# Review Chap. 5 Applying Newton’s laws

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Review Chap. 5 Applying Newton’s laws
PHYS 218 sec Review Chap. 5 Applying Newton’s laws

The purpose of this chapter is to make you be familiar with application of Newton’s laws of motion.
Therefore, this chapter deals with various examples and problems. There are lots of different examples in end-of-chapter problems and you should solve as many problems as possible. Several important examples, such as the Atwood’s machine, are not covered by the text but are covered by end-of-chapter problems. Pay special attention to these problems. First draw all forces acting on the system. Then draw free body diagrams for each object. Decompose forces into x and y directions, if needed. The sum of these forces should give ma according to Newton’s second law.

Particles in Equilibrium
Ex 5.1 We are neglecting the mass of the rope. Since this system is in equilibrium.

Gymnast + Rope as a composite body
Ex 5.2 Same as Ex 5.1, but with a massive rope Gymnast + Rope as a composite body Gymnast Rope Action-reaction pair

Ex 5.3 Two-dimensional equilibrium Engine Ring O O

Ex 5.4 An inclined plane

Coordinate systems may differ for m1 and for m2
Ex 5.5 Tension over a frictionless pulley Coordinate systems may differ for m1 and for m2

Moving downward with decreasing speed
Dynamics of Particles Tension in an elevator cable: Obtain the tension when the elevator slows to a stop with constant acceleration in a distance of d. Its initial velocity is v0. Ex 5.8 Moving downward with decreasing speed

Free body diagram for woman
Apparent weight in an accelerating elevator: A woman is standing on a scale while riding the elevator in Ex 5.8. What’s the reading on the scale? Ex 5.9 Free body diagram for woman Apparent weight Apparent weightless

Ex 5.10 Acceleration down a hill: What’s the acceleration?

Massless, inelastic string
Two bodies with the same magnitude of acceleration: Find the acceleration of each body and the tension in the string Ex 5.12 Massless, inelastic string The two masses are connected, so their accelerations are equal frictionless m1 has no motion in its y-direction Coordinate systems may differ for m1 and for m2

Ex 5.12 (cont’d) The equation of motion for m1 in y-direction gives no information on a or T The final answers should be written in terms of quantities given in the problem, i.e., m1, m2 and g in this case

Contact force (other contact force is normal force)
Friction forces Contact force (other contact force is normal force) Friction and normal forces are always perpendicular to each other Microscopically, these forces arise from interactions between molecules. Therefore, frictionless surface is an idealization. Direction of frictions force: always to oppose relative motion of the two surfaces Empirically, it is known that the magnitude of friction force is proportional to the magnitude of normal force, This symbol means ‘proportional to’. Kinetic friction The kind of friction that acts when a body slides over a surface Depends on many conditions such as velocity, etc

Static friction The kind of friction that acts when there is no relative motion. See Fig in p.151 of the textbook. Static friction is always less than or equal to its maximum value. (because there is a critical value where the object starts to move) (E.g, if there is no applied force, the static friction is zero.) Rolling friction The friction force when an object is rolling on a surface

The value of q which gives the minimum value of T when mk =1
Minimizing the kinetic friction: What is the force to keep the crate moving with constant velocity? Ex 5.15 The value of q which gives the minimum value of T when mk =1

Ex 5.16 & 5.17 Toboggan ride with friction Some special cases

Fluid resistance & terminal speed
Fluid resistance: the force that a fluid (a gas or liquid) exerts on a body moving through it Origin: Newton’s 3rd law: The moving object exerts a force on the fluid to push it out of the way, then the fluid pushes back on the body (action-reaction) Direction: always opposite the direction of the body’s velocity relative to the fluid. Magnitude: Called air drag Terminal speed

See the solution of quiz problems
Terminal speed See the solution of quiz problems

Velocity with fluid resistance (I)
The LHS is the integral over v and the RHS is the integral over t.

Velocity with fluid resistance (II)

Dynamics of circular motion
Any force, such as gravity, tension, friction, etc can take the role of centripetal force. As a centripetal force, the net force should give the centripetal acceleration and so must be written as

In this problem, the tension takes the role of the centripetal force.
Conical pendulum: Tension & Period the bob moves in a horizontal circle with constant speed v Ex 5.21 We point the positive x-direction toward the center of the circle since the centripetal acceleration is pointing this direction. fixed In this problem, the tension takes the role of the centripetal force. R

Rounding a flat curve: maximum speed at which the driver can take the curve without sliding?
Ex 5.22 There should be no motion along the radial direction. So the friction force exerting on the car is static friction. The friction force exerted by the road on the car should work as centripetal force. If there is no force including friction, the car would go straight (even if you turn the handle) and will be off the road.

Ex 5.22 (cont’d)

A banked road will look like this.
Rounding a banked curve: the bank angle b at which a car can make the turn without friction? (the car has a constant velocity v) Ex 5.23 A banked road will look like this. The normal force has non-vanishing x-component. And it takes the role of the centripetal force for this motion.

Banked curves and the flight of airplanes
To make a turn

Ex 5.24 Uniform circular motion in a vertical circle: a Ferris wheel At top At bottom R Apparent weight At top: n < w At bottom: n > w