PWS 1 1. Which object has the largest resultant force acting on it? , Ans: C 2. The graph shows the speed–time graph for a parachutist who jumps from a plane but does not At which point does he open his parachute? Ans: B [Because when he opens his parachute at point B, upward force air resistance will be more than the downward force weight, so resultant force acts upward but his motion was downward so deceleration will occur, speed will start decreasing . When speed decreases air resistance also start decreasing and at D air resistance and weight becomes equal and the person moves with constant velocity with the parachute. At AB person moves with constant velocity with out parachute.] 3. A plastic cube, of density p, is compressed so that each of its sides becomes one-half of its original length. What is the new density of the material of the cube? A 0.125p B 0.50p C 2.0p D 8.0p Ans : D [Mass is same for both of the cubes. But volume is different . if l is the length of one side of the first cube, then volume is l3 . The side of 2nd cube is l/2 . The volume of 2nd cube is (l/2)3 = l3/8 Mass= density x volume After calculation u will see that density of 2nd cube is 8 times more than first cube. ] 4. A ball starts from rest and rolls down a steep slope. The ball then rolls along rough horizontal ground. Which graph shows the speed of the ball at different times? Ans: D [Steep slope gives constant acceleration and rough horizontal road gives deceleration] 5. A car travels at 60 km / h on a straight road. The road is dry. The driver applies the brakes suddenly. The table shows the thinking distance, the braking distance and the stopping distance. The same car is driven by the same driver at 60 km / h on the same road when it is wet. The driver applies the brakes suddenly again. What is the effect of the wet road on the thinking distance, the braking distance and the stopping distance? Ans : A 6. What causes the frictional effects experienced by a cyclist to increase? A using a lighter bicycle B travelling at greater speed C a smoother tyre surface D a road surface covered in oil Ans : B [More speed more air resistance] 7. Water is added to a measuring cylinder containing 100 cm3 of liquid paraffin. (The density of paraffin is 0.80 g / cm3 and that of the water is 1.0 g / cm3.) As the water is added, the level of the paraffin rises to 150 cm3. The paraffin and water do not mix. What finally is the total mass of liquid in the measuring cylinder? A 130 g B 140 g C 167 g D 175 g Ans: A [ mass= density ×volume , volume of water = (150- 100)cm3 Mass of water = 1×50= 50 g , mass of paraffin = 0.80 ×100 = 80 g Total mass = (50+80)= 135 g 8. A student lists three changes that affect the stopping distance of a car. 1 increasing the braking force 2 increasing the friction between the tyres and the road 3 increasing the speed of the car Which change or changes increase the stopping distance? A 1 and 2 B 1 and 3 C 1 only D 3 only Ans: D 9. Two identical objects begin to fall from rest. One object falls from 200 m above the Earth’s surface and the other falls from 200 m above the Moon’s surface. One second after they have started to fall, both objects are still accelerating. There is no atmosphere on the Moon and the weight of each object is constant. Which row describes the motion of both objects at this time? Ans : A 10. A stone is placed in a measuring cylinder of water, as shown. The volume of water in the cylinder before adding the stone is 15 cm3 and the stone has a mass of 90 g. What is the density of the stone? A 1.0 g / cm3 B 1.2 g / cm3 C 1.5 g / cm3 D 6.0 g / cm3 Ans : C 11. . A boy starts at P and walks 3.0 m due north from P to Q and then 4.0 m due east from Q to R. What is the shortest distance that he must now walk to have an overall displacement of zero? A 3.0 m B 4.0 m C 5.0 m D 7.0 m 12. A student investigates the motion of a ball falling through the air. Which quantity is a vector? A the diameter of the ball B the gravitational force on the ball C the distance from which the ball is dropped D the speed at which the ball hits the ground 13. A skier slides down a slope. The diagram shows how his speed varies with time. What is his average acceleration during the 6.0 s? A 2.2 m / s2 B 4.3 m / s2 C 8.0 m / s2 Ans: B Average acceleration =( v-u) ÷ t =( 26-0) ÷ 6 D 13.0 m / s2 14. A solid object, immersed in water, hangs from an elastic thread. Three forces act on the object: its weight W, the tension in the thread T, and a force F from the water. Which equation is correct when the object is stationary? AF+W=0 BF–T=0 CF–W–T=0 DF–W+T=0 Ans: D [ weight(W) acts downward = Tension (T) acts opposite to the weight upward + Force by water also acts upward on the immersed object] 15. A car of weight 11 000 N moves with constant velocity along a horizontal road. A driving force of 5000 N acts on the car. What is the force opposing the motion of the car? A 5000 N B 6000 N C 11 000 N D 16 000 N Ans: A 16. A man with an open parachute falls to Earth at constant speed. The following forces act: P the upward force of the parachute on the man Q the upward force of the man on the Earth R the downward force of the Earth on the parachute S the downward force of the man on the parachute Which two forces are a Newton’s third law pair? A P and Q B P and R C P and S D Q and R Ans: C 17. Which diagram shows the vector addition of a 4.0 N force and a 3.0 N force? Ans : A [The line from the starting point to the ending point is the resultant force for A. B is wrong because the line is from the ending to the starting point. C is wrong because resultant force should be along the diagonal]. 18. Trolley X and trolley Y are joined by a stretched spring. Trolley X has twice the mass of trolley Y. When the trolleys are released, the acceleration of X is 2 m / s2 to the right. What is the initial acceleration of trolley Y to the left? A 1 m / s2 Ans : D B 2 m / s2 C 3 m / s2 D 4 m / s2 19. The diagram shows a car at the moment the driver sees a hazard ahead. Which expression is equal to the stopping distance? A braking distance + thinking distance B braking distance – thinking distance C braking distance × thinking distance D braking distance ÷ thinking distance Ans : A 20. A tractor pulls a trailer at a constant speed. The tractor exerts a force of 1600 N forwards on the trailer. What force is exerted by the trailer on the tractor? A0N B 1600 N backwards C 1600 N forwards D 3200 N forwards Ans : B ----------------------------------------------------------------------------------------------------------------Sometimes friction is the driving force of the movement. This is because when we walk we have to exert a backward force on the ground , and the result is a forward force from the ground ( friction ) and this pushes us forward. When an object exerts force on the ground, ground also exerts a force on the object , as stated by Newton’s 3rd law. Examples : 1. Walking , as we walk ,the friction between our shoes and the floor pushes us forward . 2. friction also pushes the tyre of a car for it to move forward. The wheels pushed the road backwards, the road must push the wheels forward, it is this reaction force (friction) acting on the wheels causes the car to move.
