Simple Dynamics question (just help to check the answer)

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In summary, the conversation discusses a physics homework problem involving a body of mass and density being dropped from a height into a lake. The question asks for the speed of the body before entering the lake, the acceleration while in the lake, and the maximum depth it sinks to. The attempted solution involves using equations for distance, force, and acceleration, but the final answer is corrected by another person. The conversation ends with a thank you.
  • #1
dawn_pingpong
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Okay I have done it, but have a feeling that there's something wrong. I don't have the solution/answer so it will be great if someone can help me check. more of part 2 (which leads to part 3)

Homework Statement


a body of mass 6.0kg and density 450kg/m^3 is dropped from rest at a height of 7.5m into a lake. Calculate
1. the speed of the body just before entering the lake
2. the acceleration of body while its in the lake
3. the maximum depth to which the body sinks before returning to surface.

Neglect any surface tension or air resistance.

The Attempt at a Solution



s=0.5gt2
sub s=7.5m, g=9.81, t=1.2365s
then v=gt, v=12.131m/s

2. Ffloat= ρgV, net force= Ffloat-mg
V=6.0/450=1/75, net force=-(1000 x 9.81 x 1/75 -6.0 x 9.81)=-71.94
then F=ma, a=-71.94/9.81=-7.3333m/s2

for me, down is positive, up is negative

3. using v2=v02+2as, v=0 when the body stops sinking.
s=10.034m, which is the distance sank.

Gahh I really think this question is rather easy, have not done dynamics for sometime, have given everything back to the teacher, sadly:(
 
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  • #2
dawn_pingpong said:
then F=ma, a=-71.94/9.81=-7.3333m/s2

Why is this?
 
  • #3
ahh, it's you again:)

Oh shoot wrong wrong it's a=F/m=-71.94/6.0=-11.99, so everything else follows is wrong. Thanks!
 

1. How do you define dynamics in science?

Dynamics in science refers to the study of how objects move and interact with each other, based on the principles of Newton's laws of motion and conservation of energy. It involves analyzing the forces acting on an object and how these forces affect its motion.

2. What is the difference between statics and dynamics?

Statics deals with objects that are at rest or in a state of equilibrium, while dynamics deals with objects that are in motion or experiencing acceleration. In other words, statics focuses on the forces that keep an object in place, while dynamics focuses on the forces that cause an object to move.

3. How do you calculate the velocity of an object in dynamics?

The velocity of an object in dynamics can be calculated by dividing the distance traveled by the time it took to travel that distance. This can be expressed in the formula: velocity = distance/time.

4. What is the significance of Newton's second law of motion in dynamics?

Newton's second law of motion states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This law is crucial in understanding how forces affect the motion of objects in dynamics.

5. How is energy conserved in dynamic systems?

In dynamic systems, energy is conserved through the principle of conservation of energy, which states that energy cannot be created or destroyed, only transferred or transformed. This means that the total energy in a system remains constant, even when energy is transferred between different forms, such as potential and kinetic energy.

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