KEY -­‐ Weight vs. Mass (Mrs. Wright’s Key) 1. In your own words, explain the difference between weight and mass. Weight is the force of gravity pulling on your mass. It is an attractive force between two masses (the object’s & the Earth’s) Mass is how much matter makes up an object 2. Why are weight and mass used interchangeably on Earth? The acceleration due to gravity is the same everywhere on earth. Therefore, your weight is the same everywhere on Earth & so is your mass (however, your weight on Earth doesn’t equal your mass). However, if we were an interplanetary species and traveled back and forth between Earth & Mars, then everyone (American’s included) would want to switch to mass because mass is the same everywhere in the universe, while weight changes based on each location’s gravitational field. 3. What are the units for weight? What are the units for mass? Weight – N Mass -­‐ kg 4. A 1.0 kg is about average for the typical high school physics book. a. What is the mass of the physics book? 1.0 kg b. What is the weight of the physics book? W = mg = (1.0 kg) (9.8 m/s2) = 9.8 N c. What is the force of gravity on the physics book? Force of gravity = weight = 9.8 N 5. A 500. N stoplight just turned red. a. What is the mass of the stoplight? W = mg à m = W/g = (500. N) / (9.8m/s2) = 51.0 kg b. What is the weight of the stoplight? W = 500. N c. What is the force of gravity on the stoplight? Fgravity = 500. N 6. A 65.0 kg astronaut goes to the moon where the free-­‐fall acceleration is 1/6th that on Earth. a. What is the mass of the astronaut on Earth? 65.0 kg b. What is the weight of the astronaut on Earth? W = mg = (65.0 kg) (9.8m/s2) = 637 N c. What is the force of gravity on the astronaut on Earth? Fgravity = 637 N d. What is the mass of the astronaut on the moon? 65.0 kg e. What is the weight of the astronaut on the moon? W = mgmoon = (65.0 kg) (9.8/6 m/s2) = 106 N f.
What is the force of gravity on the astronaut on the moon? Fgravity = 106 N 7. Estimate if you don’t know (or pick a general amount like 150 lbs.): a. What is your weight in pounds? 150 lbs b. Based on your answer to part “a”, what is your weight in newtons? (1 lb. = 4.448 N) " 4.448N %
150lbs × $
' = 670N 670 N # 1lbs &
c. What is your mass? W = mg à m = W/g = (670 N) / (9.8m/s2) = 68 kg d. Imagine you are standing on a bathroom scale, calibrated in newtons, in an elevator accelerating upwards. Would the scale read more, less, or equal to your answer to part “b”? Explain. The scale would read MORE than my actual weight. If it is accelerating upwards, their must be a net force pointing in the upward direction. That means the scale needs to push up on me hard enough to (1) cancel out my weight, but then (2) even harder to accelerate me upwards and create that net upwards force. 8. A boxer trains at a ski resort near Denver and then flies to New York for the fight. When he weighs in, he is disqualified because he exceeds the 600.0 N weight-­‐limit for his class by 0.4 N. The boxer claims he was just at the limit when he weighted himself before the flight and had nothing to eat or drink on the airplane. Explain the discrepancy. An objects weight has to do with the acceleration due ot gravity (g). The acceleration due to gravity varies due to altitude. It doesn’t vary a lot, but it still does. Denver is at a higher altitude than New York City. Therefore the acceleration due to gravity is slightly smaller and it would account for him weighing ever so slightly less before his trip. 9. A full moving box has a mass of 20.0 kg. If a net force of 30. N is applied to it, what is its acceleration? ΣF = ma
30.N = (20.0kg)a a = 1.5 m/s2 a = 1.5m / s 2
10. A toy car has a mass of 525 g. If the car accelerates down a ramp at 2.0 m/s2, what is the net force on the car? ΣF = ma
ΣF = (0.525kg)(2.0m / s 2 ) ΣF = 1.1N
Fnet = 1.1 N 11. A student’s backpack weighs 35 lbs. If the student kicks the backpack with 13 N of force on a friction-­‐free pond, what is the backpack’s acceleration? (see #7 for lb à N conversion) " 4.448N %
35lbs × $
à ' = 155.68N # 1lbs &
ΣF = ma
a = 0.82 m/s2 13N = (15.89kg)a a = 0.82m / s 2
W = mg à m = W/g = (155.68N) / (9.8m/s2) = 15.89 kg 12. If a 25 kg box accelerates at 5 m/s2, what is the new acceleration if it is pushed twice as hard? The net force is directly proportional to the acceleration. So if the net force doubles, the ΣF = ma acceleration doubles leading to an acceleration of 10 m/s2. 13. If the force on an object decreases by 1/3, how is the acceleration effected? The acceleration would also decrease by 1/3. See #12 for the explaination. THIS KEY IS PROVIDED FOR CHECKING YOUR ANSWERS & MAY NOT BE REPRODUCED AS YOUR OWN WORK