1. Forces3rd1.ppt
Forces3rd1.ppt

2. Extended free body diagrams
For the following situations draw separate free body diagrams (FBD’s) for each object listed. Label the forces in the form FBonC, NSonC, etc…. Link any 3rd law pairs with a dotted line.
A dog and a kid involved in tug-of-war with a rope. The dog is winning (draw FBD’s for the dog and for the kid)
A woman riding in an elevator that’s accelerating upwards (draw FBD for woman and elevator)
A mass hanging off the end of a table is attached by a string over a pulley to a frictionless cart (draw FBD’s for the cart and the mass.
Forces3rd1.ppt

3. Two blocks being pushed
Box 1 (m1 = 3 kg) and box 2 (m2 = 1 kg) contain poisonous gas. You need to push them together (you have no time to push each separately) off a cliff. You push with your maximum force of 4 N.
If the interior forces (i.e. the force of one box on the other) are too large, the boxes will break, releasing the poisonous gas. Which box should you put in front? Does it matter? For simplicity assume the cliff is frictionless.
a) Find the acceleration of the blocks.
b) Find the force between the two blocks.
c) Put the applied force on the other box. Repeat a) and b).
Forces3rd3.ppt

4. A large block
A large block (m1) is initially at rest on a frictionless surface. A coffee cup (mass m2) slides off a table onto the top of the first with an initial speed v0. There is friction between the blocks with coefficients of friction µs and µk. The mass m1 is greater than m2.
a) Draw a free body diagram for each block and link any 3rd law pairs with a dotted line.
b) Which acceleration should be larger? Explain your reasoning.
c) Find the acceleration of each block in terms of m1, m2, µk, µs, and g.
Forces3rd3.ppt

5. Block over ledge
Block B (mB) sits on rough surface and is connected to block A (mA) by a massless string, as shown. The coefficients of friction between block B and the surface are µs and µk. Find the acceleration of the blocks (assuming they slide).
Draw a free body diagram for block A and block B.
Write out Newton’s 2nd law component equations for each block in terms of the forces you have defined on your free body diagram, mA, mB, aB, and aA. No numbers yet. There should be at least 3 equations.
Write down any constraining equations.
Find the value of a in terms of some combination of µk, µs, mA, mB, and g.
Given that mA = 1 kg. mB = 2 kg, µs = 0.8, and µk = 0.2, find the numerical value of a.
What is the minimum value of mA that will cause the blocks to slip if the blocks start at rest?
Forces3rd3.ppt