2 - Mr Simnett
Newton’s Law’s of Motion
Newton’s First Law of Motion:
( If an object is at rest or the velocity of the object is constant, then the same motion will continue unless acted on by an unbalanced force.
( If the net force acting on an object is zero, that object will maintain its state of motion (constant velocity or at rest).
Examples of Newton’s First Law ( Card with a penny on it and the card gets hit.
( Table cloth magic with dishes on it.
( A car going around a corner and a passenger feels pushed against the door.
Inertia is the ability of an object to resist changes to its motion. (The motion being either at rest or a constant velocity.)
Example # 1: Include the proper FBDs with the following questions. What is the net force of an object traveling in a constant velocity? If
a) the object is a car.
b) the object is a plane.
Equilibrium:
• If an object is in equilibrium then the net force of the object is zero, therefore, the magnitude of the acceleration is zero
• Types of Equilibrium
o Dynamic equilibrium indicates constant velocity
o Static equilibrium indicates at rest
o Equilibrium ( ( F = 0, thus ( Fx = 0 and ( Fy = 0
Example # 2: Calculate the magnitudes of the tensions in the two cables in the diagram on the blackboard; which the mass is in equilibrium.
Newton’s Second Law:
(The acceleration of an object is directly proportional to the net force and inversely proportional to its mass. The direction of the acceleration must be the same as the direction of the net force.
The first sentence can be written as follows.
→ → →
( a ( ( F and a ( 1 / m
→ →
This means that a ( ( F / m
→ →
a = k ( F / m (where k = a constant)
but for this case k =1
→ → → →
a = ( F / m or ( F = ma
Reminder: 1 N = 1 kg ( m/s2
Example # 3: A 2.0 X 103 kg vehicle has an applied force of 4000 N from the engine acting on it. It has a frictional force with a magnitude of 1000 N and is driving on a level surface. What is the acceleration of the car? Draw a FBD.
→ →
Reminders: ( weight = the force of gravity acting on an object = Fg = mg
( mass is the amount of matter within an object.
Newton’s Third Law:
( For every action force there is a reaction force that is equal in size, but in the opposite direction.
→ →
( Faction = – Freaction [pic][pic]
Note: This means all forces are paired, but each force acts on a different object.
Example # 4: Hinal (m=50kg) and Payal (m = 55kg) are in space. Payal decides to sacrifice herself to save Hinal by pushing her back to the space craft. If Payal pushes on Hinal with a force of 300N [right], calculate the acceleration of both astronauts.
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