PHYSICS 103: Lecture 6

1
PHYSICS 103 Lecture 6
Agenda for Today

• Explaining Motion
• Newtons 3 Laws
• Applications of Laws

2
Inertia

• Question If I throw a ball in the air, what
  causes it to keep moving?

3
A Few Questions...

• Why does stamping your feet clean the snow off
  your shoes?
• Why does an unseatbelted driver hit the steering
  wheel when the car comes to a sudden stop?

4
NEWTONS LAWS OF MOTION
FIRST LAW Law of Inertia
An object at rest will remain at rest and an
object in motion will remain in motion unless
acted upon by an external force.
INERTIA
The quality of an object that indicates the
reluctance of an object to be altered in its
movement
(things are inherently lazy)
5
NEWTONS LAWS OF MOTION
Second LAW
The net force on an object is equal to the
product of that objects mass times its
acceleration. The acceleration is in the same
direction as the force.
F m . a
(Throwing a baseball is much easier than throwing
a bowling ball)
(mass is in units of kilograms)
6
FREELY FALLING OBJECTS
Why do things fall at the same rate?
aheavy alight
a g constant 9.81 m/s2 10 m/s2
7
Force is a vector
The net force on an object is what determines the
acceleration of the object
Is there a force acting on me?
8
Example problem Pushing on a block
10 kg
20 N
2 N
a?
9
NEWTONS LAWS OF MOTION
Third LAW
For every force that one object exerts on a
second object, there is an equal but oppositely
directed force that the second object exerts on
the first object.
(For every action there is an equal but opposite
reaction)
10
Questions

• If you push on your friend who is running away
  from you with a force of 50 N, how much force
  will he/she exert back on you?
• You are ice skating with your identical twin
  (who has the same mass). You push off your twin
  with a force of 50 N. What will happen to you
  and your twin as your pushing off each other? 1
  second after you let go?
• You now ice skate with a sumo wrestler and push
  off the sumo wrestler with a force of 50 N. What
  will happen to you? What will happen to the sumo
  wrestler?

11
Main Points from Todays Lecture

• Newtons 3 Laws of Motion
• You should understand the three laws of motion
  and think of examples of them in everyday life.
• You should be able to calculate the net force on
  an object and understand that this is what
  determines the acceleration.
• You should be able to apply Newtons laws to
  real problems, solving for the net force and
  acceleration

12
HW

• Ch 4 Q7,8,9,10,11,13,14,24
• E1,2,4,6,7