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S3 Mechanics. Mr. C.K.Yu. Mechanics. What is mechanics?. The branch of physics dealing with the study of motion. www.vibrationalrelativity.org/glossary.htm. Mechanics. What is mechanics?.

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## S3 Mechanics

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**S3 Mechanics**Mr. C.K.Yu**Mechanics**What is mechanics? The branch of physics dealing with the study of motion.www.vibrationalrelativity.org/glossary.htm**Mechanics**What is mechanics? The branch of the physical sciences concerned with the state of rest or motion of bodies that are subjected to the action of forces. www.unb.ca/web/civil/schriver/Previews/glossary.htm**Mechanics**What is mechanics? The science dealing with the motions of material bodies, including kinematics, dynamics, and statics.www.mercksource.com/pp/us/cns/cns_hl_dorlands.jspzQzpgzEzzSzppdocszSzuszSzcommonzSzdorlandszSzdorlandzSzdmd_m_06zPzhtm**Mechanics**What is mechanics? That branch of physics dealing with the action of forces on bodies which includes kinetics and statics.www.aucco.org/glossary.html**Mechanics**What is mechanics? • Mechanics is a study of the motion of objects • From Wikipedia, the free encyclopedia**Daily Life Example**School Sports Day**Analysis of Sports Day**Which of the following are important in each track event? • Length of event (e.g. 100 m, 200 m….) • Time of each competitor • Number of competitor **Analysis of Sports Day**What are we looking for in each event? • Who is the winner ? • How quick the winner is ? **Analysis of Sports Day**What item do we need to measure/record? • Length of the event ? • Time of the winner ? **Basic Quantity (Time)**Time is used to set up an time interval (間隔) for observing (觀察) the change (改變) of motion of an object. The beginning of this time interval is called initial moment. (t=0) The end of this time interval is called final moment.**Basic Quantity (Time)**Unit in calculation and Symbol Unit : second (秒) (S.I. unit system 單位系統). symbol for the unit : s Symbol : t**Basic Quantity (Time)**Other Scales (convert them into second in calculation) 3600 1 minute = s 1 hour = s 1 day = s 1 ms = s 1 μs = s 1 ns = s 60 86400 10-3 10-6 10-9**Basic Quantity (Time)**Measuring device (Timer) • Stopwatch (秒錶) • Scalar-timer (計數計時器)**Stopwatch**• Stopwatch is very convenient and easy to use. • There are about 0.2s reaction time for a person to start or stop a stopwatch after he received any message. • Stopwatch is not very accurate, even though some claimed that they are able to measure time correct to 1/1000 s. There is always a minimum error of 0.4s (0.2s+0.2s) caused by the reaction of the time-keeper in every time measurement.**Scalar-timer**• Scalar-timer is normally used with an electric circuit. • Scalar-timer can be started by breaking and stopped by making an electric current. • The electric current can be controlled by a light beam. • The reaction time error is avoided. • Scalar-timer can measure time intervals to the nearest millisecond.**Scalar-timer**Light detector 00000 Light can reach the light detector. The circuit is made and the scalar-timer stops. Light source**00000**Light reaches the light detector. The circuit is broken. The Scalar-timer stops .**00005**Light is blocked by the card and the circuit is broken. The Scalar-timer starts.**00020**Light is blocked by the card and the circuit is broken. The Scalar-timer starts.**00030**Light is blocked by the card and the circuit is broken. The Scalar-timer starts.**00040**Light is blocked by the card and the circuit is broken. The Scalar-timer starts.**00050**Light reaches the light detector again. The circuit is made and the scalar-timer stops.**00050**Light reaches the light detector again. The circuit is made and the scalar-timer stops.**00050**During the recording period (50 ms), how far did the cart move? The length of the card.**Basic Quantity (Distance,距離)**• Distance is a scalar quantity. A scalar has magnitude or size only. • Distance is the length of path an object travelled and is measured along the path. • Distance can be measured with a ruler, rule or slide-rule.**Unit and Symbol for distance**Unit : metre (米) Symbol for unit : m Symbol : s**5.4 m**B A**5.4 m**B A**5.4 m**B A**5.4 m**B A**Distance travelled = 5.4m +**3.6 m B A**Distance travelled = 9.0m +**5.2 m B A**Distance travelled = 14.2 m +**4.4 m B A**Basic Quantity (Distance)**Other Scales (convert them into metre in calculation) 10-2 1 km = m 1 cm = m 1 mm = m 1 nm = m 1000 10-9 10-3**N**Example 1 If a toy car starts at A and travels 10 metres to B, which is 6 metres due east of A car B A 10 The total distance travelled = ___m The total displacement travelled = 6 m east**C**5 km A > B 5 km Example 2 A car goes from town A to town C via (經過) town B. Town B is 5 km due east of town A and 5 km due south of town C.**C**5 km A > B 5 km Example 2 A car goes from town A to town C via (經過) town B. Town B is 5 km due east of town A and 5 km due south of town C. 5km+5km= 10,000 m The total distance travelled = The total displacement travelled = 7.1 km NE**Class Practice**A boy cycles once around a circular (圓形的) track of radius 40 m. What distance does he travel? He travels 80 π m**Derived Quantity (Speed)**• A derived quantity comes from two or more quantities. • Average speed is the rate of change in/of distance in one second. (How many meters the object travelled in one second.) Distance travelled within the time interval time interval Average speed = Is average speed a basic or derived quantity?**Speed**Unit : metre per second symbol for unit : m s-1 Symbol: v In mathematical equation (數學方程式) : Ds Dt Distance time v Average speed =**Example**A car travelled 10 m in 2 s. What is its speed ? As Ds = 10 m , Dt = 2 s By v = Ds / Dt ∴ v = 10 m / 2 s = 5 ms-1 The speed of the car is 5 ms-1.**Measuring Speed**As speed is a derived quantity from distance and time, measuring speed requires measuring both the distance travelled and the time interval. We can use stopwatch to measure time, rule to measure distance and calculate the average speed by the equation v = Ds/Dt.**Measuring Speed**To measure both distance and time together, we can use the ticker-tape timer.**Ticker-tape timer**a metal strip (金屬條) a metal bolt (螺絲閂) It operates (操作) with a 12 V a.c. power supply.**Ticker-tape timer**When the metal strip vibrates, the bolt at the end hits the carbon paper disk and dots are marked on the paper strip (ticker-tape) under the carbon paper.**Ticker-tape timer**The number of dots produced in one second is called the frequency of the timer. no. t No. time f frequency = The time, T, between two dots is one tick time and is : 1 f T Period = Usually, the metal strip hits the ticker-tape 50 times in a second, that is, its frequency is 50 Hz.**Ticker-tape timer**Unit of frequency : Hertz (Hz) Symbol for frequency is : f Example, there were 60 dots produced in 3 s, thus, the frequency : f = no./t = 60 /3s = 20 s-1 = 20Hz.

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