Force and evolving energySlide 2
1 In a pile up, which causes more prominent harm? A A huge auto. B A quick auto. C A huge and quick moving auto.Slide 3
2 An auto slams into a settled divider. In what manner can the harm be decreased? A It crashes at fast. B It has a milder guard at the front with the goal that it stops in a more drawn out time. C It has a hard guard with the goal that it stops in a brief span.Slide 4
1 What is energy? Encounter lets us know that: a substantial , quick auto causes more noteworthy harm than an overwhelming , moderate auto or a light , quick auto. Both mass and speed are vital physical amounts for concentrating on moving articles .Slide 5
1 What is energy? Physicists have characterized an amount called energy ( 動量) of a moving article as... Force = mass speed = m v Unit: kg m s 1 A vector amountSlide 6
Example 1 Calculating energy Calculate the force of the accompanying items in kg m s –1 . ( an) A 20 g shot moving at 400 m s –1 . Energy of the projectile = 0.02 400 = 8 kg m s –1 ( towards the privilege )Slide 7
90 = 0.05 3.6 Example 1 ( b) A 0.05 kg tennis ball moving at 90 km h –1 . Energy of the tennis ball 0.05 kg 90 km h –1 = 1.25 kg m s –1 ( towards the left ) + veSlide 8
v u = m ( ) t m v m u = t 2 Momentum and constrain is additionally identified with the compel on a protest . Newton\'s second law can be communicated as far as the adjustment in force . F = m an underlying change in energy last Slide 9
m v m u = t 2 Momentum and drive change in energy Force = time Newton\'s second Law can be restated as: The net constrain following up on a protest is equivalent to the rate of progress of energy of the question.Slide 10
Q1 Two rams battle... Two rams battle . On the off chance that smash A wins, Which of the accompanying must be valid? A m A > m B A > B C v A > v B A B m B m A v A v BSlide 11
Q2 Vincent & Esther... Vincent & Esther are skating. Who is right? A Vincent. B Esther. C Neither. We have an indistinguishable from the results of our m & v are equivalent. 2.0 m s –1 2.5 m s –1 I don\'t think in this way, our headings are distinctive. 40 kg 50 kg Esther VincentSlide 12
Q3 A 0.4-kg football... A 0.4-kg football is moving at 20 m s –1 . What is the force of the football? Force of the football = m v = ___ __ ____ _ __ ______ = _________ __ 0.4 20 8 kg m s –1Slide 13
= v 2 gs = v 2 10 1.8 Q4 A stone of mass 1.5 kg... A stone of mass 1.5 kg is dropped from a stature of 1.8 m. ( a) What is the adjustment in force of the stone when it achieves the ground? v 2 u 2 = 2 as = 6 m s –1 Momentum change of the stone = m v = __________= _________ 9 kg m s –1 1.5 6Slide 14
Q4 A stone of mass 1.5kg... A stone of mass 1.5 kg is dropped from a stature of 1.8 m. ( b) Why does the energy of the stone change? It is on account of a ____________ drive follows up on the stone for a time of _________. gravitational timeSlide 15
3 Impact When a tennis ball is hit by a racket, a huge compel follows up on the ball in a brief timeframe. How might we think about the constrain amid effect? ReproductionSlide 16
30 m s 1 20 m s 1 0.05 kg Example 2 Average compel following up on tennis ball What is the normal constrain following up on the ball? time of effect t = 0.005 s + veSlide 17
m v m u F = t 0.05 30 0.05 ( 20) = 0.005 Example 2 Average constrain following up on tennis ball mass m = 0.05 kg u = 20 m s 1 v = 30 m s 1 t = 0.005 s + ve = 500 N to one side 500 NSlide 18
Experiment 8a Investigating the effect of compel Set up the accompanying mechanical assembly : Start information logging . Settle a little spring on the compel sensor. marginally push it from rest so it moves down the runway & crashes on the compel sensor.Slide 19
Experiment 8a Investigating the effect of drive From the v-t diagram, discover the speed of the trolley prior and then afterward affect. From the F-t diagram, discover the region under the chart. Contrast this territory and the adjustment in energy . VideoSlide 20
a Force-time chart of effect v changes when trolley crashes on drive sensor speed Result of analysis 8a: time (s) max. F drive (N) F F v. brief time interim 1.65 s 1.60 sSlide 21
a Force-time diagram of effect When a tennis ball is being hit by a racket, the ball is distorted . Why does the compel shift amid effect? The compel of effect to max. at the point when the ball is d eformed generally . As the ball recaptures its shape , the constrain .Slide 22
m v m u F = t a Force-time diagram of effect Rearrange terms in F t = m v m u drive : result of compel & time amid which the constrain demonstrations Impulse = change in energySlide 23
a Force-time chart of effect constrain/N This F-t diagram is a straight level line. steady compel Area u nder F-t diagram = F t = drive time/s region = motivationSlide 24
a Force-time chart of effect constrain/N bend line drive shifts a progression of restricted rectangular bars time/s region of e ach bar gives the drive amid the time interim aggregate region gives the aggregate driveSlide 25
a Force-time chart of effect compel/N drive/N time/s time/s Area under F-t chart = motivation = change in energySlide 26
F t b Force of effect For a similar change in m omentum , shorter time of effect constrain same zone same drive same in bigger F , littler t littler F , bigger tSlide 27
b Force of effect Impact time relies on upon the hardness of impacting articles. Harder question shorter effect time e.g. • Golf ball\'s hardness >> tennis ball\'s shorter effect time on being struck by golf club • Design of auto: crash time in the event of accidenceSlide 28
b Force of effect VideoSlide 29
Example 3 F - t chart of an effect A trolley hits and bounce back from a compel sensor and the F-t diagram is acquired. The zone under chart is 0.46 N s. 87654321 constrain (N) 1.75 1.80 1.85 1.90 1.95 2.00 time (s)Slide 30
Example 3 F - t diagram of an effect ( a) What is the most extreme compel following up on the sensor amid effect? Greatest drive = 7.8 N range under the chart = 0.46 N s 87654321 7.8 N constrain (N) 1.75 1.80 1.85 1.90 1.95 2.00 time (s)Slide 31
87654321 1.75 1.80 1.85 1.90 1.95 2.00 Example 3 F - t diagram of an effect ( b) Find the adjustment in force of the trolley. region under the chart = 0.46 N s compel (N) got by information logging program time (s) Area under the diagram = 0.46 N s Change in energy = 0.46 kg m s –1Slide 32
87654321 1.75 1.80 1.85 1.90 1.95 2.00 in force 0.46 0.3 time Example 3 F - t chart of an effect ( c) Hence locate the normal constrain acting amid effect. in energy = 0.46 m s –1 drive (N) time of effect = 2.05 – 1.75 = 0.3 s time (s) Average compel = 1.53 N =Slide 33
Example 4 Crumple zone of an auto An auto of mass 1500 kg moving at 20 m s 1 (72 km h 1 ) slams into a divider head-on and grinds to a halt. Figure the drive of effect on the auto in the accompanying cases. ( a) A auto has a fold zone in the front segment and it stops in 0.5 s. ( b) A auto has a solid guard in the front area and it stops in 0.02 s.Slide 34
m v m u F = t 0 1500 20 = 0.5 Example 4 Crumple zone of an auto ( a) A auto has a fold zone in the front area and it stops in 0.5 s. mass of auto = 1500 kg inverse course to auto\'s movement + ve = 6 10 4 N 20 m s 1Slide 35
m v m u F = t 0 1500 20 = 0.02 Example 4 Crumple zone of an auto ( b) A auto has a solid guard in the front segment and it stops in 0.02 s. mass of auto = 1500 kg 25 times more prominent than that in (a)! + ve = 1.5 10 6 N 20 m s 1Slide 36
Example 5 Force of effect of a falling can ( a) Calculate the speed of effect of the can on the ground. ( g = 10 m s 2 ) 0.4 kg + ve 30 m time of effect = 5 msSlide 37
Example 5 Force of effect of a falling can u = 0 a = g = 10 m s 2 v 2 u 2 = 2 as v 2 = 2 10 30 m v = 24.5 m s 1 ( or 88.2 km h 1 ) v = ? + veSlide 38
m v m u F = t (0 0.40 24.5) = 0.005 s Force of effect 1960 = Weight of can 0.4 10 Example 5 Force of effect of a falling can ( b) Find the normal drive of effect on the ground if the bounce back speed is immaterial. time of effect = 5 ms speed of effect =24.5 m s 1 mass of can = 0.4 kg = 1960 N (i.e. upwards) + ve = 490 times!Slide 39
Example 6 Thrust of a rocket A rocket pushes out 100 kg of hot gas every second. The speed of the launched out hot gas is 500 m s 1 . Figure the forward constrain (push) on the rocket. rate = 100 kg s 1 v = 500 m s 1Slide 40
m v m u F = ( 100 500) 0 = t 1 Example 6 Thrust of a rocket Let F be the constrain on gas = 50 000 N By Newton\'s third law, F = push u = 0 = 50 000 N v = 500 m s 1 rate = 100 kg s 1Slide 41
Q1 Which of the accompanying circumstances... Which of the accompanying circumstances does NOT include affect drive? A Traveling in a lift climbing at a consistent speed. B Kicking a ball. C A ping pong ball bobbing up starting from the earliest stage.Slide 42
F/N 4 3 2 1 2 t/s 12 1 2 3 4 5 0 5 Q2 A kid pushes a ball... A kid pushes a ball and the F-t diagram is demonstrated as follows. Locate the normal compel following up on the ball. Pixie u lse = region under F-t chart = _____ ___ _ __ _ = ____ ___ Average drive following up on the ball = _______ = _______ (1+5) 4 12 N s 2.4 NSlide 43
Q3 A auto of mass 1000 kg... An auto of mass 1000 kg quickens from rest to 8 m s –1 in 4 s. ( a) What is the adjustment in force of the auto? Change in force = mv mu 1000 (8 0) 8000 kg m s –1 = ________ __
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