November 3, 2011 A car rounds a hill while maintaining a constant speed. Is there a net force on the car as it rounds the hill? No – its speed is constant. Yes. It depends on the sharpness of the hill and the speed of the car. What is the average net force needed to bring a 1500kg car to rest from a speed of 100km/h within a distance of 55m? -11,000 N
Vertical Motion What happens when a person pulls upward on a 10.0kg box with a force equal the box’s weight? What happens when a person pulls upward on the same box with a 100.0N force?
Apparent Weight Loss A 65kg woman descend in an elevator that briefly accelerates at 0.20m/s2 downward. She stand on a scale that reads in kg. During this acceleration, what is her weight and what does the scale read? What does the scale read when the elevator descends at a constant speed of 2.0m/s?
Pulleys A 6kg mass and 4kg are attached on a pulley system. Find the force of tension on the string connecting them. Find the acceleration of the two masses. 4kg 6kg
Mechanical Advantage A mover is trying to lift a piano (slowly) up to a second- story apartment. He is using a rope looped over two pulleys as shown. What force must he exert on the rope to slowly lift the piano’s 2000N weight?
November 7, 2011 A force of 20N right is applied on a 4kg object to slide it across a frictionless surface. Find the acceleration. 5m/s2 right Oh snap! The object actually accelerates at only 3m/s2 to the right. Guess the surface is not frictionless. What is the force of friction? Ff = 8N left What is the coefficient of kinetic friction between the two surfaces? 0.204
Coefficient of Friction An object that weighs 20N requires an applied force of 6N to start it moving across a surface. What is the coefficient of static friction? 0.30 If the weight of the object is doubled, what force is needed to set it in motion? 12N
Pushing and Pulling A 10.0kg box is pulled along a horizontal surface by a force of 50.0N. The coefficient of kinetic friction is 0.30. Calculate the acceleration. Now, the same box is pulled along the same surface but the 50.0N force is at a 30˚ angle. Again, calculate the acceleration. Now, the same box is pushed at a 30˚ angle with a force of 50.0N. Calculate the acceleration.
November 8, 2011 Two boxes are connected by a cord running over a pulley. The coefficient of kinetic friction between box A and the table is 0.20. We ignore the mass of the cord and pulley, and any friction in the pulley. Find the acceleration of the system. 1.4m/s2
The Skier The 73kg Neil in the picture has just begun descending the 30˚ slope. Assuming the coefficient of kinetic friction is 0.10, calculate the acceleration and the speed he will reach after 4.0s. 4.0m/s2 16m/s
Elephant and the Feather Suppose that an elephant and a feather are dropped off a very tall building from the same height at the same time. We will assume the realistic situation that both feather and elephant encounter air resistance. Which object - the elephant or the feather - will hit the ground first?
Not Exactly Shocking, but… 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 of gravity than the feather and therefore falls faster. Both elephant and feather have the same force of gravity, yet the acceleration of gravity is greatest for the elephant. Both elephant and feather have the same force of gravity, yet the feather experiences a greater air resistance. Each object experiences the same amount of air resistance, yet the elephant experiences the greatest force of gravity. Each object experiences the same amount of air resistance, yet the feather experiences the greatest force of gravity. The feather weighs more than the elephant, and therefore will not accelerate as rapidly as the elephant. Both elephant and feather weigh the same amount, yet the greater mass of the feather leads to a smaller acceleration. The elephant experiences less air resistance and than the feather and thus reaches a larger terminal velocity. The feather experiences more air resistance than the elephant and thus reaches a smaller terminal velocity. The elephant and the feather encounter the same amount of air resistance, yet the elephant has a greater terminal velocity.
Elevator Consider a person standing in an elevator that is accelerating upward. The upward normal force exerted by the elevator floor on the person is _________ the downward weight. larger than identical to smaller than
Tricky Elevator A person stands on a bathroom scale in a motionless elevator. When the elevator begins to move, the scale briefly reads only 0.75 of the person’s regular weight. Calculate the acceleration of the elevator. 2.45m/s2 down
Hoisting A 12.0kg bucket is lowered (?) vertically by a rope in which there is 163N of tension. What is the acceleration of the bucket? 3.75m/s2 up
November 17, 2011 A constant force is exerted on a cart that is initially at rest on an air track. Friction between the cart and the track is negligible. The force acts for a short time interval and gives the cart a certain final speed. To reach the same final speed with a force that is only half as big, the force must be exerted on the cart for a time interval __________ that stronger force. four times as long as twice as long as equal to half as long as a quarter of
A constant force is exerted for a short time interval on a cart that is initially at rest on an air track. This force gives the cart a certain final speed. Suppose we repeat the experiment but, instead of starting from rest, the cart is already moving with a constant speed in the direction of the force at the moment we begin to apply the force. After we exert the same constant force for the same short time interval, the increase in the cart’s speed is equal to two times its initial speed. is equal to the square of its initial speed. is equal to four times its initial speed. is the same as when it started from rest. cannot be determined from the information provide.
The Crate’s Orientation You are pushing a wooden crate across the floor at a constant speed. You decide to turn the crate on end, reducing by half the surface area in contact with the floor. In the new orientation, to push the same crate across the same floor with the same speed, the force that you apply must be ________ as the force required before you changed the crate’s orientation. four times as great twice as great equally as great half as great one-fourth as great
Tilting Tables An object is held in place by friction on an inclined surface. The angle of inclination is increased until the object start moving. If the surface is kept at this angle, the object slows down. moves at uniform speed. speeds up. none of the above.