Home Page Textbooks Physics: Principles and Problems Page 245: Section Review

Physics: Principles and Problems

9th Edition

Elliott, Haase, Harper, Herzog, Margaret Zorn, Nelson, Schuler, Zitzewitz

ISBN: 9780078458132

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Page 245: Section Review

Exercise 26

Step 1

1 of 1

Since most of the mass of the disk is located on the outer part (at the rim), this increases the disk’s moment of inertia. So when the disk is thrown and spinning, its angular momentum is much bigger than what it would be if the mass was biggest in the center.Objects that have a bigger angular momentum will move through the air with more stability in the long run.

Exercise 27

Step 1

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Given: $F_{g1}=24.5,,rm N$, $F_{g2}=36.8,,rm N$, $l=1,,rm m$, $phi=30^o$

We can use conservation of energy:

$$
mglsinphi=frac{mv^2}{2}rightarrow v=sqrt{2glsinphi}=sqrt{2cdot9.81cdot1cdotsin30^o}
$$

$$
boxed{v=3.13,,rm m/s}
$$

We can use conservation of momentum:

$$
begin{align*}
m_1v&=(m_1+m_2)v’\
frac{F_{g1}}{g}v&=(frac{F_{g1}}{g}+frac{F_{g2}}{g})v’\
F_{g1}v&=(F_{g1}+F_{g2})v’\
v’&=frac{F_{g1}v}{F_{g1}+F_{g2}}\
&=frac{24.5cdot3.13}{24.5+36.8}
end{align*}
$$

$$
boxed{v’=1.25,,rm m/s}
$$

Result

2 of 2

a) $v=3.13,,rm m/s$

b) $v’=1.25,,rm m/s$

Exercise 28

Step 1

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The momentum is conserved in this collision, in this system we have to include ball, racket and tennis player. When a tennis player hits the ball he transfers some momentum to the ball to total momentum is equal before and after the collision.

Result

2 of 2

The momentum is conserved.

Exercise 29

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First, let us analyze the system. In the system, there is the athlete, the long pole and the crossbar. The athlete jumps over the crossbar by using the pole as their support.

When the athlete is currently at the top of the pole, over the crossbar, the point of the contact of the pole to the ground reacts has opposite force that is equal. The force that reacts **from the ground to the pole** has a vertical component and that component of force changes vertical momentum over some time.

Exercise 30

Step 1

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Resulting momentum is zero that means that before the collision sum of momentum has to be zero, we know that they run towards each other so their direction is opposite and to total momentum would be zero they must have the same magnitude of momentum.

Result

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Same magnitude of momentum but in opposite direction.

Exercise 31

Step 1

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In first case in our system we have ball, skateboard and you, if the total momentum is conserved you would have to move after the collision.

In the second case in the system we have ball earth an you, now to have conserved momentum we do not have to move because standing on the ground we transfer momentum to the earth.

Result

2 of 2

In first case earth is not in the system but in the second it is.

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Page 245: Section Review

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