# Newton's 3rd Law

Newton's 3rd Law

1) 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 a normal contact force).

True / False?

2) The two forces of a 3rd law pair always act on different bodies.

True / False?

3) 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.)

True/ False?

4) According to Newton's 3rd law, the force on the (smaller) moon due to the (larger) earth is

a) greater in magnitude and antiparallel to the force on the earth due to the moon.

b) greater in magnitude and parallel to the force on the earth due to the moon.

c) equal in magnitude but antiparallel to the force on the earth due to the moon.

d) equal in magnitude and parallel to the force on the earth due to the moon.

e) smaller in magnitude and antiparallel to the force on the earth due to the moon.

f) smaller in magnitude and parallel to the force on the earth due to the moon.

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#### Solution Preview

Newton's 3rd Law is a consequence of conservation of momentum. When two bodies interact then the momenta of the bodies can change, but the total momentum stays the same. The force on a body is the time derivative of the momentum, and this is thus opposite to the time derivative of the other body's momentum, and thus the force exerted on the other body is equal but opposite in direction.

Question 1) is a bit abstract, so let's start first with the other questions.

2) The 3rd law pair can be part of one body.

3) If the masses of the 3rd law pair are not equal then, applying the second Law yields that the accelerations ...

#### Solution Summary

A detailed solution is given. The expert examines Newton's 3rd Law.