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5Analysis of Forces
5.1Newton’s Third Law
Newton created the modern concept of force starting from his insight
that all the effects that govern motion are interactions between two objects:
unlike the Aristotelian theory, Newtonian physics has no phenomena in
which an object changes its own motion.
Is one object always the “order-giver” and the other the “order-fol-
lower”. As an example, consider a batter hitting a baseball. The bat defi-
nitely exerts a large force on the ball, because the ball accelerates drastically.
But if you have ever hit a baseball, you also know that the ball makes a force
on the bat — often with painful results if your technique is as bad as mine!
How does the ball’s force on the bat compare with the bat’s force on the
ball. The bat’s acceleration is not as spectacular as the ball’s, but maybe we
shouldn’t expect it to be, since the bat’s mass is much greater. In fact, careful
measurements of both objects’ masses and accelerations would show that
m
ball
a
ball
is very nearly equal to –m
bat
a
bat
, which suggests that the ball’s force
on the bat is of the same magnitude as the bat’s force on the ball, but in the
opposite direction.
Rockets work by pushing exhaust gases out
the back. Newton’s third law says that if the
rocket exerts a backward force on the gases,
the gases must make an equal forward force
on the rocket. Rocket engines can function
above the atmosphere, unlike propellers and
jets, which work by pushing against the sur-
rounding air.
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