Chapter
4
Newton's Three Laws
Part
6
Newton's Third Law of Motion
Newton's
Third Law: For every force that acts on an object, there is a
reaction force acting on a different object that is equal in
magnitude and opposite in direction.
Example
7
Two
groups of canoeists meet in the middle of a lake. After a brief
visit, a person in canoe 1 pushes on canoe 2 with a force of
46 N to separate the canoes. If the mass of canoe 1 and its occupants
is 
,
and the mass of canoe 2 and its occupants is 
,
(a) find the acceleration the push gives to each canoe. (b) What
is the separation of the canoes after 1.2 s of pushing?
Free-body diagram
Solution
Example
8
Two
boxes–one large and heavy, the other small and light–rest on
a smooth, level floor. You push with a force 
on
either the small box or the large box. Is the contact force between
the two boxes (a) the same in either case, (b) when you push
on the large box, or (c) larger when you push on the small box?
(c)
The contact force is larger when you push on the small box.
Example
9
A
box of mass 
rests
on a smooth, horizontal floor next to a box of mass 
.
If you push on box 1 with a horizontal force of magnitude F =
20.0 N, (a) what is the acceleration of the boxes? (b) What is
the force of contact between the boxes?
Solution
Video
What
happens if the relative position of the boxes is reversed? (a)
What is the acceleration of the boxes? (b) What is the force
of contact between the boxes? (c) What is the total force acting
on box 2?
Solution:
(a)
1.33 m/s^2
(b)
13.3 N
(c)
6.67 N
Weight
