Some physics questions (Kinematics)

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1

<p>Hi there,
I just answered a couple of questions about kinematics and got some wrong.
My book only states the correct answer without explanations and I am
almost positive that all questions I supposedly got wrong are just typos in the answer key...</p>

<p>So I wanted to double check with you guys:</p>

<p>1) Compared to the inertia of a 1-kilogram mass, the inertia of a 4-kilogram mass is
a) one-fourth as great
b) one-sixteenth as great
c) 16 times as great
d) 4 times as great</p>

<p>-> My solution: d)
Official answer key suggested c). c) makes no sense to me at all!</p>

<p>2) An 80-kilogram skier slides onward on waxed skis along a horizontal surface of snow at constant velocity while pushing with his poles. What is the horizontal component of the force pushing him forward? (frictional coefficient: 0.05)
a) 0.05N
b) 0.4N
c) 40N
d) 4N</p>

<p>-> My solution: Friction = F(N) * k = 785 * 0.05 = 40N - So 40N are needed in order to<br>
move at a constant velocity.
Answer key suggested a) 0.05N.</p>

<p>3) A carpenter hits a nail with a hammer. Compared to the magnitude of the force the hammer exerts on the nail, the magnitude of the force the nail exerts on the hammer during contact is
a) less
b) greater
c) the same</p>

<p>-> My solution: a) less - because the nail most likely will hit further in the wood (or
whatever) and will NOT exert the same force on the hammer. Then the nail wouldn't
move, that is not realistic!
Answer key suggests c) the same.</p>

<p>4) An object with a mass of 0.5 kilogram starts from rest and achieves a maximum speed of 20 meters per second in 0.01 second. What average unbalanced force accelerates this object?
a) 1000N
b) 10N
c) 0.1N
d) 0.001N</p>

<p>-> My solution: v = a<em>t -> 20m/s : 0.01s = 2000 m/s^2
F = m</em>a =0.5kg * 2000 m/s^2 = 1000N a)
Answer key suggested c) 0.1N.</p>

<p>5) A baseball bat moving at high velocity strikes a feather. If air resistance is neglected, compared to the force exerted by the bat on the feather, the force exerted by the feather on the bat will be
a) smaller
b) larger
c) the same</p>

<p>-> My solution: a) smaller
Answer key suggested c) the same...</p>

<p>I know, these questions are not in standard SAT form but they are still dealing with content of the SAT II Physics.
I would appreciate if someone could confirm that the answer key is wrong and my answers are logical.</p>

<p>Thanks in advance!</p>

2

<p>5 should be C, I would think. Newton’s Third Law states that for every force, there is an equal and opposite reaction.</p>

3

<p>@314159265:
Well, that might be right if things are at rest (e.g. weight and normal force) but in this example a baseball bat is hitting a feather! When I push against a wall the wall exerts the same force on me because it is not moving. If it would move then the wall wouldn’t exert the SAME force back on me.
So when a baseball bat hits a feather, the feather will definitely not be able to exert the same force back on the baseball bat. That is how I see it. Any other opinions?</p>

4

<p>Q3 and Q5 - The force will be the same. Read ‘action-reaction pair’</p>

<p>Q4, I’m getting the same answer as you.</p>

<p>Q2, velocity is constant, hence average acceleration is 0. Imagine the free body diagram of the system, there are 3 forces: Normal, Weight, Friction. Normal and weight (mg) cancel out each other. Only other force left is is friction = uN = 0.05*800 = 40N.</p>

<p>I’m getting the same answer as you.</p>

<p>Q1, Sorry, can’t remember the concept on this one… BUt I think it should be (D).</p>

5

<p>I think your answers are correct. Because the only doubtful one is 5, but they didnt tell us the motion of the 2 objects after the collision, so assuming practically, the feather would move in the same direction as the motion of the incoming bat, so I guess the force has to be smaller. Cuz if it’s the same, then they 2 should just stop…which makes no sense.</p>

6

<p>For Q5,</p>

<p>at the impact time (instantaneous time), both the bat and the feather apply an equal but opposite force on each other. The difference however is in the acceleration.
Let the equal force be F.
Mass of Bat be ‘M’ and mass of feather be ‘m’
Acceleration of Bat be ‘A’ and acceleration of feather be ‘a’</p>

<p>A=F/M … a=F/m
As M >> m,
Therefore, acceleration of feather will be greater than acceleration of bat.
And hence you will see a greater movement exhibited by the feather than by the bat.</p>

<p>For this question, don’t imagine a player swinging a bat and hitting a feather mid-way in the swing. Imagine that a bat is moving towards a feather automatically and ‘stops’ once it hits the feather.</p>

7

<p>Okay, so I am still struggling with Q3 and Q5.</p>

<p>I know about Newton’s Third Law. But if Newton’s Third Law would always be the case then there would be no objects on Earth which move because there would be no unbalanced forces (since every force would be canceled out by an equal and opposite force). </p>

<p>Q3: When I hit a nail with a hammer, then the nail moves further into the wood, right?!
So if the nail would exert the same force on the hammer as the hammer exerts on
the nail, the nail would not move further into the wood, right? </p>

<p>Q5: It doesn’t make sense that the bat and the feather just stop. If that would be the
case shouldn’t they state that in the task? I understand what TIZIL7 said but the bat
and feather will most likely move in the same direction after the collision - not in
opposite directions with different accelerations…</p>

<p>I just don’t understand it. Where is the LOGIC?
Just assuming they would exert the same force on each other (either the bat and the feather or the hammer and the nail) what makes these objects behave like we know it in reality and not just stop when they hit each other?</p>

8

<p>no offense Amad3us, but do you know anything about physics? like have you ever taken a physics class? it really all does make sense when you actually know how things work (what you learn in your typical classical mechanics class)</p>

9

<p>I’ll try to help too.</p>

<ol>
<li>You are correct in saying that the nail will move, and that once the collision takes place, the hammer will not move very much. But again, remember F = ma. Even though the same force occurs, the acceleration that occurs will be different because the nail has a much smaller mass. Equal forces are occurring (every action has an equal and opposite reaction!), but the smaller mass of the nail makes it move significantly more.</li>
</ol>

<p>If that didn’t click, think of how gravitation works between, let’s say earth and the sun. the formula G<em>m1</em>m2 / d^2 gives the magnitude of the force between them. That means the sun pulls on the earth with that much force, AND the earth pulls on the sun that much. The sun has a huge mass compared to the earth, so it hardly moves. But the earth’s mass is tiny relative to the sun, so it moves a ton. </p>

<p>Any better?</p>

10

<p>Question 1 should be D. The key must be wrong. Mass is a measure of inertia.</p>

<p>I think you’re right on number 4 and 2.</p>

<p>And OtherWindow is correct about questions 3 and 5.</p>

11

<p>@leapinlizards:
Well, it’s pretty hard to get anything else out of your response than offense. At least, you could have tried to explain something if you are so adept in physics. But be assured, back in Germany I took a physics class and finished my final exam with a 1+ (which is btw even harder to reach than an A+ in the U.S.) which is the best grade you can get in Germany. It’s just a long time ago and since I am reviewing everything in English right now, it is a bit different.
I had an error in my way of thinking but thanks to OtherWindow’s explanation I am cleverer now.</p>

<p>@OtherWindow and hahalolk:
Thanks for your explanations. It helped.</p>

12

<p>I explained the same thing as OtherWindow :slight_smile:
Though a little more mathematically :D</p>

13

<p>How would #2 be 40N??
80*.05 = 4N</p>

14

<p>^Because the frictional force is coefficient<em>normal force, not coefficient</em>mass.</p>

15

<p>Ok thanks 3141519265</p>

16

<p>@Tizil7
Lol, yep you did, I didn’t read all the posts before I posted :p</p>

17

<p>Amad3us, your answers to 1, 2, and 4 are correct.<br>
3 and 5 are both examples of Newton’s 3rd Law. If A exerts a force F on B, then B exerts a force -F on A.
So when two objects interact, they exert equal magnitudes of force on each other. The effects the two objects will differ if the objects have vastly different masses.</p>