Monday 27 Sep:

 Target: Understanding Buoyancy & Two Body Tension

Journal:

1.   A  block of metal weighs 45 N in air and 25 N in the water at the bottom of the pool.  What is the buoyant force of the water?

a)  20 N

b) 25 N

c)  45 N

d) 70 N

2.   A  block of metal weighs 45 N in air and is floating in the pool.  What is the buoyant force of the water?

a)  20 N

b) 25 N

c)  45 N

d) 90 N

3. A barge made out of metal with a mass of 1kg is floating on the water. It sinks to the bottom. When is the force of the water pushing up on the metal barge (force of buoyancy) greatest?

a. when it is floating

b. when it has sunk?

4. Two equal masses. One is Pb and one is Al. When placed in water, which one experiences the greater buoyant force?

a. Pb

b. Al

Quiz Free Response

1.    Density (heart)    

D = mass/Volume

 Go over Response Q

Two Body Tension

Steps to Solve:

1. Add masses together (total mass of "system")

2. Use Newton's 2nd Law:  F_net = (m_total)(a)    (to calculate the  acceleration of "system")

3. Draw "Free Body Diagram" of at least one of the blocks

4. Find "Net force" (F_net) on a block, calculate the remaining force.

Example:

Using the example above (3kg on table and 1kg hanging) , find the acceleration and the tension in the string.

Answer:

1.     total mass of "system" is 3-kg  +  1-kg   =   4-kg

2. F_net = (m_total)(a)     -->

(10N)  = (4-kg)(a)  -->   a = 2.5m/s²

3.                                T =  ?         

                     1-kg

                        W =  10N

 4. F_net  = (1-kg)(2.5m/s²)  =   2.5N

Since the block is accelerating downward, then tension (T) must be less than weight. Therefore, tension is equal to 7.5N

        W - T =  2.5N

        T  = 10 - 2.5 = 7.5N