a) The slingshot’s speed
2. What is the formula for constant speed? Speed = distance / time
3. Symbolically, what is the distance around a circle? the circumference(2pr)
4. What is the formula for the speed if one is moving around in a circle? Speed = 2pr / time
5. What is the formula for the centripetal acceleration? ac = v2 / r
6. What does centripetal mean? Center seeking
7. What is the formula for centripetal force? Fc = mac or Fc = mv2 / r
8. A person swings a 0.65 kg rock in an old fashion slingshot around
in a vertical circle. The length of the slingshot is 0.85 m. If the person
swings the rock around once in 0.55 seconds, determine:
a) The slingshot’s speed
b) The slingshot’s tension when the rock is at the bottom of the
swing
c) The slingshot’s tension when the rock is at the lowest point of
the swing
9. Suppose the same person swung the rock around horizontally at an angle of 20 degrees below the horizontal.
Draw the free body diagram of the rock in a sling below
How many forces are in the y? Two
Is it moving in the y direction? No
What condition is the y direction in? Equilibrium
How many forces are in the x direction? one
What part of the circle does the horizontal force point? towards the center
What does the horizontal force equal? the centripetal force
Write an equation in the y direction. Tsinq = mg
What can you solve for? Solve for it! Tension! T = mg/ sinq = 18.6 N
Write an equation in the x direction. TCosq = Fc
Solve for the radius of the circle using Geometry then solve for
the velocity of the rock
What is the basic formula for work? Work equals Force times displacement
What one thing must you remember conceptually about the formula above? Force and displacement MUST be on the same axis
Can work be negative? If so, the force must be what? Yes, it could be friction or any other resistive force
If the force is at an angle, and the displacement along the + X:
What must you do to the force before you apply the formula?
You must break it into components and use
the horizontal component in the formula
A man pushes down on a lawnmower with a force of 50 N at 55 degrees
above the horizontal along the +x.
a) If the displacement is 6 m, how much work does the man do on
the lawnmower?
Work equals a change in what? Kinetic energy (DKE)
b) If the lawnmower starts from rest, how fast is it moving at the
end of the 6 m if its mass is 100 kg?
A crane pulls a 250 kg box 35 m upward with a constant speed in 5.5 seconds.
a) What is the formula and units for Power? Power = Work divided by Time or force times velocity; Power is measure in WATTS
b) What is the force in this case? The box's weight
c) Solve for Power.
What is the formula for potential energy? PE = mgh
What is the formula for kinetic energy? KE = 0.5mv2
What is the unit for energy? JOULES
Suppose the crane in the problem above has a giant 400 kg ball at
the end of a 15 m cable.
If the ball is above the ground in such a way that the cable makes
an angle of 25 degrees with the vertical, determine:
Draw a sketch of this situation below
What do you have to subtract to find the height? L - LCos 25 = h
Solve for the height 15 - 15Cos25 = h =1.40 m
What kind of energy does it have at this point? Potential
If it suddenly was released, what kind of energy would it have at the bottom? Kinetic
Solve for the velocity at the bottom
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What is the formula for momentum? p = mDv
Symbolically, show the conservation of momentum formula below
p(before) = p(after)
What would the formula look like if you had an ELASTIC collision
between 2 objects
m1vo1 + m2vo2
= m1v1 + m2v2
What would the formula look like if you had an INELASTIC collision
between 2 objects
m1vo1 + m2vo2
= mtvt
What would the formula look like if you had 1 object that split into
2 objects.
mtvt = m1v1
+ m2v2
A 675 kg car traveling at 15 m/s strikes another car at rest. The
second car ( 525 kg) moves at what velocity if the 1st car comes to rest?
What velocity would BOTH cars move had they become connected?
A loaded cannon with ball has a mass of 250 kg. What is the velocity
of a 5 kg ball if the cannon moves backwards at 0.5 m/s?
A 145 kg linebacker moving at 1.5 m/s to the west strikes a 165
Kg runningback who momentarily stops before the collision. The linebacker
moves at 32 degrees N of W after the collision and the runningback moves
at 66 degrees S of W.
Draw the sketch for this situation below
Due to the angles you must use the what to solve this problem?
You must use the components!
Write an equation for the X direction
see pic below
Write an equation for the Y direction
see pic below
Solve simultaneously for the velocity of each player
see pic below
