Newton’s 3rd Law
Action and Reaction
Force Pairs
A force is always applied by a secondary
object
 The object to which the force is applied
also exerts a force back on the initial
object
 Whenever there is one force, there is a
second force

Newton’s 3rd Law
Whenever one object exerts a force on a
second object, the second object exerts an
equal and opposite force back on the first
 For every action, there is an equal and opposite
reaction
 The “action” and “reaction” forces act on
different objects, this results in a different
acceleration for both

Example

Two carts, one with a mass of 3 kg, and
the other with a mass of 1 kg are sitting
end to end on a desk. One the “explodes”
and exerts a force of 50N on the other,
what is the acceleration of each?
Tension

Force exerted on a lightweight string,
cord, or cable
Example

Two blocks are attached by a string; block
A is on the left and has a mass of 20 kg,
block B on the right and has a mass of 10
kg. Block A is pulled with a force of
60 N (W). Find the acceleration of the
two blocks and the tension on the string
between the two.
Example
An elevator uses a
counterweight as
shown. The mass of
the counterweight is
1000kg and the mass
of a loaded elevator
is 1150kg. Calculate
the acceleration of
the elevator and the
tension in the cable.
Example
A mover is lifting a piano
up to a second-story
apartment. He is using
a rope looped over
two pulleys as shown.
What force must he
exert on the rope to
lift the 1600kg piano at
a constant speed?
Example
A small 50g mass hangs from a thin string in a car
and can swing like a pendulum as shown. What
is the angle and force of tension on the string
when the car
a) accelerates forward at 1.20 m/s2
b) moves at a constant velocity of 90 km/hr