1) READ , fill blanks and draw the situation (word bank direction, friction, magnitude, air resistance, mass, tension, normal force, weight, inertia/ REMEMBER: IF AN OBJECT IS AT REST OR IF AN OBJECT IS MOVING AT A CONSTANT SPEED, THAT MEANS: THERE IS NO NET FORCE ACTING ! IF FORCES ACT ON HTE OBJECT, THEY MUST BALANCE EACH OTHERS ! (long the vertical or horizontal)
FORCES
CAN BALANCE EACH OTHER. A book on a table is at rest, not moving, even
if 2 forces are acting on it: the normal force and the weight/ They
balance each other, net force = 0. A car moving at a constant speed
experiences at least 4 forces: weight = normal force and motrice force
= friction. The net force is 0.
misconception:
" Force is action " WRONG. If the net force is 0 there is no acceleration. The object moves at constant speed or stays at rest. A)
A book on a table is at rest. The weight is balanced by
________________. The 2 forces have the same ___________ but
opposite ___________. B) A hamster, dropped from the windown, is
falling at a constant speed. This is because its weight is balanced by
_________________. C) A car is moving at a constant speed, this is
because, along the horizontal, the force of the engine is
exactly balanced by the ____________ between the road and the wheel. Along the vertical, the ____________ balanced the ____________. D)
You are holding a mouse by its tail, ready to drop it to check if I was
right. (from the 3rd floor). the mouse is so scared, it does not move. Its body (not including the tail) is at rest because the _________ from the tail is balanced by its ________. E) you are pushing very hard on an elephant, trying to move it because it eating your flowers. The elephant is not moving. This is because, along the horizontal, your push is exactly balanced by the _________. This force is large because the _________ of the elephant is large. More pressure on the ground means more friction. We say an elephant has a large ____________. Along the vertical the __________ of the elephant is balanced by the __________.
2) Fill the blanks. banks: friction - balance - accelerate (F=ma) - surfaces - motion
When we attempt to slide or roll two objects across each other, a force that resists the ________ is produced. The force is called friction.
Friction can be caused by the irregularities of the two __________
sliding or sliding across each other. However, if you polish the
surfaces, friction will decrease only to a certain point. If the surfaces are too smooth, friction will increase again. (electric force will hold the molecules of the surface together).
The force of the engine has to balance the _________ forces between the tires and the road.
If
they each_________ each other, the car moves at a constant speed. If
the force of the engine is greater, you ____________ according to
Newton’s second law. (accelerate ? decelerate ? move at a constant speed ?) Actually, there is also air resistance that slows down the car and has to be counter balanced by the engine too/
3) Read and fill blanks. There are 2 kind of frictional forces. Let's say you are pushing on an object, first lightly then harder and even harder. The
object is not moving (large mass). This is because the frictional force
between the object and the ground is also increasing as you _________ and is balancing your
push. This is called the static frictional force. Static because
____________________________. At some point, the object moves and the frictional force decreases slightly. This is called the kinetic frictional force. The static friction is larger than the kinetic friction. COMMENT THE FOLLOWING COMIC (from DL)
4) HERE WE WILL MOSTLY study kinetic frictional force or just the friction force FK. YOU just learnt about linear relationship. When you move an object along a flat surface, THE RELATIONSHIP BETWEEN THE FRICTION FORCE FK and THE WEIGHT W IS LINEAR!!!!
so FK = u Wu is a constant and is called the coefficient of friction.
(the slope of FK vs W ) So the friction is proportional to the ________________ and the constant of proportionality is ___________.
DO THIs LAB TO FIND tHE COEFFICIENT OF FRICTION between a block and the table.
5) A) In the following picture what are the vectors representing? (someone is pushing a big object)
B) What is the net force acting Fnet on the chair along the horizontal? Fnet = ________ @ ________ Hint: Fnet = P + FK (signs!)
C) How do you know that the normal force FN = weight FW ?
D) Find FN the normal force if m = 50kg. (hint: weight = mass x 10) E)Find the coefficient of friction uK Hints FK = uK W solve for uK
6) Observe the picture:
Look at the following picture.
A) what is the net force Fnet? Fnet = ____ N @ _______
Hints: forces are vectors, they have direction. Left = negative and right = positive. Fnet = F + f
B) find the weight = ____N @ _____
C) What is the coefficient of kinetic friction uK ?
Hints: write f = uK W
D) if Fnet = m x a where m (kg) is the mass of the object and a its acceleration (m/s/s). What is the acceleration of the block ? Find the magnitude and direction.
7) Consider the following picture: (a block is pushed along a table @ left. there is friction between the table and the block)
the friction force f = _____ @ right the push F = ______ @ ___________ so the Fnet = ________ @ __________ If the mass of the block is 1 kg, weight = _______N @ _______ The coefficient of friction is ________. The normal force (balancing the weight) is _____N @ _________ and if Fnet = m a, (Newton;s second law, coming up soon) where a is the acceleration of the block, find a. (m/s/s)
9) A force of P= 170N@right is needed to keep a W= 530N wooden block sliding on a wooden floor. A) what is the friction force ? FK = ____N @ _______ (if the block moves at a constant speed, push = friction, they balance each other)
B) What is the coefficient of kinetic friction ? (hints: use Fk = uK W)
10) A cart on wheels weighs W = 2400N. The coefficient of kinetic friction between the wheels and floor is u=0.16. What force P is needed to keep the cart rolling uniformly ? Hints: there is no acceleration so forces balance each other. That means Push = FK and FK = uK Wsolve for P = FK.
11) A piano weighs W = 4700N. What force P
is needed to start the piano rolling across the floor when the
coefficient of static friction is us = 0.23 ? Use FS = uS W (static frictional force is also proportional to te weight W , the slope is us) Hints: there is no acceleration so forces balance each other. That means P = FS = frictional force .
12) A snowmobile is pulling a large sled across a snow-covered field. The weight W of the sled is 3560 N. If the coefficient of friction of the sled is uK= 0.12, what is the pulling force supplied by the snowmobile ? Hints: pulling P = frictional force so P = µk W.
13) 2 workers push in the same direction on a large pallet. The force exerted by one of the worker is F1 = + 645N. The force exerted by the other worker is F2 = 755N. The motion is opposed by a frictional force of f = -1175 N. Find the net force. Hint: Fnet = F1 + F2 + f
14) Friction is a force that always acts parallel to the surface in contact and opposite to the direction of motion.
So in that case, can you draw the frictional force acting on the skier sliding on the incline plane ? And also draw any other forces acting on the skier. Altogether, there are 4 forces. (see example with car above and comment) Which force helps the skier going down ? What is the force holding him back?
15) A 60kg crate rests on a level flour at a shipping dock. The
coefficient of static and kinetic are 0.760 and 0.410 respectively. What
horizontal pushing force is required to (A) just start the crate moving
and (B) slide the crate across the dock at a constant speed. hint: weight = mass x 10 the static frictional force is the force that opposes your push while the object is not moving but about to Fs = coeffs x weight the kinetic frictional force balnces your push when it is moving Fk = coeffk x weight
16)
A freezer weighs 206 lb. A person is attempting to push the freezer
across the roo m with a horizontal force of 60lb . but the freezer does
not move. (A) what is the static frictional force that the floor exerts
on the freezer ? (B) What can be said about the coefficient of static
friction.
17) EXTRA CREDITS When an object moves the frictional force opposes the motion if the object slides along a surface. But the frictional force is not the only force that opposes the motion. Air resistance also opposes the motion. That;s why race car have an aerodynamic design, to minimize the air resistance effect. source: race car and aerodynamics The frictional force is proportional to the weight. But air resistance, if you remember, depends not only on the surface area exposed to air (parachute) but also on the speed the object is moving. Faster the object is moving, more air resistance. In fact, the air resistance increases with the speed squared. So if the speed is multiplied by 2, the air resistance is multiplied by 4 !!! that's
why meteorite can be vaporised when entering the atmosphere. As they
speed up, attracted by gravity (F= ma , speed keeps increasing) air
resistance (and therefore friction) increases tremendously. (rub
your hand together to see what happends when 2 objects are rubbed like air and a meteorite)
A) Get your TI. use the key Y write Y1 = 2x2 2nd mode (to quit) ZOOM 6 Do you know this graph ? The relation is said quadratic. If x is multiplied by 2, y is multiplied by _____ is x is multiplied by 3, y is multiplied by ______, if x is multiplied by 4, y is multiplied by _________ y is proportional to the _________ of x
B) The table shows the air resistance (called friction here) on a bicycle at different speeds:
x= speed (m/s)
0
1
2
3
4
5
y = Friction (N)
0
10
30
70
130
200
Using your TI plot these points. steps: erase any expression in Y (Y1, Y2 ...) 2nd MODE to quit STAT EDIT enter x-values in L1 and y-values in L2 2nd MODE to quit 2nd Y and turn on PLOT1 then 2nd MODE (to quit) ZOOM 9 to plot it IS it a line ? or a parabola ? So the relationship between air resistance and speed is _____________ (see intro) That means that air resistance (y) is proportional to the _______ of the speed (x) and not just proportional to the speed. This
is always the case. if a car accelerates from 20mph to 40mph, the speed
is multiplied by _____ but air resistance is multiplied by
_______. If you accelerate from 20mph to 60mph, the speed is mulitplied by ____________ but air resistance by _____________ When
a object fall, air resistance depends not only on the surface area
(parachute opened or not) of the obejct but also on the _____ of
its speed. So as you sky dive falling to the earth, your speed increases and so does ______________. Air resistance slows you down and hopefully, you will reach the terminal _____________.
C) using your graph : How fast the cyclist travel if she exert a forward push of 180N ? (about) suppose the person is moving at a given speed. (constant speed, no acceleration so the forces balance each other). (hint : that means the push and the friction balance each other. P = friction = 180) (hint: you can graph it by hand and find for which x you have y = 180 or you can enter Y1 = 180 and find the interesection with the graph by using trace and play with up and down arrow)
D) When the cyclist lays ahead lower, she travels faster. EXplain why.
18) The relatioship between air resistance (drag force) and speed could be modeled by : Fd = 1/2 d A Cd V2 d
is the density of the air, A is the surface area of the car, Cd depends
on the aerodynamics of the car, better is the aerodynamics smaller is Cd. V is the speed of the car. For a given a car you could have F = 0.342 V2 GRaph this relationship with force betweem 0 and 40N and speed between 0 and 40mph Can you find the inverse function , V as a function of Fd ? (solve for V)
