2. NEWTON’S THIRD LAW
For every action force, there must be an equal and opposite
reaction force. Forces occur in pairs.
Action
Reaction
Action
Reaction
3. ACTING AND REACTING FORCES
The action force is exerted by
the _____ on the _____.
The reaction force is exerted
by the _____ on the _____.
bar
hands bar
hands
Action
Reaction
4. ACTING AND REACTING FORCES
In every interaction, the forces always occur only in pairs,
BUT these forces act on two different bodies.
Common definition:
- to every action there is an equal and opposite reaction
is very dangerous, so please do not use it. It is not defined what is
action and what is reaction, so it looks as if we were talking about one
body, but that’s not true.
These forces act on different bodies.
5. ACTION - REACTION
•When the rocket fuel is ignited, a hot gas is produced.
•As the gas molecules collide with the inside engine walls, the
walls exert a force that pushes them out of the bottom of the
engine.
6. ACTION - REACTION
•This downward push is the action force.
•The reaction force is the upward push on the rocket engine by
the gas molecules.
•This is the thrust that propels the rocket upward.
13. EXAMPLES
action: cannon pushes the
cannonball
reaction: cannonball pushes
the cannon (recoil)
The same force F (opposite
direction)
14. THIRD LAW
To every action, there is always an
opposed but equal reaction. More
formally, the law states that for every
force exerted by object A on object B,
there is an equal but opposite force
exerted by object B on object A.
FAB = –FBA
15. THIRD LAW
For example, when you hit your
hand against your desk, your hand
exerts a force on the desk.
Simultaneously, the desk exerts a
force of equal magnitude in the
opposite direction on your hand.
Physical contact is not necessary for
Newton’s third law; the mutual
gravitational pull between the Earth
and the Moon traverses hundreds of
thousands of kilometers of space.
16. 1 - QUIZ
A 60-kg athlete exerts a force on a 10-kg skateboard. If she receives
an acceleration of 4 m/s2, what is the acceleration of the
skateboard?
17. 2 - QUIZ
A woman riding a bicycle collides head-on with a
parked school bus. Which object feels greater force?
