Download Phys101 Term: 122 Online HW-Ch05

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 F1=
50.0 N, while the second pulls with force F2 = 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 )î + (F1y + F2y )ĵ
⇒ 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
∴ ⃗F = ((25 + 26)î + (43.3 − 15)ĵ) N = (51 î + 28.3 ĵ ) N
28.3
& 𝜃 = tan−1 (
) = 29.0°
51
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Phys101
Term: 122
Online HW-Ch05-Lec01
Q3:
Only two forces, F1 and F2 act upon a 10 kg box. One of the forces is F1 = (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 F2? (Give your answer in three significant figures form)
Ans:
Since the velocity is constant then the acceleration is zero.
Newton’s second law:
⃗Fnet = ma⃗
0
⇒ ⃗F1 + ⃗F2 = 0
⃗2 = −F
⃗1
⇒ F
⃗ 2 | = |F
⃗ 1 | = √32 + 42 = 5.00 N
and |F
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