Download An object moving at constant velocity in an inertial frame must

Phys101
Term: 122
Online HW-Ch05-Lec01
Q1:
An object moving at constant velocity in an inertial frame must:
A.
B.
C.
D.
E.
have a net force on it
eventually stop due to gravity
not have any force of gravity on it
have no frictional force on it
have zero net force on it
Ans:
𝐄
Q2:
Two students are dragging a box of mass m
across a horizontal frozen lake (frictionless
surface). The first student pulls with force F 1 =
50.0 N, while the second pulls with force F 2 =
30 N. The direction of both forces is shown in
the figure. Find the angle (in degrees) between
the resultant force and the positive x-axis. (Give
your answer in three significant figures form)
Ans:
�⃗
The resultant force F
�⃗ = F
�⃗1 + F
�⃗2
F
�⃗ = (F1x + F2x )ı̂ + �F1y + F2y �ȷ̂
⇒ F
Where F1x = F1 cos60 = 50 cos60 = 25 N
F1y = F1 sin60 = 50 sin60 = 43.3 N
F2x = F2 cos30 = 30 sin30 ≅ 26 N
F2y = − F2 sin30 = −30 cos30 = −15 N
∴ �F⃗ = �(25 + 26)ı̂ + (43.3 − 15)ȷ̂� N = (51 ı̂ + 28.3 ȷ̂ ) N
28.3
& 𝜃 = tan−1 �
� = 29.0°
51
KFUPM-Physics Department
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Phys101
Term: 122
Online HW-Ch05-Lec01
Q3:
Only two forces, F 1 and F 2 act upon a 10 kg box. One of the forces is F 1 = (3 i -4 j)
N. If the box moves at a constant velocity v = (12 i +20 j) m/s, what is the magnitude
(in N) of the second force F 2 ? (Give your answer in three significant figures form)
Ans:
Since the velocity is constant then the acceleration is zero.
Newton’s second law:
�F⃗net = ma�⃗
0
⇒ �F⃗1 + �F⃗2 = 0
�⃗2 = − F
�⃗1
⇒ F
�⃗2 � = �F
�⃗1 � = �32 + 42 = 5.00 N
and �F
KFUPM-Physics Department
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