Transcript Class #14 - Department of Physics | Oregon State University

Starting from rest on a level, horizontal, frictionless table surface, a
3.00-kg object is subjected to a single horizontal force F, which has a
magnitude of 12.0 N and is directed at an angle of 30° above the
positive x-axis (i.e. 30° north of east).
What is the object’s y-displacement at the moment when F has been
acting on it steadily for 5 seconds?
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16 m
2.
20 m
3.
25 m
4.
25 km
5.
None of the above.
Oregon State University PH 211, Class #14
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Three forces act on an object as shown.
In which direction does the object accelerate?
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Three forces act on an object as shown.
In which direction is the object moving?
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v
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v
v
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Not
enough
info.
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Force Myths
• Force of hand (or bat or racket or…) “stays with” a thrown (or
hit or…) object after contact has ceased (i.e. while it’s in flight).
• Force “transmits” through an intermediate object.
• An object’s velocity is always in the direction of the net force.
• There is a force of motion.
• Force is required to keep an object moving.
• The force required to push an object along at a constant speed is
greater than the resisting (friction) forces.
• Inertia is a force.
• Acceleration is a force.
• A force is a property of an object.
• Air doesn’t exert a force.
• Single forces can exist in isolation.
• Greater speeds require greater net forces for equal accelerations.
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A horse is (400 kg) is pulling a cart (300 kg) at a constant speed
(3.50 m/s) along a straight level road.
What is the net force on the cart?
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1.
0.00 N
2.
350 N
3.
1050 N
4.
1400 N
5.
None of the above.
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Newton’s Laws are often misquoted, misunderstood and misapplied.
Consider the cautionary tale of Sir Edward Horse…
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