Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
A physics student fits a damper to a trolley of mass that is joined to a fixed wall by a spring. The spring constant is , and the damper gives a force of times the velocity, against the motion. The student pulls the trolley from rest and lets go. The motion obeys , where is the displacement in metres and is the time in seconds. She expects the trolley to swing to and fro with falling amplitude. She wants three numbers. The first is the angular frequency of the damped swing. The second is the time between two successive peaks. The third is the height of the first peak after the start as a fraction of the starting height . Give each answer to four decimal places.
For solve . Real roots give , equal roots give , and complex roots give . With a right side, , and the constants are fitted last.
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. , , , and the starting displacement .
Plan. Solve the auxiliary equation, read off and , then find the period and the factor by which the peaks shrink.
Step 1. Roots. gives . So and .
Step 2. The period. .
Step 3. The shrink factor. The motion is . Between peaks one period passes, so the height is multiplied by .
Answer: , and .
Check. The damping is strong: after one swing the height is only of the start.
Log in to join the discussion.
Loading…