AP physics for the week of February 8, 2016
This document is on my web page
This week you should study and understand conservation of energy using springs. Here
is a basic introduction:
When you compress or stretch a spring you do work on it. Work is a very specific term
we use in physics:
 As with energy, work is measured in Joules.
 Work is the number of Joules you put into something ( positive work) or the
number of Joules you take out of something (negative work). As with energy,
work is NOT a vector. It has no direction. (This makes it easier to work with
whether you think so yet or not…)
 W = Fd (Work is Force x distance). Please note this equation is a vector (force)
times a scalar (distance) yet work is not a vector.
Example: there is a book on the ground. I do work on it by picking it up. If I give the
book 500 J of gravitational potential energy, I have done 500 J of positive work on it.
When I return the book to the floor I do 500J of negative work on the book.
Example: when I do 500 J of work by either compressing it or stretching it I have done
500 J of positive work on it. If I use the spring to launch something (e.g., a dart in a dart
gun) the spring will transfer 500J of kinetic energy to the dart. Ta dum! If I shoot the
dart straight upward it will rise until it has 500J of gravitational potential energy.
Example: When a major league pitcher wants to throw a fastball he leans way back so he
can put a force on the ball for a long distance. This is why the pitcher leans way forward
before he releases the ball. It is also why major league pitchers are often very tall with
long arms; they can put a force on the ball for a long distance
The section of your book that covers spring energy (also known as elastic potential
energy) starts on page 296. For some reason that I do not understand there are very few
spring energy problems in your textbook so I am going to find some and send them to
you.
The basic equations for elastic or spring potential energy is on your AP physics 1
equation sheet: PEspring = ½ kx2
The other equation you need is Hooke’s Law: F = kx
Please read section 8.3 in the textbook so you can understand this equation. It’s not that
hard ;-)
K is the spring constant. It is measured in units of N/m (Newtons per meter). This
should make sense to you as it tells you how many Newtons of force it takes to compress
your spring one meter. A stiff spring has a high spring constant. A flexible spring has a
small spring constant. The spring in a dart gun has a very small spring constant. The
spring in the suspension in your car is very stiff and has a very high spring constant.
X is simply the distance that you have either stretched or compressed your spring
(measured in meters).
Here are some introductory spring problems. I will also send some AP problems that
include springs.
1. Take g = 10 m/s2 and use F = kx (don’t forget Weight=mass x g) 1. A mass of
630g is hung on a spring. Using Force = mass x gravity, what is the force of the
mass, acting on the spring? Answer: 6.3N
2. If a 4N weight is hung on a spring, and it extends by 0.2m, what is the spring
constant (k)? What do you think the units are? (20 N/m)
3. What Force would be needed to extend a spring with a spring constant k=5, 0.3
m? (1.5N)
4. A spring extends by 10 cm when a mass of 100 g is attached to it. What is the
spring constant? (10N/m)
5. What will be the extension of this spring if the load is a) 4N and b) 75 g? (40cm
and 7.5cm).