Chapter 4: Forces and the Laws of Motion

1
Chapter 4 Forces and the Laws of Motion
2
Mass VS. Weight

• Mass is the quantity of matter in an object
• Mass is measured in kilograms (kg)
• Mass is different from weight
• Weight is the force of gravity on an object. It
  is an objects gravitational pull to the earth.

3
Force

• Force is any interaction between an object and
  its environment
• Measured in Newtons
• 1 kg 9.8N
• If you know the mass of an object, measured in
  kilograms, and you want to know its weight in
  Newtons, just multiply the mass by 9.8 N/kg
• Weight mass acceleration due to gravity (g)
• W mg
• Weight (N) mass (kg) x g (N/kg) g is always
  9.8 N/kg

4
Find the weight in Newtons

• 1.) A can of soup with a mass of 2 kg
• 2.) A pillow with a mass of ½ kg
• 3.) A person with a mass of 45 kg
• 4.) A car with a mass of 230 kg
• If a 1 kg bag of nails weighs 9.8 N at the
  earths surface, is it true that 1 kg of orange
  juice weighs 9.8 N?

5
1 lb 4.448 N

• If you know the weight of an object in lbs, and
  want to find the weight in Newtons, just multiply
  the weight (in lbs) by 4.448
• Find the weight of the following objects (in
  Newtons)
• 1.) 1 lb of lunch meat
• 2.) A person who weighs 125 lbs
• 3.) An 8 lb bowling ball

6
Force of gravity

• The force of gravity increases as you near
  earths center. The force of gravity can be
  defined as
• g (GME)
• G is the universal gravitational constant 6.67 x
  10-11 Nm2/kg2 (not to be confused with little g).
  
• r is the distance from the object to earths
  center.
• At high altitudes, the force of gravity is a bit
  smaller, so objects at high altitudes will weigh
  a bit less than they normally do at sea level

r2
7
Contact Forces and Field Forces

• Contact forces result from physical contact
  between two objects.
• Field forces do not involve physical contact
  between two objects. An example is gravitational
  force when an object is in free fall, Earth
  exerts some force on the object, pulling it to
  the ground, but there is no direct contact
  between the object and earth when it is in air.

8
Do Now 1/27/09

• 1.) Find the weight (in Newtons) of the following
  objects
• 1 kg of yogurt
• 5 kg book bag
• A person who weighs 176 lbs
• A dog with a mass of 10 kg
• 2.) Give an example of a contact force. Give an
  example of a field force.
• 3.) List an example of each of the following
• A force causing an object to start moving
• A force causing an object to stop moving
• A force causing an object to change its direction
  of motion

9
Net Force and Newtons First Law

• Net force is the combination of all of the forces
  acting on an object. It is the vector sum of the
  forces acting on an object.
• Newtons First Law of Motion
• An object at rest remains at rest, and an object
  in motion continues in motion with constant
  velocity (that is, constant speed in a straight
  line) unless the object experiences a net
  external force.

10
Newtons First Law of Motion

• When an object does not experience a net external
  force, that means that its net external force is
  zero. When the net force is zero, the object
  does not have an acceleration which means that
  it continues at a constant velocity, or remains
  at rest.
• Newtons First Law is also called the law of
  inertia, because inertia is the tendency of an
  object not to accelerate it is an objects
  resistance to a change in its motion
• Mass is a measure of the inertia of an object. An
  object that has a large mass has a large inertia.
  The greater the mass of an object, the less the
  object accelerates under an applied force.

11
Do Now 1/28/09

• 1.) Explain the difference between mass and
  weight.
• 2.) What is Newtons First Law of Motion? Give
  the definition and then describe it in your own
  words.
• 3.) What is the definition of net force?
• 4.) If the net force acting on an object is zero,
  is the object at rest or in motion, or possibly
  both?

12
Do Now 1/29/09

• In packet p.129 1, 3 (You do not have to
  rewrite the question.)

13

• 1.) Mary leaves her suitcase on a hill while she
  unloads the rest of her car. The hill is at a
  55 angle with the horizontal. The following
  forces act on her suitcase
• Fgravity on suitcase Fg 34 N
• Ffriction Ff 15 N
• Fground on suitcase Fground 20 N
• Fnet ?
•  
• Draw a free body diagram of the situation. Then
  find the net force acting on the suitcase.

14

• 2.) A traffic light weighing 100 N hangs from a
  vertical cable tied to two other cables that are
  fastened to a support. The upper cables make
  angles of 37 and 53 with the horizontal. The
  following is a free body diagram for the knot the
  joins the cables
•  
• Find T1 and T2. Hint The traffic light is in
  equilibrium, so S Fx S Fy 0

T1
T2
53
37
T3 -100 N
15
Do Now 2/4/09

• 1.) In your own words, define Newtons First Law
  of Motion.
• 2.) In your own words, define Newtons Second Law
  of Motion.
• 3.) A 0.8 kg football is thrown across the field
  with a net force of 85 N. What is the
  acceleration of the football?
• 4.) An 20 kg crate is pushed across the floor
  with a force of 50 N. If the crate maintains a
  constant velocity, what is the acceleration of
  the crate? What is the net force acting on the
  crate?

16
Do Now 2/5/09

• 1.) Define Newtons Third Law of Motion in your
  own words.
• 2.) The forces acting on a sailboat are 400 N
  north and 160 N west. If the boat has a mass of
  320 kg, what are the magnitude and direction of
  the boats acceleration? (Hint Find the
  magnitude and direction of the net force, then
  plug your answer into the equation a Fnet /
  mass.)
• 3.) If a small sports car collides head-on with a
  massive truck, which vehicle experiences the
  greater impact force? Which vehicle experiences
  the greater acceleration? Explain your answers.

17
Fn
TV
Fg
The normal force is a force that acts on a
surface in a direction perpendicular to the
surface.
18
Fn mg cos ?
Fn
?
Fg
?

•  The normal force is always perpendicular to the
  contact surface but is not always opposite in
  direction to the force due to gravity.

19

• 1.) A car is parked on a hill that makes an angle
  of 30 with the horizontal. If the car has a
  mass of 320 kg, find the magnitude of the normal
  force acting on the car.
• (First draw a free body diagram of the situation)
• 2.) A loading ramp makes a 45 angle with the
  ground. If a 132 kg refrigerator sits on the
  loading ramp, find the magnitude of the normal
  force acting on the refrigerator.

20
Do Now 2/6/09

• 1.) Define Newtons first law of motion
• 2.) Define Newtons second law of motion
• 3.) Which object has a greater inertia a raw egg
  or a hardboiled egg? Why?
• 4.) A dog pulls on a pillow with a force of 4 N
  at an angle of 27 above the horizontal. Find
  the x and y components of this force.

21
Coefficient of Friction

• The coefficient of friction, µ, is defined as the
  ratio of the force of friction to the normal
  force between two surfaces.
• The coefficient of kinetic friction, µ k, is the
  ratio of the force of kinetic friction to the
  normal force.
• The coefficient of static friction, µ s, is the
  ratio of the maximum value of the force of static
  friction to the normal force.
• In a word problem, the maximum value of the force
  of static friction is usually described as the
  force that sets an object in motion. The force
  of kinetic friction is described as the force
  that keeps an object in motion.