HCI Worksheet 9 Forces and dynamics

Hwa Chong Institution Sec 3 Physics 1 Name: ________________________________________ Class: __________ Date: ___________ Sec 3 Physics Worksheet 9: Forces and Dynamics SECTION A: CONCEPTUAL QUESTIONS 1 Can the velocity of an object reverse direction while maintaining a constant acceleration? If so, give an example; if not, explain. 2 Is it possible to move in a curved path in the absence of a force? Defend your answer. 3 Three identical blocks are pulled as shown on a horizontal frictionless surface. If the tension in the rope held by the hand is 30 N, what is the tension in the other ropes? 4 How does the force of gravity on a raindrop compare with the air drag it encounters when it falls at constant velocity? 5 If you hold your book horizontally with a piece of paper beneath it, then drop both. They fall together. Repeat the experiment, but place the paper on top of the book. Describe the motion of the paper relative to the book. (Try it and see!) 6 How does the terminal speed of a parachutist before opening a parachute compare to terminal speed after? Why is there a difference? 7 Why is it that a cat that accidentally falls from the top of a 50 -storey building hits a safety net below no faster than if it fell from the twentieth storey? 8 Under what conditions would a metal sphere dropping through a viscous liquid be in equilibrium? 9 If Galileo dropped two balls from the top of the Leaning Tower of Pisa, air resistance was not negligible. Assuming that both the balls were of the same size, one made of wood and one of iron, which ball hit the ground first? Why?

Hwa Chong Institution Sec 3 Physics 2 SECTION B: MULTIPLE CHOICE QUESTIONS 10 Free-fall is motion in which gravity is the only force acting. A Is a skydiver who has reached a terminal speed in free fall? B Is a satellite above the atmosphere that circles the Earth in free fall? 1 Three forces act on an object. If the object is in translational equilibrium, which of the following must be true? I The vector sum of the three forces must equal zero. II The magnitudes of the three forces must be equal. III All three forces must be parallel. A I only B II only C I and III only D II and III only E I, II, and III 2 A ball falls straight down through the air under the influence of gravity. There is a retarding force F on the ball with magnitude given by F = bv, where v is the speed of the ball and b is a positive constant. The magnitude of the acceleration a of the ball at any time is equal to which of the following? A gb− B bvg m− C bvg m+ D g b E bv m 3 An object is released from rest on a planet that has no atmosphere. The object falls freely for 4.0 m in the first two seconds. What is the magnitude of the acceleration due to gravity on the planet? A −21.0 m s B −22.0 m s C −24.0 m s D −28.0 m s E −212.0 m s a bv mg

Hwa Chong Institution Sec 3 Physics 3 4 A block of mass 3m can move without friction on a horizontal table. This block is attached to another block of mass m by a cord that passes over a frictionless pulley. If the masses of the cord and the pulley are negligible, calculate the magnitude of the acc eleration of the descending block m? A zero B g/4 C g/3 D 2g/3 E g 5 An astronaut drops an apple on the surface of the Moon . The acceleration due to gravity is 1/6 on Earth. The time it takes for the apple to fall to the ground compared with an apple dropped from the same height on Earth is A the same B 6 times as long C 6 times as long D 36 times as long The following figure is used in questions 6 and 7. 6 A plane 5 m in length is inclined at an angle of 37, as shown above. A block of weight 20 N is placed at the top of the plane and allowed to slide down. The mass of the block is most nearly A 1.0 kg B 1.2 kg C 1.6 kg D 2.0 kg E 2.5 kg 7 The magnitude of the normal force exerted on the block by the plane is most nearly A 10 N B 12 N C 16 N D 20 N E 33 N

Hwa Chong Institution Sec 3 Physics 4 8 A cart is pulled along a horizontal track by a rope that passes over a fixed pulley. As the cart moves to the right, assume the tension in the rope and the frictional forces on the cart remain constant. As the cart moves to the right, its acceleration A decreases. B increases. C remains constant. D is zero.

Hwa Chong Institution Sec 3 Physics 5 SECTION C: STRUCTURED QUESTIONS 1 A rope of negligible mass passes over a pulley of negligible mass attached to the ceiling . One end of the rope is held by Student A of mass 70 kg, who is at rest on the floor. The opposite end of the rope is held by Student B, who has a mass 60 kg and is suspended at rest. (a) The dots below represent the students. Draw and label free-body diagrams showing the forces on Student A and Student B. (b) Calculate the magnitude of the force exerted by the floor on Student A.

Hwa Chong Institution Sec 3 Physics 6 Student B now climbs up the rope at a constant acceleration of 0.25 m s-2 with respect to the floor. (c) Calculate the tension in the rope while Student B is accelerating. (d) As Student B is accelerating, is Student A pulled upward off the floor? Justify your answer. (e) With what minimum acceleration must Student B climb up the rope to lift Student A upward off the floor?

Hwa Chong Institution Sec 3 Physics 7 2 Alicia was standing on a bathroom scale placed in a lift. (a) The reading on the scales was 720 N when the lift was accelerating upwards at 2 m s−2. Calculate the actual mass of Alicia. (b) During another part of the journey, the bathroom scales read 450 N. Calculate the acceleration of Alicia at this instance. (c) The rope of the lift suddenly snaps. Before the safety devices kick in, what would the reading on the bathroom scale be? Explain your answer.

Hwa Chong Institution Sec 3 Physics 8 3 Christopher and his 12 .0 kg backpack are on the surface of a frozen lake, 20 .0 m apart. Christopher exerts a horizontal force of 6.00 N on the backpack using a light rope, pulling it towards him. Assume that there are no frictional forces on the ice and that the mass of Christopher is 80 .0 kg. (Light rope implies that we can ignore the mass of the rope in our calculations.) a Draw free-body diagrams to show the forces acting on the backpack and Christopher. Indicate which pair(s) of forces acting on these 2 objects are action-reaction pairs. B Calculate the acceleration of the backpack. C Calculate the acceleration of Christopher.

Hwa Chong Institution Sec 3 Physics 9 D What is the displacement of Christopher from his initial position do they meet?

Hwa Chong Institution Sec 3 Physics 10 4 With a 100 kg basket attached, a hot -air balloon was moving vertically downwards with a constant speed of 10 1m s− . At time t = 0 s, the hot-air balloon reached level L, 500 m above the ground. The combined mass of the hot -air balloon with the basket was 3 000 kg. Ignore the effects of air resistance. (a) State the resultant force on the hot -air balloon with the attached basket. Explain how you deduce this value from the information given. (b) Hence, deduce the upward force acting on the balloon. Ground level Reference Level L 500 m