Class 11 Force and Motion I

1
Class 11 - Force and Motion I Chapter 5 - Friday
September 17th

• Some particular forces
• Newton's third law
• Sample problems

Reading pages 87 thru 107 (chapter 5) in
HRW Read and understand the sample
problems Assigned problems from chapter 5
24, 26, 34, 38, 44, 46, 52, 56 (due Sun. Sept. 26)

• Announcement
• Physics tutoring center in NPB 1100
• Mon/Wed periods 4 to 8
• Tues/Thurs periods 4,5,8
• Fri periods 4 to 7

2
Review
Newton's 1st law If no force acts on a body,
then the body's velocity cannot change that is,
it cannot accelerate.
1 Newton is that force required to accelerate our
standardized mass (1 Kg) at a rate of 1 m.s-2.
Mass is simply the characteristic of a body that
relates a force on the body to the resulting
acceleration
Newton's 2nd law
Free-body diagrams
3
Some particular forces
Gravity

• During free fall

• Even when a mass is stationary on the surface of
  a table, gravity still acts downwards with a
  magnitude equal to mg.
• This leads to the concept of a normal or contact
  force for the mass on the table to remain
  stationary, the table must exert an upward force
  on the mass so as to exactly balance the force
  due to gravity.

4
Normal force
5
(No Transcript)
6
Friction and tension
7
Newton's 3rd law
When two bodies interact, the forces on the
bodies from each other are always equal in
magnitude and opposite in direction.
For every "action" force, there is always an
equal and opposite "reaction" force we call
these a "third-law force pair."
8
Newton's 3rd law
When two bodies interact, the forces on the
bodies from each other are always equal in
magnitude and opposite in direction.
9
Newton's 3rd law
When two bodies interact, the forces on the
bodies from each other are always equal in
magnitude and opposite in direction.
10
Newton's 3rd law
When two bodies interact, the forces on the
bodies from each other are always equal in
magnitude and opposite in direction.
Why doesn't the earth accelerate?
11
Newton's 3rd law
When two bodies interact, the forces on the
bodies from each other are always equal in
magnitude and opposite in direction.