Questions dealing with the Second Law of Motion

In summary: Remember to always draw a free body diagram and sum the forces to find the net force and acceleration. Keep up the good work!In summary, the weight of 600g of salami is 5.9 N, calculated using the formula F=mg. The weight of a 60-kg person standing on a scale in an elevator remains constant at 588N, regardless of whether the elevator is ascending or descending at a constant speed of 3.0 m/s. This is because there is no acceleration and the only force acting is due to gravity, as shown by the free body diagram. It is important to always draw a free body diagram and sum the forces to find the net force and acceleration in physics problems.
  • #1
miss_taken19
4
0
I have a couple homework problems that are causing me some trouble...

1. The weight of 600g of salami is:
a) 0.061 N
b) 5.9 N
c) 61 N
d) 5.9 kN

I found the answer to be a) 0.061N because F=mg so m=F/g which is
0.6kg/9.8m/sec.squared. The answer is actually b) 5.9 N but I can not figure out how the got this answer. Any insights to this?

Also...

2. A 60-kg person stands on a scale in an elevator. How many Newtons does the scale read when a) it is ascending at the constant speed of 3.0 m/sec.squared? b) it is desending at the constant speed of 3.0 m/sec. squared?

I am not sure how to find this out. On one hand, I think that I need to find the acceleration at these speeds. Yet I do not think that there is any acceleration when the speed is constant. Am I sort of on the right track here?

Thanks for your help!
 
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  • #2
for 1 isn't w = mg which give you .6kg *9.8 = 5.88 founded up to 5.9
for 2 what i think is this not sure if you have the answers to checkif it is correct f = ma f= 60 * 9.8 + and - 3 m/s just a guess not sure
 
  • #3
Hi there Mistaken and welcome to PF.

miss_taken19 said:
I have a couple homework problems that are causing me some trouble...

Well, you have definatly come to the right place.

miss_taken19 said:
1. The weight of 600g of salami is:
a) 0.061 N
b) 5.9 N
c) 61 N
d) 5.9 kN

I found the answer to be a) 0.061N because F=mg so m=F/g which is
0.6kg/9.8m/sec.squared. The answer is actually b) 5.9 N but I can not figure out how the got this answer. Any insights to this?

I think you are getting a lttle confused here. Is 600g a force or a mass? Is weight a mass or a force?

miss_taken19 said:
2. A 60-kg person stands on a scale in an elevator. How many Newtons does the scale read when a) it is ascending at the constant speed of 3.0 m/sec.squared? b) it is desending at the constant speed of 3.0 m/sec. squared?

I am not sure how to find this out. On one hand, I think that I need to find the acceleration at these speeds. Yet I do not think that there is any acceleration when the speed is constant. Am I sort of on the right track here?

Again, I think you getting a little confused here, you are not given any speeds; you are given accelerations, note the untis m.s-2. The scale in the lift actually measures the reaction force exerted by the lift on the perons. I think it would be helpful here if you drew a free body diagram. A good thread which previously described the mechanics is located https://www.physicsforums.com/showthread.php?t=120110".

Think about summing the forces acting; [itex]\sum F = m\vec{a}[/itex].

~H
 
Last edited by a moderator:
  • #4
Thanks for the help with the first one. It make a lot more sense now. I actually wrote the wrong units down for the second problem. The elevator is ascending and desending at 3.0 m/s not 3.0 m/sec. squared. So is there any force occurring here? I don't think so because there's no change in the velocities which means there is no acceleration. Am I right?
 
  • #5
miss_taken19 said:
Thanks for the help with the first one. It make a lot more sense now. I actually wrote the wrong units down for the second problem. The elevator is ascending and desending at 3.0 m/s not 3.0 m/sec. squared. So is there any force occurring here? I don't think so because there's no change in the velocities which means there is no acceleration. Am I right?

Spot on, so does the person's weight change?

~H
 
  • #6
No, I don't think so.Their weight in Newtons would still be 588N (60kg * 9.8m/s. squared) since no other force is acting on their bodies except for the force due to gravity.
 
  • #7
miss_taken19 said:
No, I don't think so.Their weight in Newtons would still be 588N (60kg * 9.8m/s. squared) since no other force is acting on their bodies except for the force due to gravity.

Spot on :smile:
 
  • #8
Yes! Thanks for you help :)
 
  • #9
miss_taken19 said:
Yes! Thanks for you help :)

My pleasure.
 

What is the Second Law of Motion?

The Second Law of Motion, also known as Newton's Second Law, states that the acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to its mass. In simpler terms, the greater the force applied to an object, the greater its acceleration will be, and the more massive the object, the less it will accelerate.

What is the mathematical equation for the Second Law of Motion?

The mathematical equation for the Second Law of Motion is F=ma, where F represents the net force acting on an object, m represents the mass of the object, and a represents the acceleration of the object. This equation shows that force and acceleration are directly proportional, while mass and acceleration are inversely proportional.

How does the Second Law of Motion apply to everyday life?

The Second Law of Motion is applicable in many real-life situations, such as when driving a car. The greater the force applied to the gas pedal, the faster the car will accelerate. Alternatively, if a car is overloaded with heavy objects, it will have a slower acceleration due to the increased mass. This law also explains why it takes more force to push a heavy object than a lighter one.

What is the difference between mass and weight in relation to the Second Law of Motion?

Mass is a measure of the amount of matter in an object, while weight is the measure of the force of gravity acting on an object. In the Second Law of Motion, mass is used to determine how much an object will resist acceleration, while weight is used to determine the force needed to accelerate an object.

How does air resistance affect the Second Law of Motion?

Air resistance is a force that opposes the motion of an object through the air. This force can affect the acceleration of an object, as it is a form of resistance that must be overcome by the applied force. The greater the air resistance, the more force is needed to achieve the desired acceleration, as described by the Second Law of Motion.

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