Any force that pushes something towards the centre of a circle is called **centripetal force**, **F**_{c}. This is the force that keeps something in circular motion. Centripetal Force is a “pseudo-force” in that there is always some other real force that is pulling the object in. Think of it as a way of describing or classifying other forces. Possible centripetal forces include gravity, tension, friction, the normal force, electromagnetic force.
All centripetal forces are also equal to F_{c} = ma_{c} which gives
Example: (1) How fast can you swing a 600g mass around your head on a 1.1m long string that will break when F^{T} = 40 lbs?
What is the frequency of rotation? How long does it take for 1 cycle?
(2) The moon completes one around the earth in 27.3 days. How far away is the moon? m_{E} = 5.98E24 kg
Homework: Nelson: p 126 #5 – 10
Giancoli: p132 #1, 5, 7, 12
*NOT MUCH ABOUT GRAVITY – IT WILL ALL BE DONE IN THE FIELDS UNIT
## II. Cars Going around curves
For problems involving cars going around a curve, we use centripetal force and another (real) force.
F_{c} = mv^{2}/r
The car will “want” to continue moving in a straight line, so there must be a force that pulls it in a circular path.
There are typically two forces that can do this on cars.
1. Consider a flat, unbanked, curve. The force of friction (static friction since nothing is slipping) between the tires and the road is the centripetal force keeping the car on the circular road.
(How does this work? Think of a rollerblader – to go into a left turn he pushes his feet to the right: outward. The force of friction with the road causes the road to push him to the left.)
In this case, F_{f} mv^{2}/r Which way does the > < sign go? You can have an enormous force of friction and it won’t affect things, but if it is too small, then the car will slide.
The meaning of mv^{2}/r is that this is the **minimum **force needed for a car to be going in a circle.
2. Consider a banked curve. This is very nicely worked out in the Nelson 12 textbook (section 3.2).
If we ignore friction, then the force that is keeping the car in a circular path is the component of the normal force (from the road (perpendicular to the surface remember)) that points towards the centre of the curve.
You can calculate what this component is. (some function involving force of gravity and sin or cos).
This component is then mv^{2}/r.
**Sample Problems:**
1. A flat unbanked curve. *Let’s make up a problem.*
*What are the variables? *mass, co-efficient of friction, speed, radius of the curve.
*Which three shall be given? Which one shall we solve for?* r.
*Let’s pick some numbers:*
m = 9,000 kg (a transport truck?), = 1.5, v = 90 km/hr = 25 m/s,
r = ? (I would guess a fairly wide curve – like 20 m).
*I’ll write this as a word problem:* What is the smallest radius of curvature that a 9000 kg truck can navigate at 90 km/hr if the coefficient of friction between the tyres and the road is 1.5?
Ff = F_{N} and F_{N} = Fg This frictional force is the centripetal force
= 1.5 (9000kg)(9.8 N/kg) F_{c} = mv^{2}/r
= 132,300 N Ff = mv^{2}/r
mg = mv^{2}/r
solving for r: r = v^{2}/g
Hmm… by doing it algebraically, we see that the mass cancels out. You wouldn’t notice this if you just started putting numbers in. Note that this problem does not consider the truck going so fast that it flips over. We are only considering sliding sideways off of the curve.
r = (25m/s)^{2} / (1.5)(9.8 m/s^{2})
= 42.5 m
2. A banked curve with no friction (i.e. with a sheet of ice on it – like a bobsled run – if it is curved it can still keep the vehicle on the track.
*Let’s make up a problem.*
*What are the variables?* mass, angle banked, speed, radius of the curve.
*Which three shall be given? Which one shall we solve for?*
*Choose some numbers: *m = 1000 kg, r = 15 m, v = 60 km/hr
*Write a word problem with this:* At what angle should a 30m diameter curve be banked for a 1000 kg car to be able to stay on the road at 60 km/hr assuming icy conditions?
In this case it works best if we use x and y in the horizontal and vertical directions (because Fc is horizontal to the centre of the curve).
*** x is in the direction of the net force (Fc)
We can see two important things:
Fc = F_{Nx} and F_{Ny }= Fg
Draw a force triangle for F_{N}
With some simple geometry, we can prove that the angle at the bottom of the triangle is - the same as the ramp angle.
From trig. we see that
F_{Nx} = F_{N} sin and F_{Ny} = F_{N} cos we won’t need these relationship unless we need to find F_{N}
also: tan = F_{Nx} / F_{Ny}
If we substitute in from above … tan = Fc / Fg tan = (mv^{2} /r ) / mg
.: tan = v^{2} / rg Note that we never had to calculate values for F_{N} or F_{Nx} (F_{Ny} = Fg)
Notice that the mass cancels out here too!
With numbers: tan = (16.7m/s)^{2}/(9.8 N/kg)(15m)
= 62.21 degrees.
In real life this angle would be less because friction also keeps the car on the road, and there would be speed reduction signs for such a tight curve.
__Homework:__
Giancoli p 132 # 8,9 (fairly simple) p 132 #17,18 are complicated – they combine friction with a slope. We won’t be doing this in this course. AP physics students should do these two questions.
Students think that centripetal acceleration means that the speed is always increasing.
Constant speed in a circle = no acceleration!
But **acceleration** implies that **either** speed **or** direction changes (or both).
**Example:**
“A car is going around a curve (r=30m) at a speed of 60kph. Will it slide off the road if =1.1 ?
(assume the mass is 1234 kg).
When you have a problem like this, you have too much information. What is the unknown that you are trying to find? Make one of the known numbers something that you don’t know, and solve for it, then compare the answer to the number in the question in a logical fashion.
e.g. How fast can the car go around a 30m curve with =1.1?
…
by doing this algebraically first, you can see that the masses cancel out.
v = 64 km/ph
Answer to original question: YES. The maximum speed is 64 kph, and you’re only going 60kph.
Method 2:
Find the force of friction that the car has Ff = F_{N} … = 13316N
Find the centripetal force needed to keep the car I na circle at that speed and radius
Fc = mv^{2}/r … = 11426N
Yes, there is enough friction to keep the car on the road.
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