LESSON 1.11 - REACTION
Objectives
On completion of this lesson, you should be able to:
State: Newton’s Third Law.
Define: Action, Reaction and Interaction.
Explain: how rockets work by reaction.
Describe the various interactions of forces that occur
when an object is made to move (by an externally applied
force).
Calculate: The vertical and horizontal
components of a force acting on an object.
ACTIVITIES
- Push against a wall:
Describe the action and reaction forces involved.
- Push someone sitting
in an office chair (with castors) and describe
the action and reaction forces involved.
- Suspended spring
balance: Use a piece of string to suspend a
spring balance from a point above it. Use a
second piece of string to suspend a weight from
the spring balance. Describe the tension forces
in the pieces of string.
Newton’s
Third Law: This states that when an object
exerts a force on a second object, the second object
exerts an equal and opposite force on the first object.
Action: A force
being applied.
Reaction: The equal
and opposite force to the force being applied.
Interaction: A
mutual action between objects.
Interaction
Interaction occurs when objects exert equal and opposite
forces on each other. The opposite force is called the
reaction. For example, centrifugal force is the reaction
to the centripetal force that tends to pull an object
that is moving in a straight line towards the center of a
circular path.
Newton’s Third law
Newton’s Third law states that when an object exerts
a force on a second object, the second object exerts an
equal and opposite force on the first object. This means
that when I stand on the ground, my weight exerts a force
of roughly 800 N on the ground. The ground exerts an
equal force upwards.
If I lean against the
wall, my hand pushes against the wall and the wall pushes
against my hand.
Equal forces act on
different objects
Although forces are equal and opposite, they act on
different things. The diagram of a man pushing a sled
shows the various forces at play. There are equal and
opposite forces where the man’s hands push the sled.
His push acts on the sled and this force is transferred
through the sled to overcome friction with the ground and
to overcome inertia if it is accelerating. The reaction
from the sled is transferred to the man’s body to
keep him from falling and to oppose friction between his
feet and the ground
Example 1.11.1 Reaction
A man with a mass of
Solution
The downward
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Review
Questions
- A person holding the
end of a fire hose finds it hard to hold the hose
in position while directing a large stream of
water at a fire. Why does the water leaving the
hose affect the person holding the hose?
- Can rockets travel
faster in air than in outer space because the
rocket can push against the air? Explain.
- A person who was
attempting to step from a small boat onto a pier
missed the pier and landed in the water. Explain
why the boat moved away from the pier as the
person stepped off the boat.
- A person steps onto a
pier from a boat that is tied to the pier.
Describe the forces between the person’s
foot and the boat and the forces in the rope
between the boat and the pier.
- In a tug-of-war, two
teams each exert a force of 800 N on their end of
the rope. What is the tension in the rope?
- One of the teams
practices by pulling a rope that is tied to a
sturdy tree. If the team exerts a force of 800 N
on the rope, what is the tension in the rope?
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ACTIVITIES
- Push against a wall:
Describe the action and reaction forces involved.
- Push someone sitting
in an office chair (with castors) and describe
the action and reaction forces involved.
- Suspended spring
balance: Use a piece of string to suspend a
spring balance from a point above it. Use a
second piece of string to suspend a weight from
the spring balance. Describe the tension forces
in the pieces of string.
.
.
HANDS-ON
HOMEWORK
Select one or more of the recommended activities for Lesson 1.12, collect the items needed and test
the procedure before demonstrating the activity during
the next theory lesson.
Lesson
1.11 Reaction
- The reaction force at
the nozzle of a fire hose is very large because
large quantities of water are being accelerated
from it.
- No. Rockets are
propelled by reaction forces. if anything, the
resistance of the air slows the rocket down.
- Equal and opposite
forces usually act on different things. The
person exerted a force on the boat with his or
her foot. The reaction from the boat would have
propelled the person forwards if the foot had not
acted on a boat that could move.
- The boat doesn’t
move because the pier doesn’t move. The
tension in the rope from the pier acts on the
boat. The opposite tension in the rope acts on
the pier. (Other forces keep the pier from moving
relative the ground.) The tension in the rope
balances the force from the person’s foot.
The reaction from the boat propels the person
forwards.
- 800 N in both
directions.
- 800 N in both
directions.
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