Physics Chapter 4: Motions and Force

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# Physics Chapter 4: Motions and Force - PowerPoint PPT Presentation

Physics Chapter 4: Motions and Force. Christopher Chui. Motions and Force: Dynamics. Newton’s 1 st law of motion: Every body continues in its state of rest or of uniform speed in a straight line unless acted on by a nonzero net force = law of inertia holds in inertial ref frames

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### Physics Chapter 4: Motions and Force

Christopher Chui

Physics Wk3 - C. Chui

Motions and Force: Dynamics
• Newton’s 1st law of motion: Every body continues in its state of rest or of uniform speed in a straight line unless acted on by a nonzero net force = law of inertia holds in inertial ref frames
• Mass is a measure of inertia
• Weight is the force of gravity acting on a body = mg
• Newton’s 2nd law: The acceleration of an object is directly proportional to the net force acting on it and is inversely proportional to its mass. The direction of the acceleration is in the direction of the net force acting on the object—a = SF/m or SF = m a
• Force is an action capable of accelerating an object

Physics Wk3 - C. Chui

Units for Mass and Force
• System Mass Force (incl. Weight)

SI kg newton (kg-m/s2)

cgs g dyne (g-cm/s2)

British slug pound (lb)

Physics Wk3 - C. Chui

Newton’s 3rd Law: FGP = - FPG
• Whenever one object exerts a force on a second object, the second exerts an equal and opposite force on the first
• Action and reaction act on different objects
• Inanimate objects can exert a force
• For each force, be clear on which object it acts, and by which object it is exerted
• SF = m a applies only to forces acting on a body

Physics Wk3 - C. Chui

Weight—the Force of Gravity and the Normal Force
• Weight and normal force are not action-reaction pairs
• Force exerted by the table is called a contact force
• When a contact force acts perpendicular to the common surface of contact, it is the normal force
• The net force is the vector sum of all forces acting on the object
• A free-body diagram or force diagram shows all forces acting on each object involved

Physics Wk3 - C. Chui

Problem Solving (Newton’s Laws; Free-Body Diagrams)
• Draw a sketch of the situation
• Consider only one object at a time
• Draw a free-body diagram for each body showing all forces acting on the body
• Do not show any forces that the body exerts on other bodies
• Draw the arrow for each force vector as accurately as possible for direction and magnitude
• Label each force, including the unknowns, as to its source
• Only forces acting on a given body can be included in the F = ma equation for that body
• Resolve vectors in x and y components
• For each body, the F=ma can be applied to x and y separately

Physics Wk3 - C. Chui

Friction
• Kinetic friction Ffr = mk FN
• Ffr is perpendicular to FN
• Static friction refers to a force parallel to the two surfaces when they are not sliding
• Static friction is usually larger than kinetic friction in value, Ffr < or = ms FN ;
• The equal sign refers to maximum friction

Physics Wk3 - C. Chui

Physics Assignment 3
• Read and memorize Summary on p. 103
• Do even numbered Questions on p. 103-104
• Do even numbered Problems on pp. 104 - 107
• Turn in Physics Assignment 3 seven days from today
• Prepare for Physics Quiz 3
• Physics Quiz 3 will take place 7 days from today

Physics Wk3 - C. Chui