Video Transcript
A particle moves along the ๐ฅ-axis. It is initially at rest at the origin. At time ๐ก seconds, the particleโs acceleration is given by ๐ is equal to 24 minus 19๐ก meters per second squared, where ๐ก is greater than or equal to zero. How long does it take for the particle to return to the origin?
Weโre told that the particle is moving in a straight line along the ๐ฅ-axis. Weโre told this particle is initially at rest at the origin. What this means is when ๐ก is equal to zero, the velocity of our particle is equal to zero. And when ๐ก is equal to zero, the displacement of our particle is also equal to zero. Weโll call the velocity ๐ฃ of ๐ก and the displacement ๐ of ๐ก. Weโre then given the acceleration of our particle for values of ๐ก greater than or equal to zero; the acceleration is 24 minus 19๐ก meters per second squared.
Weโre tasked with finding how long it takes for our particle to return to the origin. Since we chose our function ๐ of ๐ก to be the displacement of our particle from the origin, our particle will be at the origin when this displacement is equal to zero. So we want to find an expression for our displacement function ๐ of ๐ก. Solving this equal to zero will give us all of the times where our particle is at the origin. And weโre interested in the first time it returns to the origin where ๐ก is greater than zero, since we already know it starts at the origin.
To help us find this information, letโs start by recalling the acceleration is the rate of change of the velocity with respect to time. And if this is true, since integrating is the opposite process to differentiating, we know the integral of the acceleration with respect to time will be equal to the velocity of our particle up to a constant of integration. Weโre given the acceleration function of our particle. So we can use this to find the velocity of our particle. By integrating our acceleration function with respect to time, we get the velocity of our particle at the time ๐ก is equal to the integral or 24 minus 19๐ก with respect to ๐ก. And this will be up to a constant of integration.
We can then integrate this by using the power rule for integration. We add one to our exponents of ๐ก and then divide by this new exponent of ๐ก. This gives us 24๐ก minus 19 over two ๐ก squared plus our constant of integration ๐. Remember, though, we were told our particle was initially at rest. This means the velocity when ๐ก is equal to zero should be equal to zero. So we can find our value of ๐ by substituting ๐ก is equal to zero into our velocity function. Doing this, we get the particleโs initial velocity is equal to 24 times zero minus 19 over two times zero squared plus ๐.
We know that the initial velocity of our particle is equal to zero. And we know 24 times zero minus 19 over two times zero squared is equal to zero. So this simplifies to give us that ๐ is equal to zero. So substituting ๐ is equal to zero into our equation for the particleโs velocity, weโve shown that the velocity of our particle at the time ๐ก is equal to 24๐ก minus 19 over two ๐ก squared. And itโs also worth noting since the acceleration function of our particle was only valid when ๐ก was greater than or equal to zero, our velocity function will also only be valid when ๐ก is greater than or equal to zero.
But remember, we want to find the displacement of our particle from the origin. So we recall the velocity of our particle will be equal to the rate of change of displacement with respect to time. We can use the same logic we did before. Weโll integrate with respect to ๐ก. This gives us the integral of the velocity of our particle with respect to ๐ก will be equal to the displacement of our particle at the time ๐ก. Remember, weโre using ๐ of ๐ก to be the displacement of our particle from the origin. But we couldโve used any point as our reference point, since weโll have a constant of integration. So letโs apply this. We have ๐ of ๐ก is the integral of ๐ฃ of ๐ก with respect to ๐ก. And weโve shown that ๐ฃ of ๐ก is 24๐ก minus 19 over two ๐ก squared.
Again, we can integrate this term by term by using the power rule for integration. We get 12๐ก squared minus 19 over six ๐ก cubed plus our constant of integration, weโll call ๐. We want to find the value of ๐. Remember, our particle was initially at rest at the origin. So when ๐ก is equal to zero, ๐ of ๐ก is equal to zero. So weโll substitute ๐ก is equal to zero. We get zero is equal to 12 times zero squared minus 19 over six times zero cubed plus ๐. And this simplifies to give us that ๐ is equal to zero. So weโll substitute ๐ is equal to zero into our expression for ๐ of ๐ก. This gives us ๐ of ๐ก is equal to 12๐ก squared minus 19 over six ๐ก cubed.
And remember, ๐ of ๐ก is a measure of the displacement of our particle from the origin. So when this is equal to zero, our particle is at the origin. So solving this equation equal to zero will tell us how long it takes our particle to return to the origin. Remember, our displacement function will only be valid when ๐ก is greater than or equal to zero. And in our case, weโre interested in the solution where ๐ก is positive, since we already know the particle was at rest at the origin when we started. To solve this equation equal to zero, weโll start by taking out a shared factor of ๐ก squared. This gives us ๐ก squared times 12 minus 19 over six ๐ก is equal to zero. We know if the product of factors is equal to zero, then one of our factors must be equal to zero.
When ๐ก squared is equal to zero, ๐ก is equal to zero. But we already know weโre looking for values of ๐ก greater than zero. So we must have our other factor of 12 minus 19 over six ๐ก is equal to zero. We can solve this for ๐ก. We get ๐ก is equal to 72 divided by 19. And all of our units were given in terms of meters and seconds. So we can give this a unit. Itโs 72 over 19 seconds. And this is our final answer. So weโve shown if a particle is moving along the ๐ฅ-axis which is initially at rest at the origin. And at time ๐ก seconds, the particleโs acceleration is given by ๐ is equal to 24 minus 19๐ก meters per second squared, where ๐ก is greater than or equal to zero. Then it will take 72 over 19 seconds for our particle to return to the origin.