PHY131H1S – Class 12 (half-way
through the course…)
Today:
•  Action / Reaction
Pairs
•  Newton’s Third Law
•  Ropes and Pulleys
Team Homework due
in Practicals this
week! Please hand
to your T.A. at the
start of practicals.
At the end of Class 11 I asked:
•  A cyclist is pushing on his pedals, and
therefore accelerating to the left.
•  What is the direction of the force of
static friction of the ground on the back
wheel?
A.  Left
B.  Right
C.  Up
D.  Down
E.  zero

a
€
1
4 discussion questions in a set. [2 / 4]
•  A cyclist is pushing on his pedals, and
therefore accelerating to the left.
•  What is the direction of the force of
rolling friction of the ground on the
back wheel?
A.  Left
B.  Right
C.  Up
D.  Down
E.  zero

a
€
4 discussion questions in a set. [3 / 4]
•  A cyclist is pushing on his pedals, and
therefore accelerating to the left.
•  What is the direction of the force of
static friction of the ground on the front
wheel?
A.  Left
B.  Right
C.  Up
D.  Down
E.  zero

a
€
2
4 discussion questions in a set. [4 / 4]
•  A cyclist is pushing on his pedals, and
therefore accelerating to the left.
•  What is the direction of the force of
rolling friction of the ground on the front
wheel?
A.  Left
B.  Right
C.  Up
D.  Down
E.  zero

a
€
3
•  The entire Earth accelerates toward the Moon, due to this
pulling force.
•  To find the total acceleration, you use the force as
calculated for the centre-to-centre distance.
•  Since FG = GMm/r2, the force on the ocean nearer to the
moon will be greater, so it will accelerate more than the rest
of the Earth, bulging out.
4
•  Since FG = GMm/r2, the force on the ocean nearer to the
moon will be greater, so it will accelerate more than the rest
of the Earth, bulging out.
•  Similarly, the force on the ocean further from the moon will
be less, so it will accelerate less than the rest of the Earth,
remaining behind, forming a bulge.
•  In general, tidal effects tend to stretch objects both toward
and away from the object causing the tides.
Identifying Action / Reaction Pairs
•  Consider an accelerating car.
•  Action: tire pushes on road.
•  Reaction: road pushes on tire
5
Identifying Action / Reaction Pairs
•  Consider a rocket accelerating upward.
•  Action: rocket pushes on gas.
•  Reaction: gas pushes on rocket
Identifying Action / Reaction Pairs
•  Action force: man
pulls on rope to the left.
•  Reaction force?
A. 
B. 
C. 
D. 
E. 
Feet push on ground to the right.
Ground pushes on feet to the left.
Rope pulls on man to the right.
Gravity of Earth pulls man down.
Gravity of man pulls Earth up.
6
Identifying Action / Reaction Pairs
•  Consider a stationary man pulling a rope.
•  Action: man pulls on rope
•  Reaction: rope pulls on man
Identifying Action / Reaction Pairs
•  Action force: gravity of
Earth pulls ball down.
•  Reaction force?
A. 
B. 
C. 
D. 
E. 
Feet push ground down.
Ground pushes feet up.
Gravity of Earth pulls man down.
Gravity of ball pulls Earth up.
Manager yells at player to the left.
7
Identifying Action / Reaction Pairs
•  Consider a basketball in freefall.
•  Action: Earth pulls on ball
•  Reaction: ball pulls on Earth
Test Question from PHY131 Spring 2009
scale
•  Action force: Bob stands on a
bathroom scale. There is an action
force on Bob, which is gravity.
Gravity pulls Bob down.
•  Reaction force?
A.  Feet push down on scale (normal).
B.  Scale pushes up on Bob (normal).
C.  Earth pulls down on scale.
D.  Scale pushes down on Earth.
E.  Gravity of Bob pulls up on Earth.
14% of the class got this correct on the test in 2009…
8
Identifying Action / Reaction Pairs
•  Action force: Bob stands on a
bathroom scale in an elevator, which is
accelerating upward. There is an action
force on Bob, which is gravity. Gravity
pulls Bob down.
•  Reaction force? (equal and opposite)
a
scale
A. 
B. 
C. 
D. 
E. 
Feet push down on scale (normal).
Scale pushes up on Bob (normal).
Earth pulls down on scale.
Scale pushes down on Earth.
Gravity of Bob pulls up on Earth.
The Massless String Approximation
Often in physics problems the mass of the string or rope is
much less than the masses of the objects that it connects. In
such cases, we can adopt the following massless string
approximation:
9
Pulleys
All three 50 kg blocks are at rest. Is the
tension in rope 2 greater than, less than,
or equal to the tension in rope 1?
A.  Equal to
B.  Greater than
C.  Less than
10
In the figure to the
right, is the
tension in the
string greater
than, less than, or
equal to the
weight of
block B?
A.  Equal to
B.  Greater than
C.  Less than
11