CONCEPTUAL PHYSICS, Ch. 6, p. 83, Review Questions, 1 – 18
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In the interaction between a hammer and the nail it hits, is a force exerted
on the nail? Ans. Yes. ON the hammer? Ans. Yes. How many forces occur
in this interaction? Ans. Two forces, action and reaction.
When a hammer exerts a force on a nail, how does the amount of force
compare with that of the nail on the hammer? Ans. They are equal and
opposite according to Newton’s Third Law.
When you walk along a floor, what pushes you along? Ans. The floor
pushes back on your feet.
When swimming, you push the water backward- call this action. What is
the reaction force? Ans. The water pushing you forward.
If the action is a bowstring acting on an arrow, identify the reaction force.
Ans. The arrow pushes back on the bowstring.
When you jump up, the world really does recoil downward. Why can´t this
motion of the world be noticed? Ans. The world has such a large mass
that is acceleration downwards is negligible.
When a rifle is fired, how does the size of the force of the rifle on the bullet
compare with the force of the bullet on the rifle? Ans. The forces are the
same, equal and opposite. How does the acceleration of the rifle compare
with that of the bullet? Defend your answer. Ans. The rifle will recoil
backwards with less acceleration because it has a larger mass. F = Ma
8. How can a rocket be propelled above the atmosphere where there is no air
to “push against”? Ans. The rocket doesn´t push against the air, the
rocket pushes against the hot gases coming out of its thrusters. The hot
gases push back propelling the rocket forward.
9. In the interaction between an apple and an
orange, how many forces are exerted on
the apple? Ans. One. The orange pulling
on the apple. On the orange? Ans. One,
the apple pulling the orange. Are these
forces equal in strength? Ans. Yes. Are
these forces opposite in direction? Ans.
Yes.
10. Consider the orange system. Do action and reaction forces cancel each
other in the orange system? Ans. No. They act on different objects – the
apple and the orange. Does the orange system accelerate? Ans. Yes,
towards the apple.
11. Consider the orange-apple system. Do action and reaction forces cancel
each other in this system? Ans. Yes, if you take them both together. One
force pulls to the right and another equal and opposite force to the left.
Do the orange and apple accelerate away from each other, or do they
remain together? Ans. They remain together.
12. A. In the horizontal direction, how many forces are exerted on the cart?
Ans. Two, the horse pulling the cart forward and friction pulling the cart
backwards. B. What is the net horizontal force on the cart? Ans. Force of
pull of horse forward – Force of friction on cart backwards
13. A. How many horizontal forces are exerted on the horse. Ans. Two: floor
pushing horse forward, and cart pulling back
the horse (due to friction of the cart). B. What
is the net horizontal force on the horse? Ans.
Ffp - Fpc B. How many horizontal forces are
exerted by the horse on the other objects? Ans.
Two. One on the ground and one on the cart.
14. A. How many horizontal forces are exerted on the horse-cart system?
Ans. Two: Fp and Ff. The force pulling the system forward and the
friction holding the system back. B. What is the net horizontal force on
the horse-cart system? Ans. Fp  Ff
15. In order to increase its speed, why must the horse push harder against the
ground than it pulls on the wagon? Ans. Otherwise the net force would be
zero and no acceleration would happen.
16. If you hit a wall with a force of 200 N, how much force is exerted on you?
Ans. 200 N is exerted back on you.
17. Why can´t you hit a feather in midair with a force of 200 N? Ans. It cannot
exert a reaction force of 200 N back on you.
18. How does the saying “You get what you give” relate to Newton’s third
law? Ans. If you give a hug you get one back. If you kiss somebody, they
kiss you back. Eye for an eye, tooth for a tooth. If you hit somebody,
they hit you back with an equal and opposite force.