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Physics help?? Why is gravity POSITIVE in this problem?
A large fish hangs from a spring balance supported from the roof of an elevator. If the elevator has an upward acceleration of 2.45m/s^2 and the balance reads 60.0N , what is the true weight of the fish?
I know the answer is W=48N since
F=ma = m(g+a)
60N = m (9.8 +2.45)
m=4.9
W= mg
W= (4.9)*(9.8)=48N
BUTTTT I don't understand why you add 9.8 + 2.45. Why isn't g negative it like (2.45) + (-9.8) since the acceleration is upwards and gravity is always downwards?????????????????? this doesn't make any sense to me.
can someone explain this to me??
6 Answers
- oldprofLv 710 years agoFavorite Answer
You are right, it doesn't make any sense...until you think about the physics. And the physics you should know from riding in elevators yourself is that when an elevator lurches upward, you momentarily feel heavier. Thus g plus a, the two forces add up as in w = W + F = m(g + a).
And conversely, when you reach your destination at that elegant penthouse and the elevator slides to a stop, you momentarily feel lighter. Thus, when decelerating, we have g minus a, the deceleration detracts from your weight as in w = m(g - a).
Note we rode up for the above. But what about g and a starting back down? Think about how you felt in those elevators you've ridden in when returning to the ground floor.
My point is this...pay attention to and understand the physics and you will know what the signs should be. [See source.]
Source(s): If you've never ridden on an elevator, you can also work out the signs through math logic. We start with assuming weight W = mg; where g > 0 and always pointing downward. Then net weight is w = W - F = m(g - a) where F = ma is the accelerating force. And there you are: When accelerating upward, a < 0 because it's pointing in a direction opposite to g; the elevator is getting faster upward while gravity is pulling downward. In which case w = m(g - a) = m(g - (-a)) = m(g + a) QED. But when decelerating upward, a > 0 because it's acting in the same direction as g > 0. So w = m(g - a) = m(g - (+a)) = m(g - a) QED. - DuckLv 710 years ago
Gravity _is_ acting downward. The elevator is actually providing all of the force. The elevator has to overcome the force of gravity (9.8 m/s^2) and adds an additional acceleration (2.45 m/s^2). The actual acceleration is, in fact, 12.25 m/s^2 but gravity cancels the 9.8 m/s^2 part of acceleration.
- ?Lv 710 years ago
If I understand what you're asking, negative g's are caused by downward acceleration, not upward. This is why when NASA wants to simulate zero g they put people in an airliner and fly it towards the ground. Accelerating upwards creates positive g's.
- 10 years ago
Spring scales actually measure the Normal force on an object (reaction to) therefore the apparent weight is the total normal force (reaction to weight which is positive and the upward acceleration which in this problem is also positive)
Source(s): 30 year teaching hs physics - How do you think about the answers? You can sign in to vote the answer.
- 10 years ago
the elevator is going upward so due to the force of inertia te fish tendency will be to go downwards .similarly the gravity is also pulling it downwards therefore it will be added as bot of them are in the same direction
- Anonymous4 years ago
it easily relies upon on your physique of reference. in case you outline up as useful, then g is destructive (and the preliminary speed of the object tossed up is useful). in case you outline down as useful, then g is useful (and the preliminary speed of the object ossed up is destructive). Whichever you %, you will get a similar answer because of the fact the signs and indicators will cancel one yet another out.
