1. Accelerating Frames

2. Goal of the class
To understand how to calculate forces and accelerations in accelerating frames
Question of the day: why happens if I weigh myself in a lift (elevator)?

3. Weighing Scales What do bathroom scales measure?
Draw a box on some scales and then add a force
Mass and weight are incorrect. The normal force is right Fn = mg + Fapplied
Scales show Fn/g for kgs

4. Apparent Weight in Lift
Fnet = ma
Fn-mg = ma
Fn = ma + mg ; ma +mg = apparent weight
Scale reading = Fn/g = m(1+a/g)
For 60kg man his apparent weight is 66 if a = 1m/s2 m

5. Accelerating Trains! Case 1 when train not moving Mg = T
Case 2 when a is present ->
All objects move to the right with a ->
By Newton’s 2nd
Fnet = ma ->
But mg and T act vertically so there must be a horizontal force when train is in motion
Θ is angle from vertical
T cos Θ = mg 1
T sin Θ = ma 2
Tan Θ = a/g
Case 3 – Mass on Spring
When train stationary spring is in equilibrium position not strecthed or compressed
Case 4 – a ->
When accelerating
Fnet = ma
Kx = ma
X = ma/k
Case 5 – frictionless
Coffee cup on table doesn’t move a=0
Case 6 - friction
Max value of fs = mu_s Fn = mu_s mg
Ma < mu_s mg

6. Apparent Weight in Lift
Imagine a lift moving downward with a constant velocity of 2m/s. A mass of 5kg hangs from the ceiling attached to a spring of force constant k = 500 N/m.
What is the extension of the spring?
Suddenly the lift cable breaks, what is the equilibrium extension now?
a) Kx = mg
X = 0.1m
b) X = 0
As person on scales mg – Fn = ma, but a =g so Fn = 0

7. Homework Set (x) Read Chapter 5 on accelerating frames
Answer questions from page 144 on
Answer 5 questions from general problems section with a difficulty rating of 2 or 3, but don’t hand these in.