Frictional forces and FOR

Friction: joining the x and y axis When looking at an object that moves across a surface, we can draw a FBD that includes gravity and normal force in the picture When looking at an object that moves across a surface, we can draw a FBD that includes gravity and normal force in the picture

But how does it really affect motion? If you think logically about it, gravity, and normal force don’t contribute directly to the horizontal motion of an object on a surface – since the motion is ultimately peripindicular to the axis of motion If you think logically about it, gravity, and normal force don’t contribute directly to the horizontal motion of an object on a surface – since the motion is ultimately peripindicular to the axis of motion

Gravity  normal force  friction The connection between the y-axis forces in any object sitting on a surface is friction The connection between the y-axis forces in any object sitting on a surface is friction Since friction depends on a portion of normal force – which is determined by gravity – it is important to know how they relate to one in another in different questions Since friction depends on a portion of normal force – which is determined by gravity – it is important to know how they relate to one in another in different questions

Try to remember… What is friction? What is friction? How do we know it exists? How do we know it exists? What factors affect friction? What factors affect friction? How does friction affect motion? How does friction affect motion?

What is friction? Friction is generated by the interaction of the irregular surfaces of different objects that are in contact with each other Friction is generated by the interaction of the irregular surfaces of different objects that are in contact with each other Imagine a surface with golf-ball sized bumps interacting with one that has tennis-ball sized bumps Imagine a surface with golf-ball sized bumps interacting with one that has tennis-ball sized bumps

Irregular surfaces have raised and sunken surfaces Irregular surfaces have raised and sunken surfaces Two irregular surfaces in contact that try to slide across each other will resist motion because the raised surfaces fall into the sunken surfaces Two irregular surfaces in contact that try to slide across each other will resist motion because the raised surfaces fall into the sunken surfaces Therefore, additional energy is needed to move the push the raised surfaces out of the sunken surfaces Therefore, additional energy is needed to move the push the raised surfaces out of the sunken surfaces

How do we know it exists? Think about the process of pushing an object across the floor Why does the box come to stop without any visible force acting in the opposite direction? Which one of Newton’s Laws does this violate?

Friction comes into play Therefore, another force that is not inherently visible must be present to explain this – or else the phenomenon of sliding objects coming to a stop would violate Newton’s first law of inertia Therefore, another force that is not inherently visible must be present to explain this – or else the phenomenon of sliding objects coming to a stop would violate Newton’s first law of inertia

What factors affect friction? What do your car brakes do to your car? What do your car brakes do to your car? Why do car brakes work differently in the rain and snow? Why do car brakes work differently in the rain and snow? What does this tell you about the nature of friction? What does this tell you about the nature of friction?

Friction stops cars At least, let’s hope so or else it means that you’re slamming into something else At least, let’s hope so or else it means that you’re slamming into something else When you hit the brakes on your car, you are actually stopping your wheels from turning When you hit the brakes on your car, you are actually stopping your wheels from turning But you aren’t directly controlling the forces that cause you to come to a stop But you aren’t directly controlling the forces that cause you to come to a stop

Stopping the wheels Means that you are essentially allowing the rubber on your wheels to come into contact with the pavement Means that you are essentially allowing the rubber on your wheels to come into contact with the pavement The rest is dependent on the friction between the asphalt and the rubber The rest is dependent on the friction between the asphalt and the rubber So why do rain and snow affect braking? So why do rain and snow affect braking?

Change in surfaces When it rains or snows, the nature of the road surface changes When it rains or snows, the nature of the road surface changes The bumpy surfaces on the asphalt are not as accessible anymore – because they are covered by another substance with a different surface The bumpy surfaces on the asphalt are not as accessible anymore – because they are covered by another substance with a different surface Therefore, the contact surfaces change, changing friction Therefore, the contact surfaces change, changing friction

Friction depends on surfaces Therefore, the amount of friction you can generate is dependent on the types of surfaces that are in contact Therefore, the amount of friction you can generate is dependent on the types of surfaces that are in contact

How does friction affect motion? So how much frictional force is generated to oppose motion? So how much frictional force is generated to oppose motion? Think about moving two identical cardboard boxes across the same floor Think about moving two identical cardboard boxes across the same floor One has a greater mass than the other One has a greater mass than the other Which one is easier push across the floor? Which one is easier push across the floor? Why? Why?

Remember – I didn’t ask you to lift them up Newton’s second law states that the heavier one would be harder to move Newton’s second law states that the heavier one would be harder to move Is that related directly to the mass if we are looking at horizontal motion? Is that related directly to the mass if we are looking at horizontal motion? Why is a heavier object harder to slide across the floor? Why is a heavier object harder to slide across the floor?

Normal force Remember – the equation for the calculation of friction includes normal force Remember – the equation for the calculation of friction includes normal force Therefore, frictional force is a fraction of normal force Therefore, frictional force is a fraction of normal force The amount of friction generated is affected by the normal force acting on the box The amount of friction generated is affected by the normal force acting on the box

Normal not gravitational Be aware: normal force is NOT always equal to gravitational force! Be aware: normal force is NOT always equal to gravitational force! You must be careful when calculating questions and be aware of this! You must be careful when calculating questions and be aware of this!

For example: which one of these situations can you use F f = F N µ?

Mass lifted upwards by a crane

Coin sliding upwards against a wall

How does friction affect motion? Think about the objects in these two situations – they are both at rest Think about the objects in these two situations – they are both at rest In which situation is the force of friction present? In which situation is the force of friction present?

Static friction vs. kinetic friction Static friction: Static friction: Prevents the initiation of movement Prevents the initiation of movement Kinetic friction: Kinetic friction: Prevents motion Prevents motion Which one always has a greater value? Which one always has a greater value? (Think about accelerating a car vs. keeping a car moving at a constant speed – which one requires more force?) (Think about accelerating a car vs. keeping a car moving at a constant speed – which one requires more force?) Static friction is always greater Static friction is always greater

The coefficient of friction What exactly is µ as a measurement? What exactly is µ as a measurement?

To summarize: Friction: Friction: Always opposes motion Always opposes motion Is dependent on the surfaces in contact and on normal force Is dependent on the surfaces in contact and on normal force Static friction is greater than kinetic friction Static friction is greater than kinetic friction

Let’s see what you know A force is applied to an object that sits on a surface, but it doesn’t move. A force is applied to an object that sits on a surface, but it doesn’t move. Is friction present? Is friction present? What is the value of the applied force compared to the static frictional force? What is the value of the applied force compared to the static frictional force?

Frames of reference and force questions Since we are dealing now with dynamics – you may come across questions that are contained within non-inertial frames of reference Since we are dealing now with dynamics – you may come across questions that are contained within non-inertial frames of reference Anyone remember what that means? Anyone remember what that means?

How would you explain the motion of the ball?

Non-Inertial FOR To explain the motion of objects in non-inertial FOR, you may have to define a force that is not obviously visible in the FOR that you are viewing the event in order to keep the event in line with Newton’s laws To explain the motion of objects in non-inertial FOR, you may have to define a force that is not obviously visible in the FOR that you are viewing the event in order to keep the event in line with Newton’s laws This is similar to Newton’s definition of gravity (things fall – but where’s the force?) This is similar to Newton’s definition of gravity (things fall – but where’s the force?) Notice that although we can describe gravity mathematically and predict what it will do – Newton’s descriptors of gravity didn’t give us an explanation of what it actually IS Notice that although we can describe gravity mathematically and predict what it will do – Newton’s descriptors of gravity didn’t give us an explanation of what it actually IS