Lecture 13

Newton’s 2nd Law (continued)

QUIZ

• Find this vector’s magnitude and angle with respect to the positive x axis and draw it

• Find the x- and y-components of the vector

20 / 40 /x yv m s v m s

0(2 ,45 below +x-axis)d km

Which of these is not a force discussed in this chapter?

1 Tensio

n Force

2 Orth

ogonal

F

orce

3. Norm

al Force

4. Thru

st

25% 25%25%25%• 1 Tension Force• 2 Orthogonal

Force• 3. Normal Force• 4. Thrust

Newton’s 2nd Law

NET

NET

Fa

m

F ma

Units

• Weight is a force

21 unit of force = kg* 1 Newton

1 lb = 4.45 Newtons = 4.45 N

What is the weight of a 170 lb person in N?

m

s

What is the weight of a 170 lb person in Newtons?

756N

38.2 N

1,666 N

17.3 N

25% 25%25%25%1. 756N2. 38.2 N3. 1,666 N4. 17.3 N

Using Newton’s 2nd Law

• Need FNET

• Make a free body diagram• Use particle model, object is a point dot

located at the origin of coordinate system• Draw vectors representing each force acting

on object• Draw vector representing FNET

Problem 4:34

• Free body diagram of your physics textbook sliding across the table

Free body diagrams

• Make diagram of object showing all forces acting on it (as vectors). For now object will be represented by a point.

• Decide on the x and y-axes• Find x- and y-components of each force• Find

• Show FNET on diagram

, 1 2

, 1 2

...

...NET x x x

NET y y y

F F F

F F F

Newton’s 3rd Law

• If object A exerts a force FAB on object B, the object B exerts a force FBA = - FAB on object A.

n

W

Newton’s 3rd

• Objects need not be in contact.• Example gravitational force sun exerts on the

earth.• One magnet on another.

Problem

What is a? What force does the 7 kg object exert on the 28kg object?

Vice versa? M=28 kg

M=7 kgF = 35 N

Quiz

An elevator, lifted by a cable, is going up at a steady speed. • Identify the forces acting on the elevator. Make a free body

diagram.• Is T greater than, equal to, or less than w? Or is there not

enough information to tell?

Slide 4-23

Special Homework due Monday

• Workbook, section 5.2, pages 5-3 and 5-4, problems 7-12

Problems due Today

• Reading: 4: 5-8• Problems: 4: CQ1, MC21, 4, 7, 8, 9, 10, 11

Problems due Monday

• Reading: 5: 1-5• Problems: 4: 13, 14, 17, 18, 20, 25, 29, 30, 31,

33, 35, 38, 65