The concept that additionally depends on location in a gravitational field is [mass] [weight].

Transcription

1 Nae Date CONCEPTUAL 'hys;, PRACTICE PAGE Mass and Weight..k Learning physics is learning the connections aong concepts in nature, and also learning to distinguish between closely-related concepts. Velocity and acceleration, previously treated, are often confused. Siilarly in this chapter, we find that ass and weight are often confused. They aren't the sae! Please review the distinction between ass and weight in your textbook...);1.:. To reinforce your understanding of this distinction, circle the correct answers below: Coparing the concepts of ass and weight, one is basic-fundaental-depending only on the internal akeup of an object and the nuber and kind of atos that copose it. The concept that is fundaental is [ass] [weight]. The concept that additionally depends on location in a gravitational field is [ass] [weight]. [Mass] [Weight] is a easure of the aount of atter in an object and only depends on the nuber and kind of atos that copose it. It can correctly be said that [ass] [weight] is a easure of "laziness" of an object. [Mass] [Weight] is related to the gravitational force acting on the object. [Mass] [Weight] depends on an object's location, whereas [ass] [weight] does not. In other words, a stone would have the sae [ass] [weight] whether it is on the surface of Earth or on the surface of the Moon. However, its [ass] [weight] depends on its location. On the Moon's surface, where gravity is only about 1/6 th Earth gravity [ass] [weight] [both the ass and the weight] of the stone would be the sae as on Earth. While ass and weight are not the sae, they are [directly proportional] [inversely proportional] to each other. In the sae location, twice the ass has [twice] [half] the weight. The Standard International (SI) unit of ass is the [kilogra] [newton], and the SI unit of force is the [kilogra] [newton]. In the United States, it is coon to easure the ass of soething by easuring its gravitational pull to Earth, its weight. The coon unit of weight in the U.S. is the [pound] [kilogra] [newton]. When I step on a weighing scale, two forces act on it; a downward pull of gravity, and an upward support force. These equal and opposite forces effectively copress a spring inside the scale that is calibrated to show weight. When in equilibriu, y weight = g. Support orce thanx to Daniela Taylor 11

2 CONCEPTUAL "Y5;C PRACTICE PAGE Converting Mass to Weight Objects with ass also have weight (although they can be weightless under special conditions). If you know the ass of soething in kilogras and want its weight in newtons, at Earth's surface, you can take advantage of the forula that relates weight and ass. Weight = ass x acceleration W=g due to gravity This is in accord with Newton's 2 nd law, written as = a. When the force of gravity is the only force, the acceleration of any object of ass will be g, the acceleration of free fall. Iportantly, 9 acts as a proportionality constant, 9.8 Nlkg, which is equivalent to 9.8 /s 2. Saple Question: fro = a, we see that the unit of How uch does a 1-kg bag of nails weigh on Earth? force equals the units [kg x /5 2 ]. the units [/s 2 1:: [N/kg]? W = g = (1 kg)(9.8 rn/s") = 9.8 /s" = 9.8 N....~ or siply, W = g = (1 kg)(9.8 Nlkg) = 9.8 N. ",::;r Answer the following questions: elicia the ballet dancer has a ass of 45.0 kg. 1. What is elicia's weight in newtons at Earth's surface? 2. Given that 1 kilogra of ass corresponds to 2.2 pounds at Earth's surface, what is elicia's weight in pounds on Earth? 3. What would be elicia's ass on the surface of Jupiter? 4. What would be elicia's weight on Jupiter's surface, where the acceleration due to gravity is 25.0 /s 2? Different asses are hung on a spring scale calibrated in newtons. The force exerted by gravity on 1 kg = 9.8 N. 5. The force exerted by gravity on 5 kg = N. i.r9.8n 6. The force exerted by gravity on kg=98n. Make up your own ass and show the corresponding weight: The force exerted by gravity on kg = N. By whatever eans (spring scales, easuring balances, etc.), find the ass of your physics book. Then coplete the table. OBJECT MELON APPLE MASS t kg WEIGHT 1 N BOOK A RIEND 60 k9 12

3 Nae Date CONCEPTUAL ~!r;~ PRACTICE PAGE A Day at the Races with a = / In each situation below, Cart A has a ass of 1 kg. Circle the correct answer (A, B, or Sae for both). 1. Cart A is pulled with a force of 1 N. Cart B also has a ass of 1 kg and is pulled with a force of 2 N. Which undergoes the greater acceleration? [A) [B) [Sae for both) 3. Cart A is pulled with a force of 1 N. Cart B has a ass of 2 kg and is pulled with a force of 2 N. Which undergoes the greater acceleration? [A) [B) [Sae for both) 2. Cart A is pulled with a force of 1 N. Cart B has a ass of 2 kg and is also pulled with a force of 1 N. Which undergoes the greater acceleration? [A) [B] [Sae for both] Age e~jt~~t~ B~:r~ 4. Cart A is pulled with a force of 1 N. Cart B has a ass of 3 kg and is pulled with a force of 3 N. Which undergoes the greater acceleration? [A] [B] [Sae for both] Aq~ (;J~ 3Uj~ A~ (;J~Jt!~[)!+- B~l~:t»- 5. This tie Cart A is pulled with a force of 4 N. Cart B has a ass of 4 kg and is pulled with a force of 4 N.. Which undergoes the greater acceleration? [A) [B) [Sae for both) Aq~.(;J2::: == S ~ ; ~j t~ ~~3t::t)g 6. Cart A is pulled with a force of 2 N. Cart B has a ass of 4 kg and is pulled with a force of 3 N. Which undergoes the greater acceleration? [A) [B) [Sae for both) A~ (;J~;}r~;t~ B~J~332J thanx to Dean Baird 13

4 CONCEPTUAL frysic PRACTICE PAGE Chapter 4 Newton's Second law of Motion Dropping Masses and Accelerating Cart 1. Consider a 1-kg cart being pulled by a 10-N. According to Newton's 2 nd law, acceleration of the cart is a = = 10 N = 10 /s2. 1 kg This is thescees the.cccelerctlon of free fa1t/g-becausea fcorceeq:ual to thecarfsweighf accelerates it. 2. Consider the acceleration of the cart when the is due to a 10-N iron weight attached to a string draped over a pulley. Will the cart accelerate as before, at 10 /s 2? The answer is no, because the ass being accelerated is the ass of the cart plus the ass of the piece of iron that pulls it. Both asses accelerate. The ass of the 10-N iron weight is 1 kg-so the being accelerated (cart + iron) is 2 kg. Then, The puh~ changes only thedireetion of the force. a = = 10 N 2 kg = 5/s2. Don't forget; the of a syste includes the. ass of the h~ing iron. Note this is half the acceleration due to gravity alone,g. So the acceleration of 2 kg produced by the weight of 1 kg is 9/2. a ind the acceleration of the 1-kg cart when two identical 1O-N weights are attached to the string. a = = Here we siplify and say 9 = 10 /s 2. 14

5 CONCEPTUAL PRACTICE PAGE Dropping Masses and Accelerating Cart-continued b. ind the acceleration of the 1-kg cart when the three identical 10-N weights are attach to the string. a = = = /s2. c. ind the acceleration of the 1-kg cart when four identical 10-N weights (not shown) are attached to the string. a = d. This tie 1 kg of iron is added to the cart, and only one iron piece dangles fro the pulley. ind the acceleration of the cart. a = = /s2. The force due to gravity on Q IT\QSS is g. So gravitatiomi forc.e on lkgis (1 kg)(10 /s 2 ) = 10 N. e. ind the acceleration of the cart when it carries two pieces of iron and only one iron piece dangles fro the pulley. a = = = = /s". 15

6 CONCEPTUAL flrsl PRACTICE PAGE Dropping Masses and Accelerating Calt-continued 1. ind the acceleration of the cart when it carries 3 pieces of iron and only one iron piece dangles fro the pulley. a = = = = /s 2. g. ind the acceleration of the cart when it carries 3 pieces of iron and 4 pieces of iron dangle fro the pulley. a= = = /s2..:il MOSS. of ccrt 15 ll<g. - of 10-N Iron is else 1 kg. h. Draw your own cobination of asses and find the acceleration. a = = = = /52. 16

7 N~e D~ _ _ CONCEPTUAL flysi, PRACTICE PAGE orce and Acceleration 1. Skelly the skater, 25 kg, is propelled by rocket power. a. Coplete Table I (neglect resistance). TABLE I!=ORCE 100 N 200 N ACCELERATION 10 /s1 b. Coplete Table 11 for a constant 50-N resistance. TABLE 11 I ORCE ACCELERATION 50 N o /S 2 WON 200N 2. Block A on a horizontal friction-free table is accelerated by a force fro a string attached to Block B of the sae ass. Block B falls vertically and drags Block A horizontally. (Neglect the string's ass). ( ( Circle the correct answers: a. The ass of the syste (A + B) is [] [2 ]. b. The force that accelerates (A + B) is the weight of [A] [B] [A + B]. c. The weight of B is [g/2] [g] [2 g]. d. Acceleration of (A + B) is [less than g] [g] [ore than g]. A ~.. B e. Use a = f..- to show the acceleration of (A + B) as a fraction of g. If B were allowed to foil by itself, not dragging A, then wojldn't its occeleroilon be g? bl~ t< Yes, because the force tret accelerates it woukl only be oding on its own ass - not twice t~ oss! To better U'"'derstond this. consider 3 ord 4 00 the other side! 17

8 CONCEPTUAL fty,; PRACTICE PAGE orce and Acceleration-continued A 3. Suppose Block A is still a 1-kg block, but B is a low-ass feather (or a coin). a. Copared to the acceleration of the syste of 2 equal-ass blocks the acceleration of (A + B) here is [less] [ore] B and is [close to zero] [close to g]. b. In this case, the acceleration of B is [practically that of free fall] [nearly zero]. A 4. Suppose A is the feather or coin, and Block B has a ass of 1 kg. a. The acceleration of (A + B) here is [close to zero] [close to g]. b. In this case, the acceleration of Block B is B [practically that of free fall] [nearly zero]. 5. Suarizing we see that when the weight of one object causes the acceleration of two objects, the range of possible accelerations is between [zero and g] [zero and infinity] [g and infinity]. 6. or a change of pace, consider a ball that rolls down a unifor-slope rap. a. Speed of the ball is [decreasing] [constant] [increasing]. b. Acceleration is [decreasing] [constant] [increasing]. c. If the rap were steeper, acceleration would be [ore] [the sae] [less]. d. When the ball reaches the botto and rolls along the sooth level surface, it [continues to accelerate] [does not accelerate]. ~,--\.~ r 18

9 N~e ~~ CONCEPTUAL fly./e PRACT~CE PAGE riction 1. A crate filled with delicious junk food rests on a horizontal floor. Only gravity and the support force of the floor act on it, as shown by the vectors for weight Wand noral force N. a. The net force on the crate is [zero] [greater than zero]. b. Evidence for this is <' ~f -~. f 11 w N._~ ~ - f w p 2. A slight pull P is exerted onthe crate, not enough to ove it. A force of friction f now acts, a. which is [less than] [equal to] [greater than] P b. Net force on the crate is [zero] [greater than zero]. 3. Pull P is increased until the crate beqins to ove. It is pulled so that it oves with constant velocity across the floor. a. riction fis [;Iess than] [equal to] [greater than] P b. Constant velocity eans acceleration is [zero] [ore than zero]. c. Net force on the crate is [less than] [equal to] [ore than] zero. <1\/ N ~ 4. Pull P is further increased and is now greater than friction f a. Net force on the crate is [less than] [equal to] [greater than] zero: b. The net force acts toward the right, so acceleration acts toward the [left] [right]. 5. If the pullinq force P is 150 N and the crate doesn't ove, what is the agnitude of f? 6. If the pulling force Pis 200 N and the crate doesn't ove, what is the agnitude of f? _ 7. If the force of sliding friction is 250 N, what force is necessary to keep the crate sliding at constant velocity? 8. If the ass of the crate is 50 kg and sliding friction is 250 N, what is the acceleration of the crate when the pulling force is 250 N? 300 N? 500 N? 19

10 CONCEPTUAL fty. PRACTICE PAGE Chapter :::s:;~~r:~;:::s 4 Newton's Second Law of Motion ~ froastationary Q ~ R e 0 helicopter. Various stages of fall are shown in positions a through f. Using Newton's 2 nd law, W '" 1000 N f;ndbro::::,e:tj:::hpo~jon (answer in the blanks to the right). You need to know that Bronco's ass is 100 kg so his weight is a constant 1000 N. Air resistance R varies with speed and cross-sectional area as shown. Circle the correct answers: 1. When Bronco's speed is least, his acceleration is [least] [ost]. 2. In which position(s) does Bronco experience a downward acceleration? [a] [b] [cl [d] le] [f] 3. In which position(s) does Bronco experience an upward acceleration? [a] [b] [cl [d] [e] [f] b1.~::nn c d R'" 1200N 4. When Bronco experiences an upward acceleration, his velocity is [still downward] [upward also]. R'" 2000N 5. In which position(s) is Bronco's velocity constant? [a] [b] [cl [d] [e] [f] 6. In which position(s) does Bronco experience terinal velocity? [a] [b] [cl [d] [e] 7. In which position(s) is terinal velocity greatest? [a] [b] [cl [d] [e] R"'1000N 8. If Bronco were heavier, his terinal velocity would be [greater] [less] [the sae]. 20