10. the momentum of a system is conserved only when...
a. no internal forces are acting on the system.
b. the system is not moving.
c. no forces are acting on the system.
d. no net external force acting on the system.
e. the system has zero momentum.
11. a cannon recoils from launching a cannonball. the speed of the cannons recoil is small because the...
a. impulse on the cannon is less than the impulse on the cannonball.
b. cannon has far more mass than the cannonball.
c. the momentum of the cannon is unchanged.
d. force against the cannon is relatively small.
12. the force of an apple hitting the ground depends upon...
a. the speed of the apple just before it hits.
b. the time of impact with the ground.
c. whether or not the apple bounces.
d. air resistance on the apple as it falls.
e. all of the above.
13. when you jump off a step, you usually bend your knees as you reach the ground. by doing this, the time of the impact is about 10 times more than what would be in a stiff - legged landing, and the average force on your body is reduced by...
a. less than 10 times.
b. about 10 times.
c. more than 10 times.
14. two objects, a and b, have the same size and shape, but a is twice as heavy as b. when they are dropped simultaneously from a tower, they reach the ground at the same time, but a has a higher...
a. acceleration.
b. speed.
c. momentum.
d. all of the above.
15. a 1 n apple falls to the ground. the apple hits the ground with an impact force of...
a. 1 n.
b. 2 n.
c. 4 n.
d. 10 n.
suppose a girl is standing on a pond where there is no friction between her and the ice. to get off the ice, she can...
a. bend over, touch the ice in front of her, and then bring her feet to her hands.
b. walk very slowly on tiptoes.
c. get on her hands and knees and crawl off the ice.
d. throw something in the direction opposite to the way she wants to go.

Question

Accepted Answer

### Question 10
# Brief Explanations:
The law of conservation of momentum states that the total momentum of a system remains constant if no net external force acts on the system. Internal forces don't affect the total momentum (they cancel out), a system can be moving and have conserved momentum, forces can act but net external force must be zero, and zero momentum isn't a condition for conservation. So the correct option is d.
# Answer:
d. no net external force acting on the system.

### Question 11
# Brief Explanations:
According to the law of conservation of momentum, the momentum of the cannon - cannonball system is conserved (initial momentum is zero). Let the mass of the cannon be $M$, velocity be $V$, mass of the cannonball be $m$, velocity be $v$. Then $M V=-m v$ (negative sign for opposite direction), so $V =-\frac{m v}{M}$. Since the cannon has far more mass ($M\gg m$), its recoil speed $V$ is small. Impulse on cannon and cannonball is equal (Newton's third law), momentum of cannon changes, and the force on cannon is equal to the force on cannonball. So the correct option is b.
# Answer:
b. cannon has far more mass than the cannonball.

### Question 12
# Brief Explanations:
The force of impact can be related to impulse ($F\Delta t=\Delta p$). The change in momentum ($\Delta p$) depends on the speed just before hitting (to find initial momentum) and if it bounces (change in momentum is larger if it bounces). The time of impact ($\Delta t$) affects the force ($F = \frac{\Delta p}{\Delta t}$). Air resistance affects the speed just before hitting, so it also affects the impact force. Thus, all factors (a - d) affect the impact force, so the correct option is e.
# Answer:
e. all of the above.

### Question 13
# Brief Explanations:
From the impulse - momentum theorem $F\Delta t=\Delta p$. If $\Delta t$ increases by 10 times (from stiff - legged to bent - knees landing) and $\Delta p$ (change in momentum) remains the same (assuming same initial and final velocities), then $F$ is inversely proportional to $\Delta t$. So if $\Delta t_{new}=10\Delta t_{old}$, then $F_{new}=\frac{F_{old}}{10}$, meaning the force is reduced by about 10 times.
# Answer:
b. about 10 times.

### Question 14
# Brief Explanations:
Both objects have the same acceleration (acceleration due to gravity, neglecting air resistance as they have same size and shape, so air resistance effects are similar and can be neglected here). They reach the ground at the same time, so they have the same speed. Momentum $p = mv$, since $m_A = 2m_B$ and $v_A=v_B$, then $p_A = 2m_Bv=2p_B$, so A has higher momentum. Acceleration and speed are same for both, so the correct option is c.
# Answer:
c. momentum.

### Question 15
# Brief Explanations:
The impact force depends on the change in momentum and the time of impact. The weight of the apple is 1 N, but when it hits the ground, the impact force is not just its weight. The apple is accelerating downwards before hitting, and when it hits, the ground exerts a force greater than 1 N to stop it (change its momentum). However, without knowing the time of impact or the speed, we can't calculate the exact value, but among the options, the impact force is greater than 1 N. But if we consider that in some cases (like a very short time of impact), the impact force can be much larger, but from the given options, the most reasonable (assuming a typical impact) is that the impact force is greater than 1 N, and among the options, 10 N is a possible value (as the impact force is usually greater than the weight for a falling object hitting the ground).
# Answer:
d. 10 N.

### Question (last one about the girl on ice)
# Brief Explanations:
According to the law of conservation of momentum, if the girl throws something in the opposite direction of where she wants to go, the momentum of the thrown object will give her an equal and opposite momentum, allowing her to move. Bending over, touching the ice, or crawling won't work because there's no friction to provide a force for her to move. Walking on tiptoes also requires friction, which is absent. So the correct option is d.
# Answer:
d. throw something in the direction opposite to the way she wants to go.

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Question 10

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