Dynamics – Newton’s 3rd Law
http://aplusphysics.com/courses/honors/dynamics/N3Law.html
Unit #3 Dynamics
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Objectives and Learning Targets
1. Identify action-reaction pairs.
1. Apply Newton’s 3rd Law to determine
unknown forces.
1. Use vector diagrams to analyze mechanical
systems (equilibrium and nonequilibrium).
Unit #3 Dynamics
Newton’s 3rd Law of Motion
• Newton’s 3rd Law of Motion, commonly referred to as the Law of
Action and Reaction, describes the phenomena by which all forces
come in pairs. If Object 1 exerts a force on Object 2, then Object 2
must exert a force back on Object 1. Moreover, the force of Object 1 on
Object 2 is equal in magnitude, or size, but opposite in direction to the
force of Object 2 on Object 1. Written mathematically:
This has many implications, some of which aren’t immediately obvious.
For example, if you punch the wall with your fist with a force of 100N,
the wall imparts a force back on your fist of 100N (which is why it
hurts!). Or try this. Push on the corner of your desk with your palm for
a few seconds. Now look at your palm... see the indentation? That’s
because the corner of the desk pushed back on your palm.
Unit #3 Dynamics
Newton’s 3rd Law of Motion
• Although this law surrounds your actions everyday, often times
you may not even realize its effects. To run forward, a cat pushes
with its legs backward on the ground, and the ground pushes the
cat forward.
• How do you swim? If you want to swim forwards, which way do
you push on the water? Backwards, that’s right. As you push
backwards on the water, the reactionary force, the water pushing
you, propels you forward.
• How do you jump up in the air? You push down on the ground,
and it’s the reactionary force of the ground pushing on you that
accelerates you skyward!
Unit #3 Dynamics
Action – Reaction Pairs
Unit #3 Dynamics
Action – Reaction Pairs
• As you can see, then, forces always come in pairs. These
pairs are known as action-reaction pairs. What are the
action-reaction force pairs for a girl kicking a soccer ball?
The girl’s foot applies a force on the ball, and the ball applies
an equal and opposite force on the girl’s foot.
• How does a rocket ship maneuver in space? The rocket
propels hot expanding gas particles outward, so the gas
particles in return push the rocket forward. Newton’s 3rd
Law even applies to gravity. The Earth exerts a gravitational
force on you (downward). You, therefore, must apply a
gravitational force upward on the Earth!
Unit #3 Dynamics
Sample Problem #1
Question: Earth’s mass is approximately 81
times the mass of the Moon. If Earth exerts a
gravitational force of magnitude F on the
Moon, the magnitude of the gravitational
force of the Moon on Earth is
1. F
2. F/81
3. 9F
4. 81F
Answer: (1) The force Earth exerts on the Moon is the same in
magnitude and opposite in direction of the force the Moon
exerts on Earth.
Unit #3 Dynamics
Sample Problem #2
Question: A 400-newton girl standing on a dock exerts a force of
100 newtons on a 10,000-newton sailboat as she pushes it away
from the dock. How much force does the sailboat exert on the girl?
Answer: 100N. The force the girl exerts on the sailboat is the
same in magnitude and opposite in direction of the force the
sailboat exerts on the girl.
Unit #3 Dynamics
Sample Problem #2
Unit #3 Dynamics
Sample Problem #2
Question: A carpenter hits a nail with a hammer.
Compared to the magnitude of the force the hammer
exerts on the nail, the magnitude of the force the nail
exerts on the hammer during contact is
1. less
2. greater
3. the same
Answer: (3) the same per Newton’s 3rd Law.
Unit #3 Dynamics