Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova

Lectures 13, 14, 15

H

Coefficient of friction:

1) Find the force of friction if the block is at rest.2) What is the velocity of the block when it reaches the bottom?

Newton’s 3rd Law

For every action there is an equal, but opposite, reaction

Skater• Skater pushes on a wall • The wall pushes back

– Equal and opposite force

• The push from the wall is a force– Force provides an

acceleration – She flies off with some

non-zero speed

P m1 m2

No friction

m1

Free body diagram

m1

m1g

P

N1

F21

m2g

F12

N2

m2

F12=F21

Quiza) A crate of mass m is on the flat bed of a pick up truck. The coefficient of friction between the crate and the truck is . The truck is traveling at the constant velocity of magnitude V1. Draw the free body diagram for the crate.

b) The truck starts to accelerate with an acceleration ac. Draw the free body diagram for the crate, if the crate does not slip.

A small block, mass 2kg, rests on top of a larger block, mass 20 kg. The coefficient of friction between the blocks is 0.25. If the larger block is on a frictionless table, what is the largest horizontal force that can be applied to it without the small block slipping?

F

gm1

1N

1NF

1N

gm2

2N

1N

A block of mass 20 kg is pushed against a vertical surface as shown. The coefficient of friction between the surface and the block is 0.2. If θ=300, what is the minimum magnitude of P to hold the block still?

θ

P

V0

A block of mass m is given an initial velocity V0 up an inclined plane with angle of incline θ. Find acceleration of the block if a) = 0b) non-zero

A wedge with mass M rests on a frictionless horizontal tabletop. A block with mass m is placed on the wedge and a horizontal force F is applied to the wedge. What must the magnitude of F be if the block is to remain at a constant height above the tabletop?

F

A Problem With First Year Physics Strings and Pulleys

m1

m1, m2 are givenm2>m1

m2String is massless and unstretchable

Find accelerations of m1 and m2 (assume no friction in pulley)

Block 1, of mass m1, is placed at rest on an inclined plane. It is attached by a massless, unstretchable string to block 2, of mass m2. The pulley is massless and frictionless and just changes the direction of the tension in the string. The coefficient of friction between the plane and m1 is the constant .

2

1

Determine what range of values for mass m2 will keep the system at rest.

Find acceleration if mass 2 goes down.

F

Find F necessary to drag the box 1 at constant speed.

1

2

Friction everywhere.

The advantage of a pulley

What minimum force F is needed to lift the piano of mass M?

The Elevator Problem

Have a great day!

Reading: Chapter 7Hw: Chapter 6 problems and exercises