Conceptual Physics Concept Test Questions Friction The following slides were created through a collaboration of the Industry Initiatives in Science and Math Education (IISME) and Stanford University

If we could eliminate that friction for her.. As Dora walks on the ground, the bottom of her shoes experience a friction force with the ground. If we could eliminate that friction for her.. a. she would stay in one place. b. she would walk as she normally does. c. she would be able to walk faster. Motivating Misconception: Friction always hinders motion.  Thus, you always want to eliminate friction. (Hapkiewicz 1992) Correct Answer: A Without friction, the Earth would not be able to push back on Dora to get her to move forward.  Friction is the resistive force that holds her foot fast to the Earth as she pushes on the Earth and the Earth pushes back on her. Related Topics: Newton's 3rd Law, friction. Image Credit: Dora image taken without permission from: dorathexplorertvshow.com

a. the snake would not be able to go anywhere. The motion that snakes make when moving from one place to another is called sidewinding. If we could eliminate the friction a snake encounters with the ground when sidewinding.. Motivating Misconception: Friction always hinders motion.  Thus, you always want to eliminate friction. (Hapkiewicz 1992) Correct Answer: A Slithering snakes, like walking people, must use the reaction force that the ground supplies to them when they use friction to push against the ground in order to move forward. Related topics: Newton's 3rd law, friction. Image credit: magma.nationalgeographic.com a. the snake would not be able to go anywhere. b. the snake would move as it normally does. c. the snake would be able to move faster than it normally can.

a. he would be able to move up the hill much faster. A bicycle rider is climbing up a gentle hill.  If we could eliminate the friction he encounters with the ground.. a. he would be able to move up the hill much faster. b. there would be no difference in his motion because the wheels are rolling. c. he would still go up the hill, but more slowly. d. he would not be able to go anywhere. e. he would slow to a stop, then eventually slide down the hill. Motivating Misconception: Friction always hinders motion.  Thus, you always want to eliminate friction. (Hapkiewicz 1992) Correct Answer:  E Without the reaction foce of friction with the ground pushing back on the rolling tires, the wheels would spin freely and gravity would be the dominant force, bringing the bicycle and rider back down the hill. Related Topics: Newton's 3rd law, friction, gravity Image Credit: Taken without permission from: evacassidy.com

A skydiver is falling with a constant velocity of 60 ms-1. Choose the correct list of forces acting on the skydiver. Motivating Misconception: Passive forces do not exist. (Brown and Crowder) Correct Answer: B If the skydiver has reached terminal velocity, the net force on him is zero.  There must therefore be an upward force that is equal and opposite to the force of gravity that would otherwise accelerate him downward. Related Topics: Gravity, net force, acceleration, air resistance. Image Credit: Skydiver taken without permission from bitsandpieces1.blogspot.com a. Gravity down only b. Gravity down and air pushing up c. Gravity down and air pushing down d. There are no forces on this skydiver.

b. He would slide for a long time, but eventually stop. Coach Diego Maradonna is known for getting easily excited.  Imagine if he were sliding on a frictionless pond of ice instead of on the soccer field. a. He would slide forever at constant velocity - or at least until he hit a tree. b. He would slide for a long time, but eventually stop. c. He would slide just a little farther than he does on the grass, but still stop. d. He would slide at constant velocity only if somebody kept kicking him from behind. Motivating Misconception: A force is needed to keep an object moving with a constant speed. (Hapkiewicz 1992) Correct Answer: A In a frictionless environment no force is required to maintain constant velocity. Related Topics: Newton's 1st law, velocity, friction. Image Credit: Taken without permission from coolmarriedguy.com

If it were possible for a sky diver to experience no net force during a fall, what would be the motion state of the sky diver? a. Not moving b. Slowing down c. Constant velocity d. Accelerating e. That's not possible Motivating Misconception: An object will slow down if there is no net force. (Brown and Crowder) Correct Answer: C This is the case when sky divers reach terminal velocity.  No net force simply means that the motion state will no longer change.  This is the case when an object has constant velocity. Relevant Prerequisite knowledge: Free fall, gravity, air resistance, net force. Image Credit: Taken without permission from  fotosearch.com

The friction between a sliding baseball player and the ground is from.. a. forces between atoms in the player and atoms in the ground that never touch each other. b. roughness in the player's uniform or body and roughness on the ground. c. irregularities that exist on every surface that catch each other when any two surfaces come in contact. Motivating Misconception: Frictional forces are only due to irregularities in surfaces moving past one another. (Hapkiewicz 1992) Correct Answer: A Electrostatic forces between atoms cause friction between surfaces in very close proximity to each other.  These forces are attractive and resist separation even in the smoothest of surfaces. Related topics: Friction, electrostatic force. Image Credit: media.mlive.com

If we could smooth these surfaces at the microscopic level.. A microscopic view of two apparently smooth surfaces in contact with each other shows that they are actually rough. If we could smooth these surfaces at the microscopic level.. a. friction would be greatly reduced. b. friction would be reduced slightly because surfaces that appear smooth to the naked eye already have very little friction. c. there might be even more friction. Motivating Misconception: Frictional forces are only due to irregularities in surfaces moving past one another. (Hapkiewicz 1992) Correct Answer: C Friction is a property that depends less on the roughness of a surface than on the electrostatic adhesion between any two surfaces.  Surfaces that feel and look smooth might adhere with incredible friction. Related Topics: Friction, electrostatic force. Image Credit: Friction image taken without permission from: hasdeu.bz.edu.ro

a. the same friction force acts on each of them. An elephant and a mouse are ice skating.  They have a contest to see who can glide the farthest before stopping.  Both of them start gliding with the same velocity. Assume that the skates all have the same coefficient of friction.  They tie because.. Motivating Misconception: Frictional forces are only due to irregularities in surfaces moving past one another. (Hapkiewicz 1992) Correct Answer: B A mathematical analysis of the situation shows that the mass from the kinetic energy term cancels with the mass in the work term in each equation. 1/2mv2 = umgd The solution for d is independent of m.  For a common coefficient of friction, u, and a common velocity, v, the solution for the distance will be identical.  Qualitatively, this is best described by choice B. Related Topics: Friction, Inertia. Image Credits: Elephant taken without permission from: imageenvision.com Mouse taken without permission from: supercoloring.com a. the same friction force acts on each of them. b. even though the elephant has more inertia, there is also more friction between her skates and the ice. c. the friction is the same between any pair of similar skates and the ice.

An object is held in place by friction on an inclined surface 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 the angle at which it began to slide, the object __________ 1. slows down. 2. moves at uniform speed. 3. speeds up. 4. none of the above Likely Misconception: When an object just barely overcomes friction it moves at constant speed. Answer: 3.  As the tilt of the surface is increased at a certain angle the object starts sliding. Until that angle is reached, the object is at rest, and the net force on it is zero. For the object to start sliding from rest, there must be a net force on it; if the net force is no longer zero, the object will accelerate. Relevant Prerequisite Knowledge:  friction, Newton's second law, gravitational force. Adapted from Eric Mazur’s Peer Instruction

A man is pushing to the right on a box, but it is not moving. Select the choice below that correctly identifies the forces on the box. a. Force of man pushing to the right only. b. Force of man pushing to the right and force of friction to the left. c. Force of man to the right and force of friction to the right. d. Force of friction only. e. There are no forces acting on the cube. Motivating Misconception: Passive forces don't exist (tables don't exert a normal force) (Brown and Crowder) Correct Answer: B If the box is not moving, the net force on the box must be zero.  Consequently the force with which the man pushes to the right must be matched by and equal force supplied by friction to the left. Related Topics: Net force, Newton's 3rd law, friction. Image Credit: Man taken without permission from: coreis.co.uk

You are pushing a wooden crate across the floor at constant speed You are pushing a wooden crate across the floor at 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 about ____________ as the force required before you turned it on end. 1. four times as great 2. twice as great 3. equally great 4. half as great 5. one-fourth as great Likely Misconception: Friction force is always proportional to surface area. Answer: 3. The force is proportional to the coefficient of kinetic friction and the weight of the crate. Neither depends on the size of the surface in contact with the floor. Relevant Prerequisite Knowledge: Concept of friction as retarding force, equation for friction. Adapted from Eric Mazur’s Peer Instruction