vectors, lows of physic

Forces and Newton’s Laws of Motion
Topics:
• Motion and Newton’s first law

•
•
•
•
•

What is a force?
Identifying forces
Newton’s second law
Free‐body diagrams
Newton’s third law
N t ’ thi d l

Sample q p question:
These ice boats sail across the ice at great speeds. What gets the boats moving in the first place? What keeps them from going even faster?
Slide 4-1

What Causes Motion?

In the absence of any forces acting on it, an object will continue moving forever. Motion needs no “cause.”
Slide 4-8

Seat Belts: An Application of Newton’s First Law

Slide 4-9

What Is a Force?
A force... force ... is a push or pull.

... is a vector.

... acts on an object. object

... requires an agent. agent

... is a contact force or a long-range force.

Slide 4-10

Force Vectors

Slide 4-11

→

A Short Catalog of Forces: Weight w

→

Slide 4-12

→

Spring Force Fsp

Slide 4-13

Tension Force T

Slide 4-14

Normal Force n

Slide 4-15

→

Friction fk and fs

Slide 4-16

Drag D and Thrust Fthrust

Slide 4-17

Identifying Forces – The free body diagram

Slide 4-18

Example
A block is dragged uphill by a rope Identify all rope. forces acting on the block.

Slide 4-19

Example
Block A hangs from the ceiling by a rope Another block B hangs rope. from A. Identify the forces acting on A.

Slide 4-20

Exercise
A ball hanging from the ceiling by a string is pulled ball, string, back and released. Identify the forces acting on it just after its release.

Slide 4-21

Newton’s Second Law

Slide 4-22

Example
An elevator, lifted by a cable, is going up at a steady speed elevator cable speed. • Identify the forces acting on the elevator.
• Is T greater than, equal to, or less than w? Or is there not enough i f h information t t ll? ti to tell?

Slide 4-23

Free-Body Diagrams

Slide 4-24

Newton’s Third Law

Slide 4-25

Identifying Forces for Interacting Objects

Slide 4-26

Checking Understanding
10-year-old
10 year old Sarah stands on a skateboard Her older brother skateboard. Jack starts pushing her backward and she starts speeding up.
The force of Jack on Sarah is
A. greater than the force of Sarah on Jack.
B. equal to than the force of Sarah on Jack.
C. less th th f
C l than the force of Sarah on Jack. fS h
J k

Slide 4-27

Answer
10-year-old
10 year old Sarah stands on a skateboard Her older brother skateboard. Jack starts pushing her backward and she starts speeding up.
The force of Jack on Sarah is

B. equal to than the force of Sarah on Jack.

Slide 4-28

Quiz
1.
1 What is a “net force? net force?”

Slide 4-2

Quiz
2.
2 List at least three of the steps used to identify the forces acting on an object.
A.
A Identify “the system” and “the environment ” environment.” B. Draw a picture of the situation.
C. Draw a closed curve around the system.
D. Locate every point on th b
D L t i t the boundary of thi curve where th d f this h the environment touches the system.
E. Name and label each contact force acting on the object.
F. N
F Name and l b l each l d label h long-range f force acting on the object. ti th bj t

Slide 4-3

Quiz
3.
3 Which of these is not a force discussed?
A. The tension force.
C. The orthogonal force.
B. The normal force.
D. The thrust force.

Slide 4-4

Reading Quiz
4.
4 An action/reaction pair of forces
A. point in the same direction.
B. act on the same object.
C.
C are always long range forces. long-range forces
D. act on two different objects.

Slide 4-6