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1. At a given instant in time, a 14-kg rock that has been dropped from a high cliff experiences a force of air resistance of 40 N. What are the magnitude and direction of the acceleration of the rock? (Do not forget the gravitational force!)

magnitude ________ m/s?

direction _____downward/upward_____?

2. A 7.0-kg block being pulled across a table by a horizontal force of 55 N also experiences a frictional force of 10 N. What is the acceleration of the block?

_______ m/s2?

3. An upward force of 45 N is applied via a string to lift a ball with a mass of 2.5 kg.

(a) What is the net force acting upon the ball?

magnitude __________?

direction __downward/upward________?

(b) What is the acceleration of the ball?

magnitude ________?

direction __upward/downward?

Sep 10th, 2013

Acceleration due to gravity = 9.8m/s2 (10m/s2 could be used to simplify numbers)

1) Force = mass*acceleration

For rock

F = 14kg * 9.8m/s2 = 137.2 N

Deduct air resistance. 

137.2 - 40 = 97.2N

Acceleration of rock. 

acceleration = Force/mass

A = 97.2/14 = 6.94 m/s2 downward

2) again F=m*a

total force = 55-10 =45 N (horizontal force- resistance)

a = 45/7 = 6.43m/s2

a) F=m*a = 2.5*9.8 = 24.5 N

Resultant force = 45-24.5 = 20.5 N upward

b) a = F/m = 20.5/2.5 = 8.2 m/s2 upwards

Jul 4th, 2014

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