:acceleration and force Physics :acceleration and force NB p 47 NOTES

:acceleration and force Physics Warm-up

:acceleration and force Physics Warm-up

:acceleration and force Physics Free Body Diagram HO 3.4  A free body diagram is a pictorial representation of all forces acting on an object.  We consider the object to be isolated from the physical system and choose to examine only the forces directly acting ON the object, not forces applied BY the object. Explanation

:acceleration and force Physics Free Body Diagram Example Explanation Label the forces acting on the block.

:acceleration and force Physics Free Body Diagram Example Explanation Force on the block by the ground: F Normal Force on the block by the earth: F gravity Force on the block by Antonio Force on the block by the rope. Force on the block by the ground: F friction

:acceleration and force Physics Galileo’s Investigation Engagement  What are the similarities and differences between a basketball and a tennis ball?

:acceleration and force Physics Galileo’s Investigation Engagement  What will happen if the balls are released from the same height above the floor?  Try to use Physics vocabulary in your response.

:acceleration and force Physics Galileo’s Investigation Engagement  What’s going on here, why did they move?  How would you describe the motion of the balls? – Path – Speed  Again, try to use Physics vocabulary in your response.

:acceleration and force Physics Force of Gravity: Pull or Push?  Force can be described as a push or pull one object exerts on another.  One specific force is weight. Weight = the force of gravity on an object.  But what causes gravity? – The presence of mass Explanation

:acceleration and force Physics Newton’s Universal Law of Gravitation HO 3.2  Masses attract one another about their centers with a force proportional to the product of their masses and inversely proportional to the square of the distance between them (whew!). Explanation F =G m 1 m 2 ÷ d 2 F weight = mg

:acceleration and force Physics Newton’s First Law  Objects require a force to change their velocity. – Another term for this property of matter is inertia  What is the principle property of an object that determines its inertia? – mass Explanation

:acceleration and force Physics Newton’s First Law Example  If we apply the same force (push) to a bus and a bicycle, which object will experience a greater change in velocity? Explanation

:acceleration and force Physics Newton’s Second Law  Force causes a change in motion – Change in motion means change in velocity – Change in velocity means???  A change in velocity occurs in a certain amount of time.  AHA! That’s a rate. Explanation  F = ma

:acceleration and force Physics Acceleration is a Rate  The speedometer in your car indicates what property of motion?  What happens to the needle if you speed up or slow down?  The speed of the needle is a measure of your acceleration (in a straight line).  Acceleration is the rate of change in velocity. Explanation

:acceleration and force Physics Newton’s Second Law Example #1  Important note: the acceleration vector is always in the same direction as the force vector.  If we apply a 100N force to a wagon full of rocks initially at rest with a mass of 50 kg, what is its acceleration?  What does this answer tell us?  What is the velocity of the object after 1 second? – 2 seconds? – 5 seconds? Explanation  F = ma 100 N 2 m/s 2

:acceleration and force Physics Newton’s Second Law Example #2  What happens when the acceleration vector points opposite to the velocity vector? Explanation 30 m/s F - 5m/s 2 How long will it take Mighty Mikey to stop the train?

:acceleration and force Physics Newton’s Third Law  No single isolated force can exist  Equal and Opposite forces exist only between two objects Explanation Push on water WRONG!

:acceleration and force Physics Newton’s Third Law Example  You push on the water  The water pushes back on you with an equal force in the opposite direction  So why do you move? Explanation Push on water CORRECT! Push on swimmer

:acceleration and force Physics Acceleration Due to Gravity  The Tennis Ball and Basketball hit the ground at the same time because they accelerated at the same rate (~10 m/s 2 ).  What force changed their velocities?...Weight.  Weight = mass X gravity (F W =mg). Explanation W bb > W tb

:acceleration and force Physics Acceleration Due to Gravity Explanation F = weight m = mass F/m = a a = g regardless of mass!

:acceleration and force Physics Free Body Diagram Example 2  Forces on coffee filter before release (balanced forces) Explanation Weight Support from my hand

:acceleration and force Physics Free Body Diagram Example 2  Forces on coffee filter immediately after release (unbalanced force  acceleration) Explanation Weight

:acceleration and force Physics Free Body Diagram Example 2  Forces on coffee filter while falling Explanation Weight Air resistance What factors influence the magnitude of the force of air resistance?

:acceleration and force Physics So Why Different Times?  Force due to gravity is different for the two objects  Their masses are different and therefore they have different inertias  They do not accelerate at the same rate …WHY??? – We must examine ALL the forces acting on the objects in order to determine the Net Force. Explanation

:acceleration and force Physics Net Force  If you earn $2000/mo but taxes, “free” health care, social security, and other reductions total $1350/mo then your: – Gross Pay = $2000/mo – Net Pay = $650/mo Explanation Net Force FF

:acceleration and force Physics Net Force  The force of air resistance is greater for the basketball than the coffee filter.  However, the force of air resistance is negligible compared to the weight of the basketball, therefore the Net Force is toward the floor. Explanation Air resistance Weight

:acceleration and force Physics So Why Different Times?  The force of air resistance is also acting on the coffee filter and is comparable in size to the weight.  In fact, at some point, the force of air resistance is equal to the weight of the coffee filter.  When this is the case: – There is NO net force – Therefore there is no acceleration – Therefore the object moves at a constant speed  This equilibrium condition is called terminal velocity Explanation

:acceleration and force Physics Evaluation Sky Diver Physics HO 3.5  A skydiver jumped out of a plane 1 sec ago. – Label the forces on the skydiver  After some time, the skydiver has zero acceleration – What does this indicate about her speed? – What is the name for this equilibrium condition of a falling object? – Label the forces on the skydiver  What factors influence the force of air resistance on the skydiver?  BONUS: What is the terminal velocity of a human?

:acceleration and force Physics Summary  Force  Gravity & Weight  Newton’s Laws  Air resistance  Free Body Diagrams  Net Force  So what? – This applies to transportation, moving objects, sports, etc. Summary