Reading Quiz - Concordia College

1 Physics 111, Concordia College Reading Quiz Chapter 5 Physics 111, Concordia College 1. The coefficient of static friction is A. smaller than the coefficient of kinetic friction. B. equal to the coefficient of kinetic friction. C. larger than the coefficient of kinetic friction. D. not discussed in this chapter. Physics 111, Concordia College 1. The coefficient of static friction is A. smaller than the coefficient of kinetic friction. B. equal to the coefficient of kinetic friction. C. larger than the coefficient of kinetic friction. D. not discussed in this chapter. Physics 111, Concordia College 2. The drag force pushes opposite your motion as you ride a bicycle.

2 If you double your speed, what happens to the magnitude of the drag force? A. The drag force increases. B. The drag force stays the same. C. The drag force decreases. Physics 111, Concordia College 2. The drag force pushes opposite your motion as you ride a bicycle. If you double your speed, what happens to the magnitude of the drag force? A. The drag force increases. B. The drag force stays the same. C. The drag force decreases. 3. You are standing on a scale in an elevator that is accelerating upward. The Reading on the scale is A. Larger than your true weight. B. The same as your true weight. C. Smaller than your true weight. 3. You are standing on a scale in an elevator that is accelerating upward.

3 The Reading on the scale is A. Larger than your true weight. B. The same as your true weight. C. Smaller than your true weight. Physics 111, Concordia College Equilibrium Using Newton s second law Mass, weight, and apparent weight Static and kinetic friction Applying Newton s third law Chapter 5 Applying Newton s Laws Topics: Physics 111, Concordia College An object is in equilibrium when the net force acting on it is zero. In component form, this is Equilibrium If the net force is zero then the object does not accelerate. It must be at rest or have a constant velocity. Physics 111, Concordia College Equilibrium Problem Hints If the acceleration is zero the net force must be zero.

4 Draw a Free-Body diagram of the object. Break the forces into x and y components. The sum of the x-components must be zero and the sum of the y-components must be zero. Physics 111, Concordia College A 100 kg block with a weight of 980 N hangs on a rope. Find the tension in the rope if a) the block is stationary. b) it s moving upward at a steady speed of 5 m/s. Physics 111, Concordia College A wooden box, with a weight of 220 N, is pulled at a constant speed with a rope that makes an angle of 25 with the wooden floor. What is the tension in the rope? Physics 111, Concordia College A rod is suspended by a string as shown. The lower end of the rod slides on a frictionless surface.

5 Which figure correctly shows the equilibrium position of the rod? Physics 111, Concordia College A rod is suspended by a string as shown. The lower end of the rod slides on a frictionless surface. Which figure correctly shows the equilibrium position of the rod? Physics 111, Concordia College A ball weighing 50 N is pulled back by a rope to an angle of 20 . What is the tension in the pulling rope? Physics 111, Concordia College Dynamical Systems Physics 111, Concordia College 2nd Law Hints Draw a free body diagram of the object. Find the components of the forces. Determine if any of the acceleration components are known. Write out the 2nd law in component form. Physics 111, Concordia College A sled with a mass of 20 kg slides along frictionless ice at m/s.

6 It then crosses a rough patch of snow which exerts a friction force of 12 N. How far does it slide on the snow before coming to rest? Physics 111, Concordia College Macie pulls a 40 kg rolling trunk by a strap angled at 30 from the horizontal. She pulls with a force of 40 N, and there is a 30 N rolling friction force acting on trunk. What is the trunk s acceleration? Physics 111, Concordia College Mass, Weight and Apparent Weight Physics 111, Concordia College Mass and Weight w = may = m( g) w = mg Physics 111, Concordia College Mass and Weight Units An object with a mass of kg has a weight of Newtons on the surface of Earth. Newtons is the equivalent of lbs. kg is the equivalent of x 10-2 slugs.

7 MKS English Mass kilogram (kg) slug (sl) Weight Newton (N) pound (lb) Physics 111, Concordia College Find the x- and y-components of w in each of these three coordinate systems. Physics 111, Concordia College A 75 kg skier starts down a 50-m-high, 10 slope on frictionless skis. What is his speed at the bottom? Physics 111, Concordia College The same burglars push the 1000 kg safe up a 20 frictionless slope with a horizontal force of 4000 N. What is the safe s acceleration? Physics 111, Concordia College Burglars are trying to haul a 1000 kg safe up a frictionless ramp to their getaway truck. The ramp is tilted at angle . What is the tension in the rope if the safe is at rest?

8 If the safe is moving up the ramp at a steady 1 m/s? If the safe is accelerating up the ramp at 1 m/s2? Do these answers have the expected behavior in the limit 0 and 90 ? Physics 111, Concordia College Measuring Weight Physics 111, Concordia College Apparent Weight When a person stands on a scale the scale reads the normal force that the scale exerts on the person. If the person is not accelerating than the normal force equals the weight and the scale reads correctly. Physics 111, Concordia College Apparent Weight Physics 111, Concordia College Apparent Weight If the person is acceleration the scale will not read the correct weight. n is what the scale reads. This is not mg and is called the apparent weight.

9 MawnFy= = mamgn+=Physics 111, Concordia College Elevator Rides Consider a person standing in an elevator that is accelerating upward. The magnitude of the normal force on the person exerted by the floor of the elevator is A. larger than B. the same as C. smaller than the magnitude of the weight force of the person. Physics 111, Concordia College Elevator A 200 kg box is placed on a scale in a 2000 kg elevator. The elevator accelerates downward at m/s2. What does the scale read while the elevator is accelerating? Physics 111, Concordia College A 50 kg student gets in a 1000 kg elevator at rest. As the elevator begins to move, she has an apparent weight of 600 N for the first 3 s.

10 How far has the elevator moved, and in which direction, at the end of 3 s? Physics 111, Concordia College Friction Models Physics 111, Concordia College Types of Friction Static - object at rest with respect to the surface. Kinetic- object sliding on surface Rolling- object rolling along surface Physics 111, Concordia College Static Friction fs max = sn Physics 111, Concordia College Kinetic Friction fk = kn Physics 111, Concordia College Friction Physics 111, Concordia College Sliding Away An object is held in place by friction on an inclined surface. The angle of inclination is increased until the object starts moving. If the surface is kept at this angle, the object A.