According to Newton s 2 nd Law If the force is held constant the relationship between mass and acceleration is direct/inverse. If the mass is held constant the relationship between force and acceleration is direct/inverse. In this situation... If the acceleration is held constant the relationship between mass and force is direct/inverse.
video Mass Amount of matter in an object Is always constant at any place and time Is measured in kilograms Is measured using a balance Is an intrinsic property of an object and is independent of any external factor Weight Force which a given mass feels due to the gravity at its place Depends on gravity at the place Is measured in Newtons Is measured using a scale Depends on: Mass of the object Mass of the object that is attracting it
To calculate weight: When you know the mass use ΣF = ma If standing on the Earth (no friction force), acceleration due to gravity is 9.8 m/sec/sec Mass = 65.0 kg Weight on earth? Weight on the moon? Weight on Jupiter?
Free Fall Falling without air resistance Gravity is the only force acting upon the object causing the object to change it s velocity Acceleration (change in velocity) due to gravity on earth is 9.8 m/sec/sec for all objects That means... When an object is in free fall it will be increasing its velocity 9.8 m/sec every second Videos what will hit the ground first; freefall on moon F=ma
a = 9.8 m/sec/sec
Free Fall neglecting friction which object the elephant or the featherwill hit the ground first True or false: The elephant and the feather each have the same force of gravity The elephant has the greatest acceleration (change in velocity) On earth, all objects (whether an elephant or a feather) have the same force of gravity The elephant and the feather have the same acceleration (change in velocity) due to gravity
Free fall cont. In the absence of air resistance, the elephant and the feather land at the same time. The force of gravity experienced by an object is dependent upon the mass of that object. RULE: all objects (regardless of their mass) experience the same acceleration (change in velocity) when in a state of free fall
Free Fall Draw a velocity vs. time graph v Draw an acceleration vs. time graph a t -9.8 m/s/s t Slope = -9.8 m/s/s
Acceleration of fall is less when air resistance (drag) acts Air resistance is the result of collisions of the object's leading surface with air molecules. Fluid friction (air drag) depends on two things: surface area & speed The greater the surface area, the greater the friction As an object falls, the friction increases as the speed increases F=ma
Falling with Air Resistance
Terminal Velocity of Tennis Ball Draw a velocity vs. time graph v t Draw an acceleration vs. time graph a t
Which object - the elephant or the feather - will hit the ground first? True or false: The elephant encounters a smaller force of air resistance than the feather and therefore falls faster. The elephant has a greater acceleration (change in velocity) due to gravity than the feather and therefore falls faster. Both elephant and feather have the same force of gravity, yet the feather experiences a greater air resistance. The feather has a smaller force of gravity so friction equals the force of gravity sooner causing a net force of zero (no acceleration); whereas the elephant has a net force that is not zero and will continue to speed up.
What is the acceleration for A? What is the acceleration for B? What is the acceleration for C? What is the acceleration for D?
Lab 4 Air Resistance Purpose: Does changing the mass of a falling object affect the velocities & accelerations observed on the motion graphs? Method: State independent, dependent and controlled variables Materials: motion detector on ceiling; laptop; coffee filters Hypothesis: Procedure: Data: 5 different velocity vs. time graphs (copied into a word document) Analysis: analyze motion on graphs and label with descriptions of motion (ie where is the terminal velocity, etc) Conclusion: Write a concluding paragraph. Include a discussion of the relationship between mass and terminal velocity. Be sure to include data to back up your statements!
Which encounters the greater force of air resistance a falling elephant or a falling feather?
Two smooth balls exactly the same size, one made of wood and the other made of iron, are dropped from a high building to the ground below. The ball to encounter the greatest force of air resistance on the way down is the: a) wooden ball b) iron ball c)... both the same
As she falls faster and faster through the air, her acceleration (change in velocity) a) increases b) decreases c) remains the same
What will be the acceleration (change in velocity) of a rock thrown straight upward at the moment it reaches the tippity-top of its trajectory (no air)?
End of Chapter Exercises 1. What is the net force on a bright red Mercedes convertible traveling along a straight road at a steady speed of 100 km/h? 2. On a long alley a bowling ball slows down as it rolls. Is any horizontal force acting on the ball? How do you know? 3. Your empty hand is not hurt when it bangs lightly against a wall. Why is it hurt if it does so while carrying a heavy load? Which of Newton's laws is most applicable here? 4. When a junked car is crushed into a compact cube, does its mass change? Its weight? Its volume? Explain. 5. What is the net force on a 1-N apple when you hold it at rest above your head? What is the net force on it after you release it? 6. If it takes 1 N to push horizontally on your book to make it slide at constant velocity, how much force of friction acts on the book? 7. A crate remains at rest on a factory floor while you push on it with a horizontal force F. How big is the friction force exerted on the crate by the floor?
1. Two basketballs are dropped from a high building through the air. One ball is hollow and the other filled with rocks. Explain in detail the fall of the two balls. 2. A parachutist, after opening the chute, finds herself gently floating downward, no longer gaining speed. She feels the upward pull of the harness, while gravity pulls her down. Which of these two forces is greater? Or are they equal in magnitude? 3. Why will a sheet of paper fall slower than one that is wadded into a ball? 4. How does the gravitational force on a falling body compare with the air resistance it encounters before it reaches terminal velocity? After? 5. Why is it that a cat that accidentally falls from the top of a 50- story building hits the ground no faster than if it falls from the 20 th story?
Chapter 3 Review 1. A bear that weighs 4000 N grasps a vertical tree and slides down at constant velocity. What is the friction force that acts on the bear? 2. How does the force of gravity on a raindrop compare with the air drag it encounters when it falls at constant velocity? 3. Upon which will air resistance be greater; a sheet of falling paper or the same paper wadded into a ball if they have both reached terminal velocity? (Careful!) 4. Aristotle claimed the speed of a falling object depends on its weight. We now know that objects in free fall, whatever their weights, undergo the same gain in speed. Why does weight not affect acceleration?