# PHYSICS:conservation of energy vs. SUVAT in spring questionsWatch

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Thread starter 9 months ago
#1
I have a question asking me to find the launch speed of a ball (mass 0.39kg) when released by a spring mechanism made of 2 springs each with force constant 25Nm^-2. they are pulled back 12 cm. the ball is initially at rest.I used f=kx=ma to find the acceleration and then v^2=u^2 2as to find vhowever I got a different answer (1.92) as the mark scheme which used conservation of energy, elastic potential energy (1/2kx^2)=kinetic energy (1/2mv^2). The answer they got was 1.4Shouldn't I have got the same answer and if not what am I doing wrong and why is it wrong to use SUVAT in this situation?
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9 months ago
#2
(Original post by izwx2000)
I have a question asking me to find the launch speed of a ball (mass 0.39kg) when released by a spring mechanism made of 2 springs each with force constant 25Nm^-2. they are pulled back 12 cm. the ball is initially at rest.I used f=kx=ma to find the acceleration and then v^2=u^2 2as to find vhowever I got a different answer (1.92) as the mark scheme which used conservation of energy, elastic potential energy (1/2kx^2)=kinetic energy (1/2mv^2). The answer they got was 1.4Shouldn't I have got the same answer and if not what am I doing wrong and why is it wrong to use SUVAT in this situation?
The problem is that the acceleration is not constant during the period where they are exerting a force on the ball, and the suvat equations require constant acceleration.
0
9 months ago
#3
(Original post by izwx2000)
I have a question asking me to find the launch speed of a ball (mass 0.39kg) when released by a spring mechanism made of 2 springs each with force constant 25Nm^-2. they are pulled back 12 cm. the ball is initially at rest.I used f=kx=ma to find the acceleration and then v^2=u^2 2as to find vhowever I got a different answer (1.92) as the mark scheme which used conservation of energy, elastic potential energy (1/2kx^2)=kinetic energy (1/2mv^2). The answer they got was 1.4Shouldn't I have got the same answer and if not what am I doing wrong and why is it wrong to use SUVAT in this situation?
As Pangol had already to point out what is wrong, I would add a little more. Why the force is not constant?
It is because spring force is not constant, |F| = kx.

Some thing to note in physics, whenever we are dealing a physics problem where the force is not constant, we would tend to use conservation of energy to solve the problem.

For this problem, we can still solve the problem via force method but you need to do integration.
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