Confusing regarding Kinematic Equations

In summary, the conversation discusses finding the net force exerted on a water skier who is pulled from rest to a speed of 13 m/s in a distance of 20m. One person suggests using the kinematic equation v^2 = u^2 + 2ad to find the acceleration, while another person suggests using the equation a = (delta V)/(delta T). The confusion arises from not having all the necessary information and not using the correct equations. Ultimately, it is important to choose the appropriate equation based on the given information.
  • #1
dolpho
66
0
"A 93kg water skier floating in a lake is pulled from rest to a speed of 13 m/s in a distance of 20m. What is the net force exerted on the skier, assuming his velocity is constant.

So I know I have to find the force I'm going to need acceleration, and to find it I'll need to use a kinematic equation. So this is where the confusion comes in.

The correct way to do it is v^2= U^2 + 2ad, and isolate the a. The answer to this would be 4.22 m/s^2. Then we can find the force by plugging it in, f=(4.22)(93)= 392 Newtons

But why couldn't I use this kinematic equation and solve for a?

x=x0 + v0t + 1/2(a)t^2)
20=0+0+1/2(a)(16)
20=8a
20/8 = a

Sorry if this seems like a dumb question but I think all the different letters and their meanings are getting a little confusing.

Or this one, acceleration = delta V / Delta T, couldn't you just do 13-0 / 4-0? so its 13/4?
 
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  • #2
where'd you get the four seconds from?
 
  • #3
SHISHKABOB said:
where'd you get the four seconds from?

Mmm, I think I accidentally put 4 seconds because I was reading something else. V= D/T, = T=D/V which is .65 seconds. But even then I couldn't solve for the equation in the other kinematic equation. Am I totally plugging in the wrong numbers into the x= Vsub0T + Xsub0 + 1/2(a)(t^2)?
 
  • #4
well, v = d/t only if the velocity is constant, and in this case the velocity is not constant, the guy is under a constant acceleration

so basically, you want to take what information you know, and pick the equation that works best with that stuff

the x = x0 + v0t + (1/2)at^2 equation doesn't work for this given set of values because we don't have the time
 
  • #5


I understand that kinematic equations can be confusing, especially with the use of different variables and equations. However, it is important to remember that each equation has a specific purpose and should be used accordingly.

In this scenario, the first equation you mentioned, v^2= U^2 + 2ad, is the correct equation to use because it relates the final velocity (v), initial velocity (U), acceleration (a), and displacement (d). This equation is specifically used when the final velocity is known and the initial velocity is zero, which is the case in this problem.

On the other hand, the second equation you mentioned, x=x0 + v0t + 1/2(a)t^2, is used to calculate the displacement (x) of an object with constant acceleration, where all variables are known except for time (t). This equation is not applicable in this scenario because the acceleration is not constant.

As for the third equation, acceleration = delta V / Delta T, it is important to remember that this equation is only applicable when the acceleration is constant. In this problem, the acceleration is not constant, so it cannot be used.

In summary, it is crucial to understand the purpose and conditions for using each kinematic equation to avoid confusion and ensure accurate calculations.
 

1. What are kinematic equations?

Kinematic equations are mathematical equations that describe the motion of an object in terms of its position, velocity, and acceleration. They are used to analyze the motion of objects in various situations.

2. How many kinematic equations are there?

There are five main kinematic equations, also known as the "big five" equations. These are: displacement equation, velocity equation, acceleration equation, time equation, and final velocity equation.

3. What is the difference between kinematic equations and dynamics equations?

Kinematic equations describe the motion of objects without considering the forces that cause the motion. On the other hand, dynamics equations take into account the forces that affect the motion of an object.

4. Can kinematic equations be used for any type of motion?

No, kinematic equations are only applicable to objects moving with constant acceleration. If the acceleration of an object is not constant, then other equations, such as the laws of motion, must be used to analyze its motion.

5. How do I solve problems using kinematic equations?

To solve a problem using kinematic equations, you must first identify the variables given and the variable you are trying to solve for. Then, plug in the known values into the appropriate equation and solve for the unknown variable. It is important to pay attention to units and use a consistent unit system throughout the problem.

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