Ch-5 Term 091 - PowerPoint PPT Presentation

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Ch-5 Term 091

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Ch-5 Term 091 Help-Session – PowerPoint PPT presentation

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Title: Ch-5 Term 091


1
Ch-5 Term 091
  • Help-Session

2
CH-5-082
  • Q15 You stand on a spring scale on the floor of
    an elevator. The scale shows the highest reading
    when the elevator


(Ans moves downward with decreasing speed)
3
CH-5-072
  • Q16. A 70 N block A and a 35 N block B are
    connected by a string, as shown in Fig 3. If the
    pulley is massless and the surface is
    frictionless, the magnitude of the acceleration
    of the 35 N block is A) 3.3 m/s2

Assume acceleration a of mass B downward mA
70/9.8 7.14 kg mB 35/9.8 3.57 kg Then For
masses A and B TmAa T-mBg - mBa Solving for
T mAamB(g-a) a mBg/(mAmB)
(3.579.8)/(7.143.57) 3.27 m/s2

4
CH-5-082
  • Q17. When a 25.0 kg crate is pushed across a
    frictionless horizontal floor with a force of 200
    N, directed 20? below the horizontal, the
    magnitude of the normal force of the floor on the
    crate is A) 313 N

?Fy N-200 sin20-mg 0 N 200 sin20 mg
2000.34259.8 313 N

5
CH-5-081
  • Q15. Two masses m1 10 kg and m2 20 kg are
    connected by a light string and pulled across a
    frictionless surface by a horizontal force F 30
    N as shown in Figure 2. Find the tension in the
    string. (Ans 10 N)

Fnet(?m)a 30(1020)a a 1m/s2 FBD of 10 kg
mass T 10a 101.010 N

6
CH-5-081
Q16. A 5.0-kg block and a 10-kg block are
connected by a light string as shown in Figure 3.
If the pulley is massless and the surface is
frictionless, the magnitude of the acceleration
of the 5.0 kg block is (Ans 6.5 m/s2
  • Assume acceleration a of 10 kg mass downward
  • Then for 10 kg mass
  • T-10g-10a T10g-10a
  • But from 5 kg mass
  • T 5a then
  • 5a10g-10a or 15 a 10 g
  • Then
  • a 10g/15 (109.8)/15
  • a 6.53 m/s2


7
CH-5-081
Q17. A 70 kg man stands in an elevator that is
moving downward at constant acceleration of 2.0
m/s2. The force exerted by the man on the
elevator floor is (Ans 546 N down)
N-mg-ma N mg-ma m(g-a) 70 (9.8-2) 546
N Force exerted by man equal and opposite to N

8
CH-5-072
Q13.Two blocks of mass m1 24.0 kg and m2,
respectively, are connected by a light string
that passes over a mass less pulley as shown in
Fig. 2. If the tension in the string is T 294
N. Find the value of m2. (Ignore friction) (Ans
40.0 kg)
  • Assume acceleration a of mass m1 upward
  • Then For mass m1
  • T-m1gm1a a(T-m1g)/m1
  • a (294-249.8)/24
  • 2.45 m/s2
  • Then acceleration a of mass m2 is downward
  • Then for mass m2
  • T-m2g-m2a Tm2g-m2a
  • Then m2T/(g-a)294/(9.8-2.45)
  • 40.0 kg


9
CH-5-072
Q16. A 5.0 kg block is lowered with a downward
acceleration of 2.8m/s2 by means of a rope. The
force of the block on the rope is(Ans35 N,
down)
  • Q14.Two horizontal forces of equal magnitudes are
    acting on a box sliding on a smooth horizontal
    table. The direction of one force is the north
    direction the other is in the west direction.
    What is the direction of the acceleration of the
    box?(Ans45 west of north)

force of block on the rope force of rope on
the block T Calculate the tension
T T-mg-ma Tm(g-a)5(9.8-2.8) 35 N
  • F1aj F2-ai RF1F2
  • Direction of R ?tan-1(-a/a)-45

10
CH-5-072
Q17.Two students are dragging a box (m100 kg)
across a horizontal frozen lake. The first
student pulls with force F150.0 N, while the
second pulls with force F2. The box is moving in
the x-direction with acceleration a (see Fig. 3).
Assuming that friction is negligible, what is
F2? (Ans 86.6 N)
Net force Fnet along x-axis Then ?Fy0 i.e
F1sin60F2sin30 F2F1sin60/sin30
50xsin60/sin3086.6 N

11
CH-5-071
Q14. Only two forces act upon a 5.0 kg box. One
of the forces is F1 (6i8j) N . If the box
moves at a constant velocity of v (1.6 i 1.2 j
)m/s, what is the magnitude of the second
force? ( Ans 10. N)
Q13 A constant force F of magnitude 20 N is
applied to block A of mass m 4.0 kg, which
pushes block B as shown in Fig. 5. The block
slides over a frictionless flat surface with an
acceleration of 2.0 m/s2. What is the net force
on block B? (Ans12
F1 (6i8j) N F1?(3664)10 N Since box is
moving at a constant velocity i.e
Fnet0F1F2 F2 F110 N
Acceleration aFnet/mAmB mB(Fnet/a)-mA
(20/2)-46 kg FB2x6mBa12 N
12
CH-5-071
  • Q16. A car of mass 1000 kg is initially at rest.
    It moves along a straight road for 20 s and then
    comes to rest again. The velocity time graph
    for the movement is given in Fig.6. The
    magnitude of the net force that acts on the car
    while it is slowing down to stop from t 15 s to
    t 20 s is (Ans 2000N)
  • Q15. An elevator of mass 480 kg is designed to
    carry a maximum load of 3000 N. What is the
    tension in the elevator cable at maximum load
    when the elevator moves down accelerating at 9.8
    m/s2? (Ans 0)
  • T-mg-ma
  • Tmg-ma3000-(3000/9.8)x 9.80

Fnet mass x acceleration a from t 15 s to t
20 s a (0-10)/(20-15)-2 m/s2
Fnetma1000x22000 N
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