Concept of Force and Newton’s Laws of Motion

8.01 W03D1

Next Reading Assignment: W03D2

Young and Freedman (repeat): 4.1-4.6, 5.1-5.3.

Experiment 1: Force and Motion Bring in Solution to Pre-Experiment

Question

Newton’s Second Law: Strategy

Treat each object in the system as a point-like object

Identify all forces that act on that object, draw a free body diagram

Apply Newton’s Second Law to each body

Find relevant constraint equations

Solve system of equations for quantities of interest

Concept Question: Normal Force Consider a person standing in an elevator

that is accelerating upward. The upward normal force N exerted by the elevator floor on the person is

1)  larger than 2)  identical to 3)  smaller than

the downward force of gravity on the person.

Source: Video of 100 ton door,

nearly frictionless motion on air sheet

Concept Question

Worked Example: Pulley and Inclined Plane

A block of mass m1, constrained to move along a plane inclined at angle ϕ to the horizontal, is connected via a massless inextensible rope that passes over a massless pulley to a bucket to which sand is slowly added. The coefficient of static friction is µs. Assume the gravitational constant is g. What is mass of the bucket and sand just before the block slips upward?

Concept Question: Varying Tension in String

A block of mass m1, constrained to move along a plane inclined at angle ϕ to the horizontal, is connected via a massless inextensible rope that passes over a massless pulley to a bucket to which sand is slowly added. The coefficient of static friction is µs. Assume the gravitational constant is g. What happens to the tension in the string just after the block begins to slip upward?

1.  Increases 2.  Decreases 3.  Stays the same 4.  oscillates 5.  I don’t know 6.  None of the above

Lecture Demo: Block with Pulley and Weight on Incline B24 http://scripts.mit.edu/~tsg/www/index.php?

page=demo.php?letnum=B%24&show=0

Concept Question: Car-Earth Interaction

Consider a car at rest. We can conclude that the downward gravitational pull of Earth on the car and the upward contact force of Earth on it are equal and opposite because

1)  the two forces form an interaction pair. 2)  the net force on the car is zero. 3)  neither of the above. 4)  unsure

Table Problem: Painter on Platform part a

Draw free body force diagrams for the plank and man. Is there something wrong with this picture from the 8.012 textbook?

Table Problem: Painter on Platform part b

The man pulls on each rope with a constant force F. The mass of the man is Mm and the mass of the plank is Mp. Find the acceleration of the painter.

Source: why you need to know physics

Worked Example: Two Blocks with Constraint

Two blocks 1 and 2 of mass m1 and m2 respectively are attached by a string wrapped around two pulleys as shown in the figure. Block 1 is accelerating to the right on a fricitonless surface. You may assume that the string is massless and inextensible and that the pulleys are massless. Find the accelerations of the blocks and the tension in the string connecting the blocks.

Table Problem: Blocks and Pulleys on Table

Two blocks rest on a frictionless horizontal surface. They are connected by 3 massless strings and 2 frictionless, massless pulleys as shown above. A force F is applied to block 1. What is the resulting acceleration of block 1?