S dch chuyn ca nh v vt

S dch chuyn ca nh v vt Vn Tun - Trng THPT Chuyn Vnh Phc

Trong phn thu knh th phn dch chuyn ca nh v vt l mt trong nhng phn tng i kh bi v hc sinh rt d b nhm ln do phi tnh ton gii phng trnh qu nhiu. Trong phn ny ti mun gii thiu mt b m t ta c th s l cc bi ton thuc dng ny mt cch d dng hn.

1. Chng minh mt b :

(*)Trong : k1 l phng i ca nh khi v tr ban u, k2 l phng i ca nh khi dch chuyn, d = d2 d1 l dch chuyn ca vt, d = l dch chuyn ca nh.

Tht vy:

Xt: d =

Mt khc: ;

( d = ( pcm.

2. Cc h qu:

a. Nu hai nh cng tnh cht ( k1.k2 > 0 ( d v d tri du ( nh v vt qua thu knh lun dch chuyn cng chiu. Tc l nu vt dch chuyn cng chiu truyn nh sng th nh cng dch chuyn cng chiu truyn nh sng. Ta c th lm r hn vn ny nh sau:

- nu cc nh u l tht: gi s vt dch chuyn li gn thu knh ( d < 0 ( d < 0 ( , v cc nh l tht nn ( tc l khong cch t nh sau n thu knh ln hn khong cch t nh trc ( nh dch chuyn ra xa thu knh.- nu cc nh u l o: gi s vt dch chuyn li gn thu knh ( d < 0 ( d < 0 ( , v cc nh l o nn ( tc l khong cch t nh sau n thu knh nh hn khong cch t nh trc ( nh dch chuyn li gn thu knh.

- trong c hai trng hp ta d dng nhn thy nu vt dch chuyn li gn thu knh tc l dch chuyn t tri sang phi th nh cng dch chuyn t tri sang phi ( nh v vt dch chuyn cng chiu (theo chiu truyn nh sng).

b. Tnh cht thun nghch ca nh sng:

- Vi cc bi ton gi nguyn v tr ca vt v mn, khi dch chuyn vt trong khong gia vt v mn ta tm c hai v tr ca thu knh cho nh r nt ca vt trn mn, phng i ca hai nh lc l k1 v k2 v s dng tnh cht thun nghch ca nh sng ta chng minh c: k1.k2 = 1 (d1 = d2, d2 = d1).- y nu dng cng thc (*) ta cng c th d dng chng minh c iu trn. Tht vy:

V vt v mn (nh) c nh ch di chuyn thu knh nn d c: d = - d ( k1.k2 = 1 (pcm)/

3. Cc bi tp p dng:

Vt tht AB t trn trc chnh ca mt thu knh hi t cho nh tht trn mn. Nu dch chuyn vt dc theo trc chnh li gn thu knh 20 cm th ta phi dch chuyn mn 10 cm dc theo trc chnh 10 cm mi thu c nh r nt ca vt trn mn, nh ny cao gp 2 ln nh kia. Tm tiu c ca thu knh.

Li gii:Theo gi thit: d = - 20 cm, hai nh c cng tnh cht ( d > 0 ( d = 10 cm.

S dng cng thc (*) ta c:

Mt khc theo gi thit: (cng dch chuyn li gn th nh phi ln ln).T y ta tm c: k1 = - v k2 = - 1.

Vi k1 = ( d1 = 3f

k2 = ( d2 = 2f

( d = d2 d1 = - f = - 20 cm ( f = 20 cm.

4. Cc bi tp t gii:

Mt thu knh hi t to nh tht S' ca im sng S t trn trc chnh.

Khi di S li gn thu knh mt khong 5cm th nh di 10cm.

Khi di S xa thu knh 40cm th nh di 8cm

( so vi v tr ban u).

Tnh tiu c ca thu knh.

Mt ngi dng my nh chp nh mt bc tranh.

Ln u tin my c t cch bc tranh mt khong bng d1, nh trn phim cao 40 mm.

T v tr ban u dch chuyn my la gn thm 100cm th phi di phim 5mm mi thu c nh r nt v cao 80mm.

Hi nu mun nh trn phim cao 160/3mm th phi di my mt khong bng bao nhiu, theo chiu no. Di phim mt khong bng bao nhiu, theo chiu no.

t mt vt nh AB trc mt thu knh phn k ta c nh AB. Nu tnh tin vt ra xa thu knh thm 30cm th nh tnh tin 1cm. nh lc u cao bng 1,2 ln nh lc sau. Tm tiu c?

Vt sng AB t vung gc vi trc chnh ca mt thu knh hi t, nh ca AB l nh tht. Dch chuyn vt li gn thu knh on 30cm th nh cng dch chuyn on 30cm, nhng ln gp 4 ln nh c.

1. Xc nh v tr ban u ca vt v tiu c ca thu knh.

2. c nh cao bng vt, phi dch chuyn vt t v tr ban u i mt khong bng bao nhiu, theo chiu no?

Mt vt sng AB t thng gc vi trc chnh ca mt thu knh cho nh tht nm cch vt mt khong no . Nu cho vt dch chuyn li gn thu knh mt khong 30cm th nh vn l nh tht nm cch vt mt khong nh c v c ln gp 4 ln nh ban u.

1. Xc nh tiu c ca thu knh v v tr ban u ca vt AB.

2. c nh cao bng vt, phi dch chuyn vt t v tr ban u i mt on bng bao nhiu, theo chiu no?

t mt vt phng nh AB trc v vung gc vi trc chnh ca thu knh. Trn mn vung gc vi trc chnh ta thu c 1 nh r nt ln hn vt v cao 4cm. Gi vt c nh, dch chuyn thu knh v pha mn 5cm th phi dch chuyn mn dc theo trc chnh 35cm mi thu c nh r nt cao 2cm.

a. Tnh tiu c ca thu knh v cao ca vt AB.

b. Vt AB, thu knh v mn ang v tr c nh cao 2cm. Gi vt v mn c nh. Hi phi dch chuyn thu knh dc theo trc chnh v pha mn mt khong bng bao nhiu li c nh r nt trn mn. Trong khi dch chuyn thu knh th nh A'B' dch chuyn th no so vi vt

Trn hnh H.3 ng thng xy l trc chnh, O l quang tm, F l tiu im ca mt thu knh hi t. Mt vt sng phng, nh c t vung gc vi trc chnh ca thu knh. Nu t vt ti A th nh cao 3cm, nu t vt ti B th nh cao 1,5cm. Hi nu t vt ti trung im I ca AB th nh cao bao nhiu?_1465037562.unknown

_1465037966.unknown

_1465038668.unknown

_1465038745.unknown

_1465038764.unknown

_1465038795.unknown

_1465038690.unknown

_1465038089.unknown

_1465038098.unknown

_1465037983.unknown

_1465037707.unknown

_1465037945.unknown

_1465037647.unknown

_1465037497.unknown

_1465037535.unknown

_1465037398.unknown