KwunTongGovernmentSecondary School
Examples & Past Paper for AL Physics
Chapter 1: Statics
1.2Different Types of Forces (Weight, Normal Reaction & Tension)
Question 1 (85/I/1)
Two bodies P and Q are connected by a light string A. The weights of P and Q are 4 N and 10 N respectively. P is connected to the roof by another light string B. If string B is cut and the bodies allowed to fall, the net force acting on P during free fall is
A.0 NB.4 NC.6 N
D.10 NE.14 N
Question 2(87/I/4)
An object of mass 3 kg is tied to another object of mass 2 kg with a string passing over a fixed smooth pulley. The weight of the pulley is negligible. When the objects move under the action of gravity, the vertical upward force acting on the axle of the pulley is
A.zeroB.10 NC.24 N
D.48 NE.50 N
Question 3(86/I/1)
An object of mass 3 kg is placed on a smooth plane inclined at 30º to the horizontal. It is connected by a light string passing over a frictionless pulley to another object of mass 2 kg, as shown above. Given that g = 10 ms-2, when the system is released, the tension in the string will be
A.18 NB.20 NC.24 N
D.25 NE.30 N
Question4(92/I/3)
A simple pendulum is swinging in a vertical plane. When it is at the position shown, which of the following diagrams best represents the forces acting on the bob? Neglect air friction.
A.B.C.
D.E.
Question5(94/II/1; 94AS/II/1)
The above figure shows a propeller-motor system connected by a light, rigid rod to a fixed point P on the ceiling. The system remains stationary when the motor is on. Which of the following diagrams correctly represents the forces acting on the propeller-motor system?
A.B.C.
D.E.
Question6(88/I/2)
A smooth block of mass 2 kg slides down a wedge. The wedge, of mass 10 kg, is placed on a horizontal table, and its inclined plane makes an angle of 30º with the horizontal. If the wedge remains stationary all the time, the normal reaction of the table acting on the wedge is
A.15NB.85 NC.100 N
D.115NE.120 N
Question7(89/I/5)
In the figure above, X and Y are blocks of mass 1 kg and 2 kg respectively. S is a spring balance of negligible mass and P is a smooth pulley fixed at the top of two smooth inclined planes. What is the reading of S when X is held stationary?
A.5 NB.10 NC.15 N
D.20 NE.30 N
Question8 (93/I/1)
The resultant of two forces F1 and F2 acting at a point can have a minimum value of 1 N and a maximum value of 7 N. When the two forces act at right angles to each other, the magnitude of their resultant is
A.3 NB.5 NC.6 N
D. NE.8 N
Question 9 (99/II/2; 99AS/II/3)
A block of mass 5 kg is placed on the inclined surface of the wedge shown above. All contact surfaces are assumed to be smooth. What is the magnitude of the horizontal force F exerted on the wedge so that it remains stationary while the block is sliding down the inclined surface with acceleration?
A.0 NB.18 NC.24 N
D.30 NE.32 N
1.2Different Types of Forces (Elastic Force)
- Combination of Springs
In Series
The same force F is applied on each spring. The extension in spring 1 and spring 2 is given by
F = k1e1F = k2e2
The force constant of the combination is
In Parallel
The two springs have the same extension e. The restoring force in spring 1 and spring 2 is
F1 = k1eF2 = k2e
The force constant of the combination is k = k1 + k2
Question10(89/I/4)
Two objects of weights 2 N and 3 N are suspended from a fixed point by two identical light springs A and B as shown in the diagram. The force constants of the springs are both 1 N/cm. What are the extensions of springs A and B?
Extension of spring AExtension of spring B
A.5 cm3 cm
B.5 cm2 cm
C.3 cm2 cm
D.3 cm5 cm
E.2 cm3 cm
Question11(96/II/5; 96AS/II/5)
The figure shows two blocks A and B, each of mass m, connected by two light springs to a fixed support. Each spring has a force constant k. What is the total extension of the system when it is at static equilibrium?
A.mg/(2k)B.mg/kC.3mg/(2k)
D.2mg/kE.3mg/k
Question12(98/II/1; 98AS/II/1)
Two identical light springs are connected with two masses of 1.0 kg and 0.5 kg as shown. Which of the following statement is/are correct?
(1)The tension in the upper spring is double that in the lower spring.
(2)The force acting on the ceiling by the whole system is 15 N.
(3)The tension in the upper spring will remain unchanged if the two masses are exchanged in position.
A.(1) onlyB.(3) onlyC.(1) and (2) only
D.(2) and (3) onlyE.(1), (2) and (3)
Question 13 (99/II/1; 99AS/II/1)
Two object A and B of equal mass m are connected by two identical light springs and are placed on a horizontal smooth surface. A horizontal force F is applied to B so that the system is in equilibrium. If the applied force F is suddenly removed, what are the magnitudes of the acceleration of each object at the instant when force F is removed?
Acceleration of AAcceleration of B
A.zero
B.zerozero
C.
D.zero
E.
1.2Different Types of Forces (Friction)
- Effect of Changing Surface Area on Friction
Changing the “surface area” of the body has little effect on the “actual area” of contact, for the same normal reaction. Thus, the limiting static friction is independent on the surface area of the body unless it is very small.
- Reducing Friction
By lubricant oil - Friction can be reduced by putting a liquid (e.g. lubricant oil) between the interfaces. Molecules in a liquid are able to slip over each other. Graphite is commonly used to coat surface to reduce friction because the planes of atoms can easily slip over one another.
By bearing - Bearing are small steel spheres inserted between a fixed support and a rotating shaft. The bearings roll as the shaft turns. This prevents sliding between contact surfaces.
By Magnetic Force - Levitation by magnetic force eliminates direct surface contact altogether.
- Role of friction
- Static friction is very useful, although sometimes we are not aware of it. Without friction, objects like plates and cups would slide off a table. The frictional push on the tyres of a car by the road enables the car to accelerate, turn around a bend and decelerate.
- As there is no relative motion between the surfaces, ______friction never contributes to energy loss.
- Since ______friction occurs when there is relative motion between two surfaces, it is responsible for nearly all the mechanical energy loss.
- Friction in a car
- When a car accelerates, ______friction exists between the tyres and the road and is responsible for the propelling force. Meanwhile, ______friction exists between the shaft and the chassis (body of the car).
- When a car undergoes normal braking without skidding, ______friction exists between the brake pads and rotating wheels, while ______friction is found between the tyres and the road.
- However, on wet or icy road, the ______friction is greatly reduced and the wheels would be easily locked, casing skidding. Skidding would also occur if the brake pads are pressed too hard.
- Once skidding occurs, the motion of the car will be out of control. It simply slides straight away. Since the turning of a car relies on the ______friction between the tyres and the road, steering is impossible.
- It is interesting to note that the ______friction does not contribute to energy loss when a car is in motion. It functions as an external force to accelerate or decelerates the car only.
- Anti-lock Braking System (ABS)
In normal driving, a moving car is stopped by the static friction between the tyres and the road. In usual, pressing the pedal harder produces a greater deceleration. But if the pedal is pressed too hard, the wheels are locked up and sliding occurs. The driver can no longer control the car’s direction. Panic follows, and the natural reaction is to stamp even harder on the brake, which makes things worse.
Anti-lock brakes are designed so that the brakes are released and applied in quick succession automatically. It consists of an electronic sensor system which can detect how rapid the wheels are decelerated. Signals from the sensor system control the hydraulic pressure on the brakes so as to achieve the best and safest deceleration.
Question 14 (80/I/1)
A body is subject to an applied driving force F and a constant resistive force. Which of the graphs below represents the variation of the acceleration a of the body with the applied driving force F?
A.B.C
D.E.
Question 15 (82/I/4)
A builder using a pulley system to lift a bucket of cement of weight 150 N exerts a steady force F and pulls 30 m of rope through the system in order to raise the bucket 10 m. The friction in the system is small but NOT negligible. The value of the force F is most probably
A.less than 50 N.B.exactly equal to 50 N.C.between 50 N and 150 N.
D.between 150 N and 450 N.E.more than 450 N.
Question 16 (84/I/1)
A horizontal force P is applied to a wooden block at rest on a rough horizontal table. P is increased uniformly from zero. Which of the following graphs best describes the variation of the frictional force F acting on the block with P?
A.B.C
D.E.
Question17(91/I/2)
An object of mass m slides with constant acceleration a down a plane making an angle with the horizontal. The frictional force acting on the object is
A.mg - maB.mg - ma sin C.mg sin - ma
D.(mg - ma) sin E.(mg + ma) sin
Question18(95/II/1)
When given a slight push, a toy car moves freely with constant velocity down a plane inclined at 20º to the horizontal. If the mass of the car is 0.5 kg, find the force parallel to the inclined plane for pulling the car up the plane with constant velocity.
A.1.7 NB.3.4 NC.4.7 N
D.6.7 NE.9.4 N
Question19(97/II/1; 97AS/II/1)
Two wooden blocks A and B are connected by a string which passes over a smooth, fixed pulley as shown. The maximum friction between any two surfaces is 2 N. If a horizontal force F is applied to block B, find its minimum value for moving B.
A.2 NB.4 NC.6 N
D.8 NE.10 N
1.4Moment of a Force
Question 20 (94/II/3)
A uniform metre rule of mass 0.15 kg is hinged to a wall at P and the other end R is connected by a wire attached to the wall at Q, vertically above P. A block X of mass 0.1 kg is hung from the rule 30 cm from R. The metre rule is horizontal. Find the moment about P produced by the tension in the wire.
A.1.45 NmB.1.05 NmC.0.75 Nm
D.0.70 NmE.0.25 Nm
Question 21(95/II/8; 95AS/II/6)
The figure shows a uniform rigid beam AB, pivoted at A, held in horizontal position by a wire attached to a wall at point C, vertically above A. The beam carries a load W. If W is shifted gradually from A towards B, which of the following quantities will increase?
(1)The tension in the wire.
(2)The horizontal compression force in the beam.
(3)The vertical component of the reaction at A.
A.(1) onlyB.(3) onlyC.(1) and (2) only
D.(2) and (3) onlyE.(1), (2) and (3)
Exercise on Moments
1. /2. /
3. /
4. /
5. /
6. /
7. /
______
Examples & Past Paper for AL PhysicsChapter 1: StaticsPage 1
Answer for Exercise on Moments
1. /2. / a)(i)B
3. /
4. / (a)(i)Upwards
(iii)Yes. The applied force is 5 times less than the force holding the cap at B.
(c)(i)A smaller force can be used.
5. /
(b)Yes. The reaction at the pivot provides the rest of the force required.
No. End B should be pulled downward by a string.
6. /
(iv)Same as the total weight of the boy and the piece of wood.
The total upward force equals to the total downward force.
(ii)No. It is because the piece of wood will turn over.
7. / a)No. It is because those with a larger mass should be placed near the housewife, or else a much larger effort has to be required.
(C)(i)Left:downwardsRight:upwards
Example 6
______
Examples & Past Paper for AL PhysicsChapter 1: StaticsPage 1