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week3-2

# week3-2 - Newtons First Law of Motion An object remains at...

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Newton’s First Law of Motion • An object remains at rest, • or in uniform motion in a straight line, unless it is compelled to change by an externally imposed force.

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Newton’s Second Law of Motion • The acceleration of an object is directly proportional to the magnitude of the imposed force • and inversely proportional to the mass of the object. • The acceleration is the same direction as that of the imposed force. F m a units: 1 newton = 1 N = 1 kg m s 2
Two equal forces act on an object in the directions shown. If these are the only forces involved, will the object be accelerated? a) Yes. b) No. c) It is impossible to determine from this figure. The vector sum of the two forces results in a force directed toward the upper right corner. The object will be accelerated toward the upper right corner.

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Two forces act in opposite directions on a box. What is the mass of the box if its acceleration is 4.0 m/s 2 ? a) 5 kg b) 7.5 kg c) 12.5 kg d) 80 kg e) 120 kg The net force is 50 N - 30 N = 20 N, directed to the right. From F=ma, the mass is given by: m = F/a = (20 N) / (4 m/s 2 ) = 5 kg.
A 4-kg block is acted on by three horizontal forces. What is the net horizontal force acting on the block? a) 10 N b) 20 N c) 25 N d) 30 N e) 40 N The net horizontal force is: 5 N + 25 N - 10 N = 20 N directed to the right.

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Quiz: A 4-kg block is acted on by three horizontal forces. What is the horizontal acceleration of the block? a)
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week3-2 - Newtons First Law of Motion An object remains at...

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