HW 4-1
Every force has one and only one 3rd law pair force. – True
The two forces in each pair act in opposite directions. – True
The two forces in each pair either both act on the same body or they can act on different bodies – False
The two forces in each pair may have different physical origins (for instance, one of the forces could be
due to gravity, and its pair force could be due to friction or electric charge). – False
The two forces of a 3rd law pair always act on different bodies. – True
Given that two bodies interact via some force, the accelerations of these two bodies have the same
magnitude but opposite directions. (Assume no other forces act on either body.) – False
According to Newton's 3rd law, the force on the (smaller) moon due to the (larger) earth is
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equal in magnitude but antiparallel to the force on the earth due to the moon
Which of the following diagrams is the correct interaction diagram for the situation described in this
problem? Each red line represents an interaction and an action/reaction pair of forces. The labels used in
the diagrams are the following:
 C = car
 T = truck
 S = road surface
 EE = entire earth
Assuming that the car pushes the truck to the right, which of the following free-body diagrams is the
correct diagram for the situation described in this problem? In each case, the diagram on the left refers to
the car, and that on the right to the truck. Dashed lines connect action/reaction pairs. The following
notation is used: f⃗ f→, n⃗ n→, and F⃗ GF→G are, respectively, friction, normal force, and gravity;
subscripts C and T stand for "acting on car" and "acting on truck"; F⃗ ConTF→ConT is the force exerted
on the truck by the car; and F⃗ TonCF→TonC is the force exerted on the car by the truck. Note that the
force vectors are not drawn in scale.
From Newton’s third law, as the car pushes the truck than the truck pushes back on the car
What is the magnitude FConTFConT of the force that the car exerts on the truck? Use the coordinate
system shown in Part B, where upward is the positive y direction and to the right, the
direction F⃗ ConTF→ConT points, is the positive x direction.
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F – ft = (m + M)a
a = (F – ft ) / (m + M)
Substitute given amounts, 2120 kg for M, 4530 N for F, 1230 for m, and 750 N for ft
acceleration = 1.128
F – Fc on = ma
Fc on = F – ma
Fc on = 4530 – (1230 * 1.128)
Fc on = 3142.119 N
A downward force of magnitude 5 N is exerted on the book by the force of – Gravity
An upward force of magnitude _____ is exerted on the _____ by the table. – 5N / book
Do the downward force in Part A and the upward force in Part B constitute a 3rd law pair? – No
The reaction to the force in Part A is a force of magnitude _____, exerted on the _____ by the _____. Its
direction is _____ . – 5N / earth / book / upward
The reaction to the force in Part B is a force of magnitude _____, exerted on the _____ by the _____. Its
direction is _____. – 5N / table / book / downward
Which of Newton's laws could we have used to predict that the forces in Parts A and B are equal and
opposite? – Newton’s 1st & 2nd Law
Which of Newton's laws could we have used to predict that the forces in Parts B and E are equal and
opposite? – Newton’s 3rd Law