Download Newton`s Laws and The Force

Newton’s Laws and The Force
Part I: Multiple Choice. Circle the letter corresponding to the BEST answer for each
question. (1 pts each, 10 pts total)
1. A scale measures your weight, which is to say the force exerted on you by the floor
below you. Given this information, what would the scale read if you were in an elevator
moving upward at a constant velocity? Compared to your normal weight, your elevator
weight would be:
a) the same
b) greater
c) less
d) zero
2. In the same problem as above, the elevator cable has broken and it is now accelerating
downward at 9.8m/s2. Your elevator weight would be:
a) the same
b) greater
c) less
d) zero
3. Which physical concept is BEST illustrated by the example of a waiter jerking a
tablecloth out from underneath a place setting, leaving the silverware and dishes in their
place?
a) friction
b) N’s 1st law c) N’s 2nd law d) N’s 3rd law e) potential energy
4. What, roughly, does an apple weigh in newtons?
a) 40-50 N
b) 10-25 N
c) 5-10N
d) more than 50 N e) less than 5N
5. Why does your foot hurt when you kick a walrus (as opposed to, say, a squirrel)?
a) friction
b) N’s 1st law c) N’s 2nd law d) N’s 3rd law e) potential energy
6. What is the weight of a 10 kg pickle on the moon (where the gravity is 1/6th that of the
Earth)?
a) 6 N
b) 60 N
c) 1.6 N
d) 16 N
e) 160 N
7. Which combinations of masses and separation distances would result in the largest
gravitational force between them?
a) 2 kg, 1 kg, 2 meters apart b) 2 kg, 2 kg, 1 meter apart
c) 2 kg, 1 kg, 1 meter apart d) 4 kg, 1 kg, 2 meters apart
8. When a woolly mammoth is moved from sea level to the top of a high mountain, what
changes? The mammoth's ________ change.
a) mass and weight
b) mass only c) weight only d) neither
e) force
9. Which of the following COULD be an example of equilibrium (only one):
a) a falling mountain goat without air resistance
b) a barn swallow banking through a tight turn at constant velocity
c) a leopard pouncing from rest
d) a goose skidding to a stop on the surface of a lake
e) ) a flying squirrel falling with air resistance
10. A rock is suspended from a string and moves downward at constant speed. Which
statement is true concerning the tension in the string if air resistance is ignored?
a) It is zero. b) It points downward. c) It is equal to the weight of the rock.
d) It is less than the weight of the rock.
e) It is greater than the weight of the rock.
Part II Questions: Answer any three of the questions below. You should as complete an
answer as possible. (5 pts each 15 points total)
11. Why do you push harder on a bicycle’s pedals when first starting out than when
moving at a constant speed? Be sure to explain both parts: the getting going and the
moving along at constant speed.
12. When driving on slick roads, why is it advisable to apply the brakes slowly? Explain
your answer using concepts of static and kinetic friction, deceleration, and stopping
distance.
13. A person hoists a block by pulling down on a rope that runs over a pulley before
attaching to the block. The person is on the ground, the block in the air. Draw a picture
that includes circles around two sets of 3rd Law pairs of forces. Make sure each force in
the pair is clearly labeled.
14. Write down Newton’s 3 laws and give a brief explanation or example of each. I need
more here than just a restatement of the 3 laws!
15. Define both weight and mass. How does mass differ from weight? If it’s possible,
how can you make an object weightless and/or massless? How do we measure both
quantities?
Part III: Problems. Do any 3 of the following. 5 points each, 15 total. Generally
speaking each problem will be grades as follows (some are a little different):
-1 point for a free body diagram which correctly shows and labels all forces acting on the
object(s).
-1 point for correctly filling in the values for the force chart.
-1 point for correctly writing down the equations that result from the force chart.
-1 point for solving the equations and getting the correct answers to the questions.
-1 point for including the correct units for the answer.
17. A young girl is pulling a toy car horizontally with a force of 165 N. Her older and
taller brother pulls the toy with a force of 225 N at an angle of 30 degrees above the
horizontal in the opposite direction. The car has a mass of 14 kg. (a) Find the magnitude
of the normal force on the car. (b) Who wins the tug of war? Answer this by finding the
net force on the toy horizontally and noting both its magnitude and direction.
18. Consider the system shown with two blocks connected by a light
rope that runs over a massless pulley. The ramp is frictionless.
(a) In terms of mA, mB, , and g, find an equation for the
acceleration of the blocks. (They will both have the same
acceleration as they are linked by the rope.) (b) If the ramp
angle is 30º, what must be the mass of mB (compared
=38°
to mA) so that the acceleration of the system is
zero?
A
19. A pair of fuzzy dice is hanging by a string from your rearview mirror. While you
accelerate from a stoplight to 40 m/s (in 8 seconds), what angle  does the string make
with the vertical? To solve this find (a) the acceleration of the car/dice. (b) Now draw a
FBD, and make a force chart etc. (Are the fuzzy dice in equilibrium?) You will then be
able to find the angle.
20. Find the net gravitational force on the mass 2m (the mass is
labeled as “P”) in the lower left corner of the square. The square
has sides of L. Express your answer in terms of m, L, and G.
21. A lunar landing craft (mass=11,400 Kg) is about to touch
down on the surface of the moon, where the acceleration due to gravity (gm, you should
know this value from now on) is 1.60 m/s2. At an altitude of 165 m the craft's downward
velocity is 18.0 m/s. To slow down the craft, a retrorocket is firing to provide an upward
thrust. Assuming the descent is vertical, find the magnitude of the thrust needed to
reduce the velocity to zero at the instant when the craft touches the lunar surface.
B
22. The figure shows three crates with masses m1=45.2 kg, m2 = 22.8 kg, m3 = 34.3 kg
on a horizontal frictionless surface. a.) What horizontal force is needed to push the crates
to the right, as one unit, with an acceleration of 1.32m/s2? b.) Find the force exerted by
m2 on m3. c.) by m1 on m2.
M
M
1
3
M
2
F
Part IV: Extra Credit! You may do one of the following questions. Up to 5 points.
23. A snowboarder (dressed as a giant mouse – they’re always in costume) at Girdwood's
Slush Cup starts from rest and slides down a frictionless 15º hill a distance of 20 meters.
At the bottom of the hill is a 10 meter long pit of slushy ice water. Assume there is an
effective coefficient of friction between the snowboard and the water of µk = 0.9. Will
the boarder make it across the pit safely or will they stall before making it across? You
must back up your answer with equations.
24. A model rocket of mass 0.250 kg is launched vertically with an engine that is ignited
at time t = 0. The engine provides a force of 20 N that acts for 2 seconds until it runs out
of fuel. Upon reaching its maximum height, the rocket deploys a parachute and then
descends vertically to the ground. (a) Draw and label free body diagrams of the rocket for
the following situations: i) While the engine is firing, ii) after the engine stops, but before
the parachute is deployed, and iii) after the parachute is deployed. (b) Determine the
magnitude of the average acceleration of the rocket during the 2 second firing of the
engine. (c) What maximum height will the rocket reach?