HCI SPECIMEN PAPER B Higher Paper

1 Name: _________________________________________ ( ) Class: ______________ HWA CHONG INSTITUTION SPECIMEN PAPER B HIGHER PHYSICS Level : Secondary Three Duration : 1 hour INSTRUCTIONS TO CANDIDATES Do not open this booklet until you are told to do so. Write your name, index number and class on the top of this page. Answer the questions in Section A and B in the spaces provided. All workings must be shown. In Section B, answer only 2 questions out of the 3 questions. Staple any graph paper used at the back of the answer script. INFORMATION FOR CANDIDATES In Sections A and B the intended marks for each question or part of a question a re given in brackets [ ]. Any working should be done in the space provided. When necessary, take acceleration due to gravity, g to be exactly 10 m s-2. ______________________________________________________________________________ This question paper consists of 14 printed pages, including this page.

2 Section A [20 marks] 1 This question is about a spider web. An experiment was carried out to measure the extension x of a thread of a spider’s web when a load F is applied to it. The results of the experiment are plotted as shown below. Uncertainties in the measurements are not shown. (a) Draw a best-fit line for the data points. [1] F / 10 N –2 –2x / 10 m thread breaks at this point 9.0 8.0 7.0 6.0 6.0 5.0 5.0 4.0 4.0 3.0 3.0 2.0 2.0 1.0 1.0 0.0 0.0

3 (b) When a load is applied to a material, it is said to be under “stress”. The magnitude P of the stress is given by FP A= where, A is the area of cross-section of the sample of the material. Use the graph and the data below to deduce that the thread used in the experiment has a greater breaking stress than steel. . . − − = = 92 6 Breaking stress of steel 1 0 10 N m Radius of spider web thread 4 5 10 m [3]

4 (c) In a particular web, one thread has the same original length as the thread used in the experiment. While making the web, the original length of the thread is extended by . − 22 4 10 m . (i) Use the graph to deduce the amount of work required to further extend the thread to the length at which it just breaks is about . − 31 6 10 J . Explain your working. [3] (ii) If the thread is not to break due to the impact of a flying insect, then the thread must be capable of absorbing all the kinetic energy of the insect as it is brought to rest by the impact. Determine the impact speed that an insect of mass 0.15 g must have in order that it just breaks the thread. =Impact speed ………………………………. [3]

5 2 This question is about the physics of falling objects. (Ignore air resistance for calculation.) (a) A 1.80 m tall geologist was walking in a cave. He s aw a drop of water fall past his face and splashed on the ground. Struck by inspiration, he waited and timed the next drop; it took 0.10 s to fall from the top of his head to the ground. Calculate the height of the ceiling above his head. Height above his head = …………………………. [4]

6 (b) At a particular fountain, two drops of water fall over the edge of the bowl into a pool h m below. The drop from the left falls directly into the pool. The drop from the right falls 1 m, hits a projection from a nearby Christmas tree and instantaneously loses all its speed and then begins to fall again. After it has fallen another metre, it hits another projection, …, and so on in equal stages until it hits the pool. How much longer does it take for the drop on the right to reach the pool? Express your answer in terms of the distance fallen on the left h , the number of stages on the right n , and the acceleration due to gravity g. [4] Water drops Water drops hitting a projection at every metre of fall. Pool h Picture for illustration only. The number of stages on the right is n not 5.

7 (c) The distance an object falls is directly proportional to the square of its time of flight . If it falls 16 feet in 1 s, how long will it take to fall 144 feet? Time= …………………………. [2]

8 Section B [20 marks] Answer two out of the three questions in this section. 3 This question is about body armor. When a high-speed projectile such as a bullet or bomb fragment strikes a modern body armor, the fabric of the armor stops the projectile and prevents penetration by quickly spreading the projectile’s energy over a large area. This spreading is done by longitudinal and tran sverse pulses that move radially from the point of impact, where the projectile pushes a cone-shaped dent into the fabric. The longitudinal pulse, racing along of the fibers of the fabric at speed lv ahead of the denting, causes the fibers to thin and stretch, with the material flowing radially inward into the dent. One such radial fiber show in the figure below. Part of the projectile’s energy goes into this motion and stretching. The transverse pulse, moving at a slower speed tv , is due to the denting. As the projectile increases the dent’s depth, the dent increases in radius, causing the material in the fibers to move in the same direction as the projectile (perpendicular to the transverse pulse’s direction of travel). The rest of the projectile’s energy goes into this motion. All the energy that does not eventually go into permanently deforming the fibers ends up as thermal energy. The graph below shows the speed v versus the time t for a bullet of mass . − 21 02 10 kg fired from a .38 Special revolver directly into the body armor. The scales of the vertical and horizontal axes are set by sv −= 1300 m s and .st −= 640 0 10 s . Take .lv −= 312 00 10 m s and the half-angle  of the conical dent to be . o60 0 . Side view of the bullet impact. The transverse radius (dent) is smaller than the longitudinal radius (thinned region). Speed – time graph of the bullet.

9 (a) Calculate the radius of the thinned region at the end of the collision. Radius= ………………………… [2] (b) Calculate the penetration depth d of the bullet into the armor. State the assumption for your calculation. d = …………………………….. [3] (c) Calculate the radius of the dent. Radius= …………………………….. [2]

10 (d) In the previous calculations, we have implicitly assumed that the person wearing the body armor remains stationary. In reality, the wearer is often thrown back due to the impact from the bullet. How will the values of the radii of the thinned region and the dent change if the wearer is moved back during the impact? Explain your answer. [3] 4 This question is about jogging on a hot day. When jogging strenuously, an average runner of mass 68 .0 kg and a surface area of 1.85 m 2 produces energy at a rate of 1300 W, 80% of which is converted to thermal energy. The jogger radiates heat, but actually absorbs more heat from the hot air than he radiates away. At such high levels of activity, the skin temperature can be elevated to 33.0 oC rather than the usual 30.0 oC. The only way for the body to get rid of this extra heat is by evaporating water (perspiring). (a) How much thermal energy per second is produced just by the act of jogging? Thermal energy per second=