Physics

1
Physics

• Chapter 4
• Sections 3-5

2
Lab Results

• Part A kept the total mass constant and increased
  the force (mass on the string) every time
• Acceleration increased as force increased
• Acceleration a force
• Also note that acceleration is in the same
  direction as the force

3
Lab Results contd.

• Part B kept the force constant (mass on the
  string) and decreased the total mass moving each
  time
• Acceleration increased as mass decreased
• Acceleration a 1/m

4
Lab Results contd.

• Acceleration of an object depends on both the
  force applied and the mass of the object
• Since a a F
• and a a 1/m then a a F/m
• or a F/m

5
Newtons Second Law

• The acceleration of an object is proportional to
  the net force, in the same direction as the net
  force, and inversely proportional to the mass
• a SF / m
• Where a m/s2 F N m kg
• ft/s2 lb
  slug

6
Sample Problem

• A 10 kg block is placed on a table. A force of 6
  N is applied horizontally to the block. What is
  the blocks acceleration?

7
Sample Problem

• What horizontal force must be applied to a 15 kg
  body in order to give it an acceleration of 5
  m/s2?

8
Newtons Third Law

• If there are two objects, A and B, the force A
  exerts on B will be equal and opposite the force
  B exerts on A
• For every action there is an equal and opposite
  reaction
• Forces always come in pairs and occur at the same
  time

9
Question

• A person leans against a wall. They apply 20 N
  of force to the wall and the wall applies 20 N of
  force to the person. Do these forces cancel each
  other out since they are equal and opposite? Why
  or why not?

10
Newtons 3rd Law contd.

• Examples
• When you walk, you push on the floor so the
  floor pushes on you.
• When you swim, you push on the water so the
  water pushes on you.
• When a book sits on a table, the book pushes
  down on the table and the table pushes up on the
  book.

11
Nonequilibrium

• Object(s) is accelerating
• a is not equal to zero
• All the steps are the same except
• Sx max and Sy may

12
Sample Problem

• A girl scout paddling a canoe pushes the water
  back with her paddle, exerting a backward force
  of 155 N on the water. Find the acceleration of
  the girl and the canoe if their combined mass is
  90 kg.

13
Sample Problem

• Three children fight over a small stuffed animal
  of mass 0.2 kg, pulling with the forces
  indicated. Find the instantaneous acceleration
  of the toy.
• 30 N
• 40 N
  
• 
  40 N

14
Sample Problem

• San Francisco is famous for its hills. A car
  (mass 1350 kg) is being projected up one of
  these hills by a drive force that is parallel to
  the hill and has a magnitude of 7200 N. The hill
  makes a 25 degree angle with the horizontal.
  Find the acceleration of the car and the force
  exerted on the car by the hill.