Transcript Dynamics Example (Pushing Blocks)

DYNAMICS
Worked Example 4 – Pushing Blocks.
56 N acts on 3 blocks as shown.
56 N
A
B
C
7 kg
14 kg
7 kg
Each block experiences 6 N of friction.
a) Calculate the acceleration of the blocks.
1st
Free body Diagram + sign convention
56 N
is -ve
Total mass =
28 kg
is +ve
18 N
F u = 38 N
a = ?
Fu= 38 N
m = 28 kg
a= ?
a = Fu
m
= 38
28
= 1.4 ms-2
b) What force does block A exert on block B ?
Blocks B and C are affected.
is -ve
Mass of B& C=
F from A
Fu= ?
a = 1.4 m/s2
m = 21 kg
21 kg
is +ve
F f = -12 N
Fu= m B+C x 1.4
= 21 x 1.4
= 29.4 N
Fu = FA + F f
FA = Fu - F f
= 29.4 – (-12)
= 41.4 N
Over to you – Problems
up to 75.
Internal forces tutorial.
Revision questions for Higher Physics, page 17, Q. 1 – 7.
Reminder - Components of forces.
Read through your Components of a Vector written notes.
Force is a vector therefore the components are found using
this method.
Components of forces tutorial.
Revision questions for Higher Physics, page 18, Q. 1 – 4.
Reminder – Forces on a slope.
Read through your forces on a slope notes. Remember –
use sin for component of weight parallel to the
slope.
Forces on a slope tutorial.
Revision questions for Higher Physics, page 18, Q. 1 – 5.
Lifts tutorial
Revision questions for Higher Physics, page 20, Q. 1 – 6.
Problems 52 – 75
52. 4900 N
53. a) (i) 0.015 ms-2 (ii) 3 x 106 N b) -0.0027 ms-2
54. OA … decreasing acceleration as air resistance increases
AB…constant velocity as air resistance balances the weight
BC … parachute opens so dramatic deceleration to lower speed
CD … constant velocity as new air resistance balances weight
DE …dramatic deceleration as parachutist lands.
55. 0.02 ms-2
56. 150 N
57. a) 120 N b) -20 N
58. a) 1200 N
b) 108 m
c) 2592 N
59. a) (ii) 7.7 ms-2
b) mass decreases as fuel is burned, air resistance decreases
as rocket leaves Earth’s atmosphere, weight decreases as rocket leaves Earth’s
gravitational field.
c) 15.9 ms-2
d) Once out of Earth’s gravitational pull weight drops to
zero so engines can be switched off and rocket will continue at a constant
velocity.
60. a) 1778 kg b) 62 424 N
62. a) 1960 N
b) 2260 N
63. a) (i) 2450 N
(ii) 2450 N
61. 28 600 N
c) 1960 N
(iii) 2950 N
d) 1660 N
(iv) 1950 N
b) 4.2 ms-2
Problems 52 – 75 (cont.)
63. c) An empty lift so resultant force would increase giving a higher acceleration.
64. 51.2 N
65. a) 0.5
a (m/s2)
b) 37.2 N, 39.2 N, 43.2 N.
t (s)
0
-1.0
66. a) 8 m/s2
b) 16N
67. a) 5100N b) 2500N c) 700N, 500N
69. a) 2 m/s2
b) 40N c) 20N
d) 10,300N
68. 24m
d) 12N
e) No – A onto B affects 2 blocks so must balance 8N of friction. B onto C
only affects 1 block so just has to balance 4N of friction.
70. a) 3.27 m/s2
73. 9.8 N
b) 6.54 N
74. 0.735 N
71. 0.541 m/s2
72. a) 225.5N
b) 0.376 m/s2
75. a) 2283 N b) 2.2 m/s2 c) 14.8 m/s
d) 8520N