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1
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2
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- Determine the mathematical relationship between an object’s weight and
its mass.
- Materials:
- Spring scales
- Triple beam balance
- Hanging masses
- Create a data table for all data and trials
- Create a graph of force vs. mass
- Compare the slope of your graph to g
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3
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- A body experiencing a non-zero net force will experience an acceleration
in the direction of the net force that is inversely proportional to its
mass.
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4
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- Which of the following statements are true and which are false? Explain your reasoning!
- The mass of an object depends on its location
- The weight of an object depends on its location
- Mass and weight are the same, but with different units.
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5
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- Weight: The force that gravity
exerts on an object with mass (m).
- This force is what causes falling bodies to accelerate at 9.81 m/s2.
- Weight is ALWAYS directed straight down toward the center of the earth
- Remember, g = 9.81 m/s2
- Units = Newtons
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6
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- Apparent weight is the weight something appears to have as a result of
an acceleration.
- For example, if you were standing on a scale in an elevator, your
apparent weight is the weight the scale would read.
- So now for some conceptual practice…
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7
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- Suppose you have a jet-powered flying platform that can move straight up
and down. For each of the
following cases, is you apparent weight equal to, greater than, or less
then your true weight? Explain.
- You are ascending and speeding up
- You are descending and speeding up
- You are ascending at a constant speed
- You are ascending and slowing down
- You are descending and slowing down
- You are descending at a constant speed
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8
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- Apparent weight can easily be calculated using the concept of net force.
- For example, if you are standing on a scale when you are at rest, what
forces are acting on you?
- (the force of gravity (your weight) and the force of the scale pushing
back up (the normal force))
- What is the net force in this situation?
- 0 N…you’re in static equilibrium
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9
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- Draw a Free-body diagram for this situation:
- Write out the vector equation:
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10
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- Since this situation is in equilibrium,
- Therefore,
- which means the scale is reading the “True weight”
- If the person standing on the scale has a mass of 65.0 kg, what is his
weight?
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11
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- If the elevator is accelerating upwards or downwards, then our problem
becomes slightly longer…
- For example, let’s say the elevator is accelerating upwards at a rate of
2.0 m/s2. What is now
different from our first example?
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12
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- Draw a free body diagram, including a vector off to the side indicating
the direction of the net force:
- Then write the vector equation:
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13
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- Since this situation is NOT in equilibrium, the following is ALSO TRUE:
- Using substitution, we can determine the size of the apparent weight
(the reading on the scale):
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14
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- Now let’s say the elevator is accelerating downwards at a rate of 2.0
m/s2. Draw the
free-body diagram for this situation:
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15
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- Again, we can write the vector equations…
- HOWEVER: the net force is now
DOWN, so it (and the acceleration) is therefore a negative value…
- Using substitution, we can determine the size of the apparent weight
(the reading on the scale):
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16
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- Determine the apparent weight of a 67.0 kg man standing in an elevator
when the elevator is:
- At rest
- Ascending and speeding up at a rate of 1.5 m/s2
- Ascending and slowing down at a rate of -1.2 m/s2
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Notes
