Need help with work and energy

In summary, the conversation discusses two physics problems involving blocks and inclines. In the first problem, block A (50kg) and block B (100kg) are connected by a string and move along a frictionless incline. The coefficient of kinetic friction is given and the change in kinetic energy of block A is calculated using work of gravitational force, work of force, and work of frictional force. However, the calculated answer does not match the given answer and the user is looking for an alternative method. In the second problem, a spring on an incline is compressed by a 2.0kg mass and the spring constant is calculated using the law of conservation of energy. The user is then trying to determine the distance the mass
  • #1
ballahboy
34
0
Two blocks, A(50kg) and B(100kg) are connected to a string. The pulley is frictionless and of negligible mass. The coefficient of kinetic friction between block A and the incline is Mk =0.25. Determine the change in the kinetic energy of block A as it moves from C to D, a distance of 20m up the incline if the system starts at rest.

What i did was i found Wg (work of gravitaional force), Wf (work of force) and Wff (work of frictional force) so i can use change of KE=Wg+Wf+Wff. For Wg i got -5897.79J, Wf = 19600J and Wff = -1956J. When i add these together, i got 11746.21J but the answer is suppose to be 3900J. What did i do wrong or do you guys prefer a different method.

Another question: A spring of length 080m rests along a frictionless 30degree incline. A 2.0kg mass, at rest against the end of the spring, compresses the spring by 0.10m. A) Determine the spring constant k . B) The mass is pushed down, compressing the spring an addtional 0.60m, and then released. If the incline is 2.0m long, determine how far beyond the rightmost edge of the incline the mass lands.

If someone could show me the steps to these problems, it would be greatly appreciated. Thanks!
 
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  • #2
nvm i got the answer for the first one using a different formula..
can someone help me on the second one?
 
  • #3
2.(a) static equilibrium. the sum of the forces which are parallel to the incline equals zero.
 
  • #4
k used KEf+PEf+PEsf=KEi+PEi+PEsi to find the constant k and i got 3920N/M. Does that look right to ne one?

wuts static equilibrium? i don't think i learned that yet
 
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  • #5
i got 98.1 N/m. i don't know how to use the law of conservation of energy to get the answer, your answer.

u learned Newton's 2nd law ?
a = 0 when the particle is not moving or moves with constant velocity. then the sum of the forces acting on the particle equals zero.

x-axis is chosen to be parallel to the incline and positive in the right direction.
consider all the forces acting on the block and use Newton's 2nd law :
[tex] -mgcos60+(-kx)=0[/tex]
x=-0.10 m because the displacement of the spring is to the left hand side(when it is compressed) which is negative relative to its initial postion when it is relax.
 
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  • #6
thanks leong, i think i got part a now
does ne one kno how to do part b?
 
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1. What is work and energy?

Work and energy are both concepts in physics that describe the ability to do work or cause changes in a system. Work is defined as the force applied to an object multiplied by the distance it moves, while energy is the ability to do work.

2. How are work and energy related?

Work and energy are directly related through the work-energy theorem, which states that the work done on an object is equal to the change in its kinetic energy. In other words, the energy of an object changes when work is done on it.

3. What are some examples of work and energy?

Examples of work and energy can be seen in everyday situations. For instance, when you lift a book off the ground and place it on a shelf, you are doing work by applying a force to the book and moving it a certain distance. The energy used to do this work comes from your muscles.

4. How is work and energy measured?

Work is typically measured in joules (J), while energy can be measured in various units such as joules, calories, or electron volts (eV), depending on the type of energy being measured. For example, kinetic energy is measured in joules, while thermal energy is measured in calories.

5. How can I calculate work and energy?

To calculate work, you can use the equation W = F x d, where W is work, F is the force applied, and d is the distance the object moves. To calculate energy, you can use the equation E = m x v^2, where E is energy, m is the mass of the object, and v is its velocity. These calculations can help you understand the amount of work and energy involved in a given situation.

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