Download - Review the Law of Interaction and balanced forces within bodies

TODAY’S OUTCOMES:
FORCE, MOTION AND ENERGY
- Review the Law of Interaction and
balanced forces within bodies with
constant motion
- Observe and plot an example of
acceleration of an object
- Study the effects of a change in
velocity on an object
14. Hannah is weighing a cantaloupe using a weightless scale. There are many forces
acting, as indicated by the diagram.
A. Some of these forces are related to others by the Law of Interaction. List these relationships
(please give the full name of each force).
Law of Interaction (often called Newton's Third Law): “For any two objects A and
B, the force of A on B is equal and opposite to the force of B on A.”
Weight of the cantaloupe ↓
Force of the scale hook on the cantaloupe ↑
Force pulling on the scale spring ↓
Scale spring pulling upward ↑
Scale + cantaloupe pulling on hand ↓
Hand pulling back on scale + cantaloupe ↑
} equal and opposite pair
} equal and opposite pair
} equal and opposite pair
If the scale has no weight, all these forces are equal in strength, and exist
in pairs in opposing directions.
B. Some of these forces are related to others by the Law
of Inertia. List these relationships.
The Law of Inertia (Newton’s First Law): In the absence of external forces, an object
moves in a straight line with constant speed. In this case, the speed is 0 !
No part of the hand-scale-cantaloupe structure is changing its speed, so
the forces acting on it must all cancel out in pairs, as listed above.
Weight of the cantaloupe ↓
Force of the scale hook on the cantaloupe ↑ equal and opposite pair
Force pulling on the scale spring ↓
equal and opposite pair
Scale spring pulling upward ↑
Scale + cantaloupe pulling on hand ↓
equal and opposite pair
Hand pulling back on scale + cantaloupe ↑
}
}
}
C. Suppose the scale has weight (this force is not indicated on the diagram).
How does this change any of the answers to parts A and B?
If the scale has weight, then one pair of the above forces (shown in red)
will be greater than the others - but this pair is STILL equal and opposite, and
will not affect the measurement on the scale.
In your lab, you pulled a cart at a constant speed up a hill
What forces were acting on the cart?
The scale pulled the cart UP the hill.
Gravity & contact against the wheels pulled
the cart back DOWN the hill.
How did these forces compare?
If the cart moves at a constant speed, these forces MUST be equal
and opposite.
In your lab, you pulled a cart at a constant speed up a hill
What if the cart is just sitting on the table?
What forces were acting on the cart?
Gravity pulls the cart downward.
The table pushes upward on the cart wheels.
How did these forces compare?
They are equal and opposite.
You also held a 200 gram mass with 2 spring
scales.
If held directly above the mass, what was
the (approximate) total force measured on the
scales?
The sum of both scale measurements totalled
about 2 Newtons.
What forces operate in this diagram?
2 Newton weight of the 200 gram mass pulling on
the rubber bands
Forces totalling 2 N. at the bottoms of the
rubber bands
Rubber bands pulling on the bottoms of the
hooks (totalling 2 N.)
Springs in the scales pulling upward (2 N. total)
Also a pair of opposing forces between hands
and the scale (not shown)
What do all these examples have IN COMMON?
CONSTANT SPEED AND DIRECTION
(zero speed is also a constant speed!)
The Law of Inertia (Newton’s First Law): In the absence of external
forces, an object moves in a straight line with constant speed.
What are “external forces” ?
Keep this question in mind during today’s activity!
Law of Interaction (often called Newton's Third Law): “For any two objects A and
B, the force of A on B is equal and opposite to the force of B on A.”
This is always true!
Always think - when you observe a force, where is
the equal and opposite force?
WHAT YOU ARE EXPECTED TO KNOW:
- The meaning of the Law of Interaction
(Newton’s Third Law), and how forces exist in
pairs
- That the Law of Inertia demands all forces must
cancel within an object with a constant speed
and direction
TODAY’S OUTCOMES:
FORCE, MOTION AND ENERGY
- Review the Law of Interaction and
balanced forces within bodies with
constant motion✓
- Observe and plot an example of
acceleration of an object
- Study the effects of a change in
velocity on an object