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# Concept Day

### FRICTION-1-1

• In this investigation, we will review several physics concepts that you have already encountered from time to time in LabLearner. Specifically, we will review velocity and acceleration.

Note: Throughout this CELL on Friction, you should always be able to identify all of the forces acting on an object. As you perform your experiments in Lab, it may be a good idea for you to stop[ and think about all of the forces acting on an object you are testing.

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### SLIDE FRICTION-1-2

• In this slide, we take a quick look at the representation of forces on a two-dimensional surface.

Note: You might notice that the concept of arrows representing both direction and magnitude is essentially an introduction to vectors. However, we will not develop this concept much further in this CELL.

• In the first illustration, the block would move to the left, since the arrow applying force to it from the right is longer than the arrow applying force to it on the left.
• In the second illustration, the block would not move as both arrows are pushing on the block with the same force but in the opposite directions. This is like pushing your two hands together in front of your chest with the same force. Pushing harder with your right arm will force your hands to your left. Conversely, pushing harder with your left arm will force your hands to your right.

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### SLIDE FRICTION-1-3

• In this slide, we are reviewing Newton’s Three Laws of Motion. The first two laws are particularly relevant to this CELL.
• Newton’s first law, which states that an object will remain at rest or move at a constant velocity unless acted upon by an external force, comes up in the very first Trial in Investigation 1.

Note: This is the subject of the next slide.

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### SLIDE FRICTION-1-4

• This slide is similar to Trial 1 in the experiment you will perform in Investigation 1 Lab.
• Newton’s first law would predict that once the coin is pushed with a finger (providing the initial force to overcome the coin’s state of rest), the coin should continue at a constant velocity forever unless some other force acts on it to slow it down and stop it.
• Why do you think the coin slows down and comes to rest.
• Could it be that some other force acts on the coin? That is correct. This force is friction.

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### SLIDE FRICTION-1-5

• This slide introduces the concept of acceleration.

Note: Students often think of acceleration as strictly a force involved in “going faster”. However, negative acceleration is every bit as important as a concept in explaining movement.

• Using Auguste Rodin’s “The Thinker” as a test subject, we essentially mimic what would happen to him if sitting in a vehicle undergoing different types of acceleration.
• The picture on the left depicts constant velocity, a condition under which acceleration would be zero.
• The representation of negative acceleration, in the middle picture, shows the response of a body moving a constant velocity that suddenly experiences negative acceleration.
• This would be analogous to stepping on the brake in a moving car.
• There is an immediate change in velocity, the vehicle slows down, and there is a tendency for the subject (the Thinker in this case) to continue moving at the previous velocity and direction, which is forward.
• This, of course, is the reason we wear seatbelts in cars.
• Positive acceleration is depicted in the final picture in this slide.
• The analogy here would be to suddenly step on the gas pedal (the accelerator) when traveling at a constant velocity.
• The tendency of the passenger’s body is to be thrown back into its seat.
• This is the type of acceleration that most people think of when they think of acceleration.

Note: It is interesting to note that in the first picture on the left, the passenger is moving at zero acceleration and maybe somewhat oblivious to the velocity at which they are moving. Once constant velocity is achieved, the passenger feels little effect of the forces moving them, regardless of the speed at which they are traveling. This is as true on a commercial aircraft traveling 600+ miles per hour as it is when cruising through a residential area in one’s car at 20 miles per hour. However, any change in velocity leading to either positive or negative acceleration is immediately detectable.

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### SLIDE FRICTION-1-6

Note: This slide, previously seen, asks you to explain the coin push in terms of acceleration.

• At first, the coin is at rest. Therefore, a positive acceleration is experienced when it is initially pushed.
• Once pushed, the kinetic energy transferred from the finger of the pusher to the coin keeps the coin moving at a certain velocity that will experience negative acceleration the moment it loses contact with the finger.

• On a flat surface, it is not possible for the coin to experience additional positive acceleration. Rather, from the moment of release, the coin will experience negative acceleration. It will do so, reducing its velocity until it comes to a full stop when it lacks kinetic energy, and is a rest once again.

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### SLIDE FRICTION-1-7

• Velocity is described as a vector in this slide.
• If we consider constant velocity in a single direction, velocity is very much like speed, which is expressed as distance divided by time.

Note: Velocity is displacement divided by time. In the first example on this slide, if the ball rolled to the 2-meter position and then back to the 1-meter position over the 6-second time interval, the total displacement would be only 1-meter and the velocity calculation would give a different answer.

Note: At this point, there is no need to differentiate between distance and displacement for our discussions and experiments on friction.