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Practice_Problems_Solutions

# Practice_Problems_Solutions - PracticeExam2 1 /.Theforce...

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Practice Exam 2 1. The mass of the Moon is about 1/80 th of the mass of Earth. The force exerted by Earth on the Moon is about 80 times that exerted by the Moon on Earth. a. True b False Solution : Since Newton’s 3 rd Law states the “every action has an equal and opposite reaction”, the forces experienced by the Earth and Moon will be equal and opposite. (b. False) 2. The force of kinetic friction between two surfaces is dependent on the relative speed of the two surfaces. a. True b. False Solution : Kinetic friction is calculated by the following: ࠵͝ " = ࠵͝ " ࠵͝ & None of the components of the formula are dependent on speed. False

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4. A force of 120N is applied to an object whose mass is 30 kg. The object’s acceleration is a. 4.0 ࠵͝ ࠵͝ - . b. 3600 ࠵͝ ࠵͝ - . c. 0.25 ࠵͝ ࠵͝ - . d. 2.0 ࠵͝ ࠵͝ - . e. 150 ࠵͝ ࠵͝ - . Solution : 4 ࠵͝ 5 = ࠵͝ = ࠵͝࠵͝ 120࠵͝ = (30࠵͝࠵͝)࠵͝ ࠵͝. ࠵͝ ࠵͝ ࠵͝ ࠵͝ . = ࠵͝
3. You ride on an elevator that is moving downward with constant speed while standing on a bathroom scale. The reading on the scale is the value of the speed. Solution : Constant speed no acceleration ⟹ ∑ ࠵͝ = 0 4 ࠵͝ D = ࠵͝ & − ࠵͝࠵͝ = 0 ࠵͝ & = ࠵͝࠵͝ The reading on the scale is ࠵͝ & which equals mg or your true weight.

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5. A catcher stops a ball traveling at 40 m/s in a distance of 20cm and feels a force of 600N against his glove. What is the mass of the ball? Solution : ࠵͝ = ∆࠵͝ −࠵͝࠵͝ = 1 2 ࠵͝࠵͝ K - 1 2 ࠵͝࠵͝ L - −࠵͝࠵͝ = − 1 2 ࠵͝࠵͝ L - 2࠵͝࠵͝ ࠵͝ L - = ࠵͝ = 2(600࠵͝)(0.20࠵͝) (40 ࠵͝ ࠵͝ ⁄ ) - = ࠵͝. ࠵͝࠵͝࠵͝࠵͝
5 3 2 5 6. Three boxes rest side by side on a smooth horizontal floor. Their masses are 5.0kg, 3.0kg, and 2.0kg, with the 3.0kg mass in the center. A force of 50N pushes on the 5.0kg mass, which pushes against the other two masses. What is the contact force between the 5.0kg mass and the 3.0kg mass? a. 50N b. 25N c. 10N d. 40N e. 0N Solution : First look at the system as a whole. 4 ࠵͝ 5 = ࠵͝ + (࠵͝ ST − ࠵͝ TS ) + (࠵͝ T- − ࠵͝ -T ) = (࠵͝ S + ࠵͝ T + ࠵͝ - )࠵͝ ࠵͝ = (࠵͝ U + ࠵͝ - + ࠵͝ T )࠵͝ ࠵͝ (࠵͝ S + ࠵͝ T + ࠵͝ - ) = ࠵͝ = 50࠵͝ 10࠵͝࠵͝ = 5 ࠵͝ ࠵͝ - . Now look just at block 5.

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