Download Force is part of an interaction

Force is part of an interaction
• Force is push or pull
• It is an interaction
between one thing and
another.
• Example;
– An apple exacts a force
on someone’s palm
(force of gravity).
– The person’s palm
exacts an upward force
of reaction.
• There are therefore two
forces acting in an
interaction.
Newton’s 3rd law of motion
• Whenever one object exerts a force on a second
object, the second object exerts an equal and opposite
force on the first object.
– One force is called Action Force
– The other is called Reaction Force
• Neither force exist without the other.
• Newton’s 3rd law can be summarized;
– To every action there is an equal opposite reaction.
Action = reaction
• Which of these two people is pulling with a bigger
force?_______
• Can the big guy pull more than the little guy could?_______
Pairs of Action and Reaction
Event
Pairs of
objects in an
interaction
Action
Reaction
Moving car
Car tires and
the Road
Tires
push road
backward
s
Road push
tires
forward
Firing a
rocket
Rocket and
Gas
Falling
boulder
Earth and the
Boulder
Firing a
cannon
Cannon and
cannon ball
Hitting a
nail with a
hammer
Nail and
hammer
Hammer
on nail
Nail on
hammer
Action –reaction pairs
do cause motion
•
If action equal reaction, how can motion be
caused by such pairs of forces?
Action and reaction Vs acceleration?
• How do equal forces (Action and reaction) produce
unequal acceleration?
• Because force is applied on objects with unequal masses.
• The cannon has a large
mass, than the cannon ball.
• The action force F is equal
to reaction force F.
Therefore; acceleration
of cannon is smaller than
that of the cannon ball.
Action and reaction Vs acceleration?
 Equal force, Unequal mass—Unequal
acceleration
Elevator problem
• Dave stands on a
bathroom scale in an
elevator . What is
Dave's mass if the
scale reads 840 N
while the elevator
accelerates upward at
2.2 m/s2
Use the formula
•Net force = apparent weight – actual weight
answer
Normal force = apparent weight
• Action = Reaction
• 840N up = 840N down
•
•
•
•
•
•
•
apparent weight= Net force +
actual weight
840 = ma + mg
Ma +mg = 840N
m (a +g) = 840N
m (2.2 + 9.8) = 840N
m (12) = 840
M = 840/12 = 70 kg
Normal
force
(Action)
Apparent
Weight
(reaction)
Net Force
(ma)
Actual
weight
(mg)
class work and Homework
• Class work: Textbook page 84, # 19- 29