Why Do My Physics Calculations Keep Giving Incorrect Results?

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SUMMARY

The discussion centers on the calculation of forces in two physics problems involving a helicopter lifting a truck and a car coming to a stop. In the helicopter scenario, the correct approach requires considering the gravitational force acting on both the helicopter and the truck, leading to a total force of 78,400 N when accounting for the upward acceleration of 2.00 m/s². In the car problem, the force required to stop the car is calculated using the equation F=ma, resulting in a force of 4,319.45 N, which is confirmed as correct by other participants. Misunderstandings stem from not accounting for all forces acting on the systems.

PREREQUISITES
  • Understanding of Newton's Second Law (F=ma)
  • Basic kinematics, including acceleration and velocity equations
  • Knowledge of gravitational force calculations
  • Ability to convert weight (N) to mass (kg) using gravitational acceleration (9.8 m/s²)
NEXT STEPS
  • Review the concept of net force and how it applies to systems with multiple forces
  • Study gravitational force calculations in detail, particularly in multi-object systems
  • Practice kinematic equations for various scenarios, including deceleration
  • Explore advanced topics in dynamics, such as friction and air resistance
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Students studying physics, educators teaching mechanics, and anyone looking to improve their problem-solving skills in force calculations.

Lorelyn
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Homework Statement



A 21000 kg helicopter is lifting a 5000 kg truck with an upward acceleration of 2.00 m/s2. Calculate the force the air exerts on the helicopter blades.

Homework Equations



F=ma

The Attempt at a Solution



I did the total force is equal to the sum of the two masses(26000kg) times the acceleration (2) which gave me 52000N. Which is apparently wrong but I have no idea why...

----------------------------------------------------------------------------------

Homework Statement




A car that weighs 14200.0 N is initially moving at a speed of 44.0 km/hr ((or 12.222)m/s) when the brakes are applied and the car is brought to a stop in 4.1 s. Find the magnitude of the force that stops the car, assuming it is constant.

Homework Equations



F=ma, Vf = Vi+ at

The Attempt at a Solution



So I solve using kinematics to get the acceleration (-12.22=(a*4.1)) so a = 2.98m/s^2. Which I then multiply by 14200/9.8 (the mass) to get the force equaling 4319.45N. Which is, again, wrong. And as with the above question I don't know why. I've done everything right as far as i can tell...
 
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helicopter

Lorelyn said:

Homework Statement



A 21000 kg helicopter is lifting a 5000 kg truck with an upward acceleration of 2.00 m/s2. Calculate the force the air exerts on the helicopter blades.

Homework Equations



F=ma

The Attempt at a Solution



I did the total force is equal to the sum of the two masses(26000kg) times the acceleration (2) which gave me 52000N. Which is apparently wrong but I have no idea why...
You found the net force, but the question asks for the force of the air. What other force acts on this system?
 
car

Lorelyn said:

Homework Statement




A car that weighs 14200.0 N is initially moving at a speed of 44.0 km/hr ((or 12.222)m/s) when the brakes are applied and the car is brought to a stop in 4.1 s. Find the magnitude of the force that stops the car, assuming it is constant.

Homework Equations



F=ma, Vf = Vi+ at

The Attempt at a Solution



So I solve using kinematics to get the acceleration (-12.22=(a*4.1)) so a = 2.98m/s^2. Which I then multiply by 14200/9.8 (the mass) to get the force equaling 4319.45N. Which is, again, wrong. And as with the above question I don't know why. I've done everything right as far as i can tell...
This one looks fine to me.
 

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