculate the moment of each force about the stated pivot.

1. (a)A 15 N push at 0.20 m from a hinge, at right angles to the door.[2]
2. (b)A 250 N load hanging 0.80 m from a nail.[2]

State what is meant by the centre of gravity of a body, and name the point where the weight of a uniform metre ruler acts.

A 240 N child sits 1.5 m from the pivot of a see-saw. Her 360 N uncle sits on the other side, balancing her.

1. (a)Calculate the child’s moment about the pivot.[2]
2. (b)Calculate the uncle’s distance from the pivot.[1]

A uniform metre ruler of weight 1.6 N rests on a pivot at its 40 cm mark. To balance it, a student hangs a weight at the 10 cm mark.

1. (a)State the distance from the pivot to the ruler’s weight, and to the hanging weight.[2]
2. (b)Calculate the size of the hanging weight.[3]
3. (c)State the size and direction of the force exerted by the pivot on the ruler.[1]

A delivery driver slides a 300 N crate on a trolley. The trolley handle gives her grip a point 1.2 m from the wheels; the crate’s weight acts 0.30 m from the wheels.

1. (a)Calculate the force she must apply at the handles to keep the trolley level while lifting the crate’s weight off the axle support.[3]
2. (b)The crate is moved so its weight acts 0.60 m from the wheels. State and calculate the new handle force.[2]

A student pushes a playground roundabout with 60 N, tangentially at its rim, 0.90 m from the central axle. Her friend pushes in the opposite rotational direction with 80 N at 0.60 m from the axle.

1. (a)Calculate both moments about the axle.[2]
2. (b)Determine the direction and size of the resultant moment.[3]

A hawker carries two baskets on a pole across one shoulder. The left basket (80 N) hangs 0.50 m from the shoulder; the right basket hangs 0.80 m away on the other side. The pole is light and symmetric.

1. (a)Calculate the weight of the right basket for balance.[3]
2. (b)Calculate the force the pole exerts downward on the hawker’s shoulder.[2]
3. (c)State one reason the hawker might prefer to carry a single heavy basket closer to the neck instead.[1]

A mobile-crane jib is 8.0 m long, pivoted at the mast. A 5000 N load hangs 6.0 m from the mast on the load side. A 12 000 N counterweight sits 2.5 m from the mast on the other side. The jib itself is uniform, weighs 1600 N, and its centre of gravity is 4.0 m from the mast on the load side.

1. (a)Calculate the total anticlockwise moment (load + jib weight) about the mast.[3]
2. (b)Determine whether the crane balances, and calculate any extra moment needed.[2]
3. (c)The load is moved to 5.0 m from the mast. Determine the new state of balance.[3]

A woman of weight 500 N stands on tiptoe on one foot. Her whole weight acts through a point 18 cm horizontally in front of her ankle joint; the Achilles tendon pulls at 12 cm behind the ankle, and the floor pushes her toes up 6.0 cm in front of the ankle.

1. (a)Taking moments about the ankle joint, calculate the upward force from the floor on her toes.[3]
2. (b)Calculate the tension in the Achilles tendon.[2]
3. (c)Comment on why the tendon force exceeds her body weight.[2]

A rectangular notice board, 1.2 m tall and 0.80 m wide, hangs from a single nail at its top edge. Its weight is 45 N and its centre of gravity is at its geometric centre. A horizontal gust pushes at the board’s middle height with 6.0 N.

1. (a)Calculate the moment of the gust about the nail.[2]
2. (b)When the board hangs tilted at rest, its weight’s moment about the nail balances the gust’s. Explain why a tilt is needed at all.[2]
3. (c)Estimate the tilt angle for small angles, using moment balance with the weight acting 0.60 m from the nail along the board.[3]
4. (d)State one change to the fixing that removes the tilt.[1]

A drink vending machine is 1.8 m tall, 0.70 m deep, and weighs 3000 N, with its centre of gravity 0.60 m above the floor at the horizontal middle of its base. It stands on a slope that tilts slowly upward at the front.

1. (a)Calculate the tilt angle at which the machine begins to topple forward.[3]
2. (b)A service engineer bolts a 40 kg ballast bar to the machine’s front base. Take g = 10 N/kg. State the effect on the toppling angle, with a reason.[3]
3. (c)State one more design change that increases stability.[1]

A student determines the weight of a uniform metre ruler with a knife-edge pivot. In experiment 1 she balances the ruler with the pivot at the 30 cm mark and a 2.4 N weight hanging from the 10 cm mark. In experiment 2 the pivot is moved to the 70 cm mark.

1. (a)Show that experiment 1 gives the ruler’s weight as 2.4 N. (The ruler’s weight acts at its 50 cm mark.)[3]
2. (b)Calculate the weight that must hang from the 100 cm mark to balance experiment 2.[3]
3. (c)A classmate claims a 4.8 N weight balanced the ruler in experiment 2. Assess the claim.[2]