Forces
Newton’s First and Second Laws
Lecture 11
Classical (Newtonian) Mechanics
• Describes the relationship between the motion of
objects in our everyday world and the forces acting on
them
• Conditions when Classical Mechanics does not apply
• very small objects (< atomic sizes)
• objects moving with very large speed (close to
the speed of light)
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Forces
• Usually think of a force as a push or pull
• Force is a vector quantity
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Forces
• May be contact (pushing and pulling) or long range (field)
force
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Units for force
• SI unit of force is a Newton (N)
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More about forces
• What if there are more than one forces acting on the
object?
• Different forces can be added vectorially
• The net force is the vector sum of all the forces acting on
the object:
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Example: Find the net force? Each grid has 2 N side.
A
B
C
F
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Newton’s First Law
An object moves with a velocity that is constant in magnitude
and direction unless a non-zero net force acts on it.
If an object is at rest then it will remain at rest or if it is moving
along a straight line with uniform speed then it will continue to
keep moving unless an external force is applied on it to change
its existing state
When there are no external forces acting on an object, the
acceleration of the object is zero.
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Newton’s First Law
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Newton’s First Law: An example
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Inertia
Because of inertia, objects resist changes in their motion
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Inertia and Mass
•
•
•
Inertia is the tendency of an
object to continue in its original
motion
Mass is a measure of the
inertia, i.e resistance of an
object to changes in its motion
due to a force
Recall: mass is a scalar
quantity
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Newton’s Second Law
• The acceleration of an object is directly proportional to
the net force acting on it and inversely proportional to its
mass.
• Since both the force and acceleration are vector
quantities, the acceleration can also be caused by
change of the direction of velocity
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Newton’s Second Law
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Newton’s Second Law
• Note:
represents the vector sum of all external
forces acting on the object.
• Since Newton’s second law is a vector equation, we can
always write it in terms of components
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Units for force
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An inertial reference frame
Frame A
𝑣1 = 0
Frame B
𝑣2 =
≠ 𝑐𝑜𝑛𝑠𝑡
𝑣3 = 𝑐𝑜𝑛𝑠𝑡
Concept test
A car rounds a curve while maintaining a constant speed.
Is there a net force on the car as it rounds the curve?
1. No—its speed is constant.
2. Yes.
3. It depends on the sharpness of the curve
and the speed of the car.
4. It depends on the driving experience of the
driver.
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Summary: Newton’s First Law
An object at rest will stay at rest, and an object in motion will stay in
motion at constant velocity, unless acted upon by an unbalanced force.
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Summary: Inertia
•
•
Inertia is the tendency of an object to resist changes in its
velocity: whether in motion or motionless.
Inertia is proportional to the mass of the object.
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Summary: Newton’s Second Law
• Force equals mass times acceleration
Acceleration
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Summary: Newton’s Second Law
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