## Newton’s Laws and Circular Motion

From Young and Freedman’s University Physics with Modern Physics Textbook, 14th edition

A stone with mass 0.8 kg is attached to the end of 0.9 m string. The string will break if its tension exceeds 60 N. The stone is whirled in a horizontal circle on a frictionless tabletop; the other end of the string remains fixed. (a) draw a free body diagram of the stone. (b) Find the maximum speed the stone can attain without the string breaking.

Analyze the problem:

A stone is tied into an end of a string and then swung horizontally to move in circle. As the stone move in circular motion then a radial acceleration is involved. The string can uphold a tension force up to 60 N so there will be a maximum radial acceleration that when exceeded the string will break.

What are the given information?

1. The stone mass, m=0.8 kg
2. The string length, L= 0.9 m
3. Maximum tension, T=60 N

What are the needed information?

1. Body-diagram for the stone.
2. maximum speed of stone so the string is not breaking

SOLUTION:

a)b)

## Newton’s Laws Application

Source: Young and Freedman’s University Physics with Modern Physics textbook, 14th edition

Blocks A, B, and C are connected using rope of negligible mass (see figure below). Both A and B weight 23.8 N each, and the coefficient of kinetic friction between each block and the surface is 0.32. Block C descends with constant velocity.

a) Draw free body diagram for blocks A and B.

b) Find tension in rope connecting blocks A and B.

c) What is the weight of block C?

d) If the rope connecting A and B were cut, what would be the acceleration of block C?

Analyze the problem:

The problem has three blocks attached by an inelastic string (so they should have the same acceleration) and pulleys mass ignored. The surface where Blocks A and B are set on have friction and the kinetic friction coefficient was given (kinetic friction coefficient is involved when bodies are in motion which is different than static friction coefficient which used when bodies are not in motion).

What are the given information?

1. Block A weight, FwA=23.8 N
2. Block B weight, FwB=23.8 N
3. Kinetic friction coefficient for all surfaces = 0.32
4. Block C velocity is constant, so acceleration, a =0 m/s2

What are the needed information?

1. Body-diagram draw for blocks A and B.
2. Tension in rope attaching block A and block B.
3. The weight of Block C
4. When rope between block A and block B were cut , then find the acceleration of block C.

SOLUTION:

Note: The acceleration shown in figure below is for part (d).

## Explosion and Collision

A space probe explodes in flight into three equal portions. One portion continues along the original line of flight. The other two go off in directions each inclined at 60º to the original path. The energy released in the explosion is twice as great as the kinetic energy possessed by the probe at the time of the explosion. Determine the kinetic energy of each fragment immediately after the explosion.

Analyze the problem:

With the word explosion we should know that it could involve conservation of momentum and kinetic energy.

What are the given information?

1. Three fragments of exploded probe that have equal masses
2. Direction of probe is φ=0º with the horizontal.
3. Directions of motion of each fragment after explosion are: fragment 1 (θ1=60º with the horizontal), fragment 2 (θ2=0º with the horizontal), fragment 3 (θ3=-60º with the horizontal).
4. Total energy released during explosion is twice the total kinetic energy of the probe.

What are the needed information?

1. The final kinetic energy for each fragment of the probe immediately after explosion.

SOLUTION:

## Two Blocks on Inclined plane

“Physics Principles with Applications”, Douglas C. Giancoli, 7th Edition, Global Edition

44. Two blocks are connected by a light string passing over a pulley of radius 0.15 m and moment of inertia I. The blocks move (toward the right) with an acceleration of 1.00 m/s2 along their frictionless inclines (See figure below). (a) Draw free-body diagrams for each of the two blocks and the pulley. (b) Determine FTA and FTB, the tensions in the two parts of the string. (c) Find the net torque acting on the pulley, and determine its moment of inertia, I.

Analyze the problem:

The problem has two blocks on inclined surfaces and they are attached by an inelastic string (so they should have the same acceleration) the pulley’s mass is not ignored (we know that because the problem mentioned that there is a moment of inertia).

What are the given information?

1. Block A mass, mA=8 kg
2. Block B mass, mB=10 kg
3. Acceleration, a = 1 m/s2
4. Inclination Angle 1, θ1=32º
5. Inclination Angle 2, θ2=61º
6. Radius of pulley, R= 0.15 m

What are the needed information?

1. Body-diagram draw for the two blocks and the pulley.
2. FTA and FTB tension forces
3. The net torque on the pulley
4. The moment of inertia of the pulley

SOLUTION:

## Newton’s laws and weightlessness

Source: “Physics for Scientist and Engineers” Serway and Jewett, 6th edition

#### 6.25 A person stands on a scale in an elevator. As the elevator starts, the scale has a constant reading of 591 N. As the elevator later stops, the scale reading is 391 N. Assume the magnitude of the acceleration is the same during starting and stopping, and determine (a) the weight of the person, (b) the person’s mass, and (c) the acceleration of the elevator.

Analyze the problem:

The problems mentions two stages of motion: (1) starting of motion and (2) stopping of motion for an elevator. The given forces are the normal forces or the readings of the scale.

What are given information:

1. Normal force (N1) when elevator is starting = 591 N
2. Normal force (N2) when elevator is stopping = 391 N

What the problem need:

1. Weight of person
2. Person’s mass
3. Acceleration of the elevator

SOLUTION:

Now, we have to recall Newton’s laws to be able to analyze the motion of man in the elevator.

By the starting of motion an acceleration of the elevator will be directed toward a specific direction and when it stops it will change to the other opposite direction.

## Newton’s Laws and Circular Motion

From Serway and Jewett Physics for Scientists and Engineers with modern physics, 9th edition

6.11 A 4.00-kg object is attached to a vertical rod by two strings, as in Figure bellow. The object rotates in a horizontal circle at constant speed 6.00 m/s. Find the tension in (a) the upper string and (b) the lower string.

Analyze the problem:

The problems states that it wants tension force in string for the above setting. Newton’s laws should be used beside the dynamic of circular motion concepts.

What are given information:

1. Strings lengths (L) = 2 m
2. Height (h) = 3 m
3. Mass of object (m) = 4 kg
4. Speed of object (v) = 6 m/s

What the problem need:

1. Tension of upper string
2. Tension of lower string

SOLUTION:

From above figure we can see that we have isosceles triangle hence we can have right triangle by drawing a perpendicular line on the base of triangle to get sides lengths 2 m, and 3/2=1.5 m, so:

Then:

The tension in the upper string and  the lower string

## Simple Dynamics of Circular Motion Problem

From Serway and Jewett Physics for Scientists and Engineers with modern physics, 9th edition

Let’s analyze the wording of the problem:

1. A conical pendulum is a swinging pendulum that can rotate around an axis. The bob (the mass) will have circular path at all angles θ, but here we need to investigate when the angle is 5°.
2. When a body moves is circular motion it must have at least a radial acceleration (centripetal acceleration).
3. The mass is connected by a wire, the wire will have a tension (because it is used to suspend the bob). The tension should have x- and y-components.

What the information give in problem:

1. The bob’s mass (m) = 80 kg
2. Length of wire (L) = 10 m
3. Angle of pendulum (θ) = 5°

What the problem need:

1. The x- and y-components of tension force.