SPH4U: Weight and Acceleration
In the world of physics, weight has a simple definition: Weight = Fg =
mg where the gravitational field strength g = 9.8 N/kg. This sometimes
differs from our everyday experience of weight which we associate with
the reading of a bathroom scale. To distinguish this, we will call the scale
reading the apparent weight. We have already learned that this reading
is actually a measurement of the force required to support the object
(often the normal force).
Date: __________________________
Recorder: ______________________
Manager: ______________________
Speaker: ______________________
Com / Know / Inq / MC: 0 1 2 3 4 5
You need: one spring scale, one mass (~200 g)
For each situation below, start by using a simple spring scale and mass to model each situation. Attach the mass
to the scale with some tape so it doesn’t fall off. Note what happens to the apparent weight readings of your
scale.
Scene 1: A 1 kg rock on a bathroom scale in an elevator moving upwards at a constant velocity of 5 m/s.
a) Spring scale prediction: What happens to the apparent weight?
b) What is the acceleration of the rock?
c) What is the weight of the rock (in Newtons)?
d) Draw a force diagram for the rock.
Scale
e) Use Newton’s 2 Law to determine the apparent weight of the
rock (in Newtons).
nd
Scene 2: A 1 kg rock on a bathroom scale in an elevator accelerating upwards at 2 m/s 2.
a) Spring scale prediction: What happens to the apparent weight?
b) What is the acceleration of the rock?
c) What is the weight of the rock?
d) Draw a force diagram for the rock.
e) Use Newton’s 2nd Law to determine the apparent weight of the rock.
Scale
Scene 3: A 1 kg rock on a bathroom scale in an elevator accelerating downwards at 5 m/s2.
a) Spring scale prediction: What happens to the apparent weight?
b) What is the acceleration of the rock?
c) What is the weight of the rock?
d) Draw a free-body diagram for the rock.
Scale
e) Use Newton’s 2nd Law to determine the apparent weight of the rock.
Imagine a journey in an elevator from the ground floor of a building to the top floor and back down. Complete
the chart.
Motion
Rest at bottom
Direction of Acceleration
Effect on Apparent Weight
Begins moving upwards
Constant velocity while moving
upwards
Slows while moving upward at top
floor
Begins moving downwards
Constant velocity while moving
downwards
Slows while moving downwards at
ground floor
State all the factors that may affect the apparent weight of an object. Think of previous activities as well!
Bonus 1: A 1 kg rock on a bathroom scale in an elevator with the cable cut. (Careful with the spring scale! Put
something soft underneath!)
a) Spring scale prediction: What happens to the apparent weight?
b) What is the acceleration of the rock?
c) What is the weight of the rock?
d) Draw a free-body diagram for the rock.
e) Use Newton’s 2nd Law to determine the apparent weight of the rock.
Scale
Bonus 2: A 1 kg rock on a bathroom scale in an accelerating elevator. The scale reads its usual value.
a) What is the normal force acting on the rock?
b) Draw a free-body diagram for the rock.
c) Write a scalar equation for the net force acting on the rock.
d) What is the acceleration of the rock?
e) Explain (don’t do!) how you could model this with the spring scale.
Scale