HCI Assignment 04a Answer Newton s laws of motion

Dynamics 1 Name: Index No: Date: Class: Assignment 04a: Newton's laws of motion 1. Ann and Tim are in the ice-skating rink. Ann pushes Tim with 20 N force. Complete the blanks to explain the immediate motion of Ann and Tim. [3] [1]: Ann moves to the …left………………… and Tim would moves to the …right…………………. [1]: When Ann exerts a … 20………………… N (leftward/rightward) force on Tim, by Newton’s 3 rd law of motion, Tim exerts a …20………………… N (leftward/rightward) force on Ann. [1]: As the horizontal … normal………………… force exerted on each of them is greater than the …frictional………………… force by the rink ground on the skates, both moved backwards. Question: Will Ann and Tim move backwards with the same magnitude of acceleration? 2. Explain why a runner moves forward when he pushes backwards on the starting block. [2] [1]: As the runner exerts a (normal) force on the block, the block exerts an equal but opposite (normal) force on the runner. [1]: This reaction force propels the runner forward. HWA CHONG INSTITUTION Sec 3 Physics Ann Tim Action: Force by Ann on Tim Reaction: Force by Tim on Ann

Dynamics 2 3. The diagram below shows a system of 3 bodies - box, table and Earth/ground. The box is resting on the table which is supported by the ground. Source: http://canu.ucalgary.ca/map/content/force/newton3/simulate/box_on_table/applet.html (a) Complete the table below, stating the type of force, the body which exerts the force and the body which the force is exerted on. [4] F1: normal force exerted by the table on the box F2: gravitational force exerted by the Earth on the box F3: normal force exerted by the ground on the table F4: gravitational force exerted by the Earth on the table F5: normal force exerted by the box on the table F6: gravitational force exerted by the box on the Earth F7: gravitational force exerted by the table on the Earth F8: normal force exerted by the table on the ground (b) Hence, identify the four pairs of action-reaction forces. [4] 1. ______F1____ and ____ F5______ 2. ______ F2____ and ____ F6______ 3. ____ F4______ and ____ F7______ 4. ____ F3______ and ____ F8______ (c) The box has a mass of 5.00 kg. It is pulled horizontally across the rough table from rest, by a constant force of 30.0 N. The speed of the box is 2.00 m s -1 when it has moved 50.0 cm. Determine the constant friction f acting on the box as it moves. [2] (2.00 m s-1)2 = 02 + 2 a (0.500 m) a = 4.00 m s-2 [1] 30.0 N – f = (5.00 kg)(4.00 m s-2) f = 10.0 N (to 3 s.f.) [1] box table Earth ground box table Earth/ ground

Dynamics 3 4. For the system below, (a) identify the action and reaction forces between the interacting free-bodies which are underlined, (b) draw and label the vertical forces which act on the given free -bodies. When you label each force, state the type of force , the body which exerts the force and the body which the force is exerted on. A 50 kg gymnast balances on a 100 kg beam which stands on the ground / Earth. (Ignore the gravitational forces between the gymnast and the beam.) The four pairs of action and reaction forces are: [4] • Fgravitational (Earth on gymnast) and Fgravitational (gymnast on Earth) • Fgravitational (Earth on beam) and Fgravitational (beam on Earth) • Fnormal (gymnast on beam) and Fnormal (beam on gymnast) • Fnormal (beam on ground) and Fnormal (ground on beam) free-body diagram of gymnast [2] free-body diagram of beam [3] Fnormal (beam on gymnast) Fgravitational (Earth on gymnast) gymnast Fnormal (ground on beam) Fnormal (gymnast on beam) beam Fgravitational (Earth on beam) Note: Fgravitational (beam on gymnast) and Fgravitational (gymnast on beam) is always assumed to be negligible.

Dynamics 4 5. A bungee jumper is attached by a n elastic bungee cord which is tied to a bridge connecting two sides of mountain rocks. The bungee jumper jumps off from rest and stretches the elastic bungee cord as he falls. (a) The diagrams below show the free-bodies of the bungee jumper and the stretched bungee cord. Draw and label the pair of action and reaction forces between the two free-bodies. When you label each force, state the type of force , the body which exerts the force and the body which the force is exerted on. [Ftension is accepted.] [2] (b) The bungee jumper is 80.0 kg . Assume acceleration due to gravit y g = 10 m s -2 and ignore air resistance. Take positive velocity to be downward direction. (i) As the bungee jumper falls, calculate his instantaneous acceleration when the force in the stretched cord is 6.00 X 102 N. [2] [1]: (80.0 kg)(10 m s-2) - 6.00 X 102 N = (80.0 kg) a [i.e. mg - Felastic = ma] [1]: a = 2.50 m s-2 (ii) State the force in the stretched cord, at a particular instant when the falling bungee jumper’s acceleration is zero. [1] 8.00 x 102 N [accept 800 N] [Since a = 0 m s-2, mg = Felastic.] (iii) The bungee jumper continues falling until he reaches the lowest point of descent. At this instant, the force in the cord is 1.20 X 103 N. Calculate his acceleration at this instant. [2] [1]: (80.0 kg)(10 m s-2) - 1.20 X 103 N = (80.0 kg) a [1]: a = - 5.00 m s-2 (to 3 s.f.) Felastic (bungee jumper on bungee cord) bungee cord Felastic (bungee cord on bungee jumper) bungee jumper

Dynamics 5 (iv) Hence, state the direction of net force and the velocity of the bungee jumper when he is at the lowest point of descent. [2] Direction of net force: ……upward………. Velocity of bungee jumper: … 0 m s-1………… [momentarily at rest before being pulled up the next instant] 6. Data-based question: Mars mission Phoenix was a robotic spacecraft on a space exploration mission on Mars under the Mars Scout Program. The Phoenix lander descended on Mars on May 25, 2008. Mission scientists used instruments aboard the lander to search for environment suitable for microbial life on Mars and to research the history of water there. During the first part of the descent, it moved vertically towards the surface of Mars with a constant speed of 5.0 m s-1. The craft has a mass of 18000 kg. Neglect air resistance in this question. Source: http://www.slipperybrick.com/wp-content/uploads/2008/05/phoenix-mars-lands-on-mars.jpg (a) During the first part of the descent, the constant upward thrust of the fuel gases burnt by the spacecraft was 66.6 kN. Draw a labelled free –body diagram, showing clearly all forces acting on the spacecraft during the descent. For each force, state the type of force and its numerical value. [2] arrows have equal vertical lengths, in opposite directions: [1] correct labelling and values for weight and thrust: [1] Note: NOT “upthrust”! Weight (Mars on spacecraft) = 66.6 kN Thrust (gas on spacecraft) = 66.6 kN spacecraft

Dynamics 6 (b) During the last part of the descent, more fuel gases were burnt and the upward thrust was increased to a constant force of 90.0 kN. (i