1. The Astronomy Picture of the Day

2. Force, Acceleration, &
Interaction

3. Outline
• Acceleration
• Newton’s Second Law of Motion
• Action & Reaction: Newton’s Third Law
of Motion

4. Acceleration
• The acceleration of an object is
defined as its change in velocity
divided by the time taken to change
it. Mathematically:
Change in velocity
Acceleration = Change in time
or using symbols
a= ∆v
∆t

5. Views of a car in uniform motion & accelerating

6. Newton’s Second Law of Motion
(Two equivalent statements)
I. The force (F) acting on an object
equals the product of its acceleration
(a) & its mass (m). Mathematically
F = m×a
II. The amount of acceleration (a) that a
body will experience is equal to the
force (F) applied divided by the
body’s mass (m):
a = F/m

7. A loaded cart will not accelerate as easily
as an empty cart

8. Newton’s Third Law of Motion
(law of action-reaction)
• When two bodies interact, they create
equal & opposite forces on each other.

9. Newton’s third law of motion: (A) When X pushes on Y, an equal push is
given to X by Y. (B) When X pushes on a much heavier person Z, an
equal & opposite push is given to each, but Z moves off at a smaller speed

10. A hammer thrower feels an equal & opposite force matching the
centripetal force on the swinging hammer

11. Questions for in-class discussion