Download 4.1 answers

Honors Physics Wks 4.1: Force as a Vector
4.1A:
1) Roy pulls a 25-kg box from rest, using a rope which makes an angle that is 35 o above the horizontal, along a horizontal
driveway. The force used to pull the crate is 150 N. If the coefficients of static and kinetic friction are 0.40 and 0.25
respectively, then find the velocity of the crate 4.0 seconds after Roy began pulling it. Make a free body diagram too.
(13 m/s)
2) A 40.0 kg crate is pulled across the ice with a rope. A force of 100 N is applied at an angle of 25o above the horizontal.
a) Assuming there is no friction, and draw a free body diagram for the crate. Find the acceleration of the crate. (2.27
m/s2)
b) Repeat part (a) if the coefficients of static and kinetic friction are 0.45 and 0.35. (0)
c) Repeat part (a) if the coefficients of static and kinetic friction are only 0.25 and 0.15. (0.95 m/s 2)
3) Sally pulls her 18 kg suitcase at a constant speed by pulling with a force of 43 N on a handle that makes an angle, ,
above the horizontal. The force of friction is 27 N.
a) What angle does the handle make with the horizontal? (51o)
Number 4
b) What is the normal force exerted on the suitcase? (143 N)
c) What is the coefficient of kinetic friction? (0.19)
4) A 5.0-kg box is pushed across a table using a force that makes a 30o angle with the
horizontal. If the box accelerates at 3.0 m/s2 and the force of friction is 10 N, then
find
a) the magnitude of the force (29 N)
b) the coefficient of kinetic friction. ( 0.16)
c) Make a free body diagram too.
5) Three forces act on a small box that has a mass of 0.8 kg. Force A is 10.0 N due North, Force B is 15.0 N due West and
force C is 15.0 N at 30o north of east. Find the acceleration of the box. (Hint…use vector addition to find the net force
on the box.) Also, assume friction is not a factor. Make a free body diagram too. Your FBD should be from a top view.
You do not need to worry about F g or FN. (Why?) (22 m/s2 at 83o N of W)
4.1B:
6) A crate weighing 768 N is pulled a long a horizontal sidewalk at a constant speed by a rope that makes an angle of 41 o
degrees with the sidewalk. If a force of 243 is applied to the rope, then what is the coefficient of kinetic friction? (0.30)
7) Two adults and a child want to push a crate in the direction marked x in
the drawing to the right. (This is a top view again…) The two adults
push with forces, F1 = 100 N and F2 = 140 N. Find the magnitude and
direction of the smallest force that the child should exert to get the crate
moving in the right direction. (Hint…since we only want acceleration
in the x-direction, what should the net force in the y-direction be?)
Oh…Assume there is no friction. Make a free body diagram too.
(16.6 N South)
8) Uh-oh…Roy is in trouble with the mob again! He is dragged (kicking
and screaming) by some thugs across the parking lot. Suppose he has a
Number 6
mass of 76 kg.
o
a) The thugs begin to drag him by his shoelaces, which are held at 40
above the horizontal, and find they need a force of at least 345 N to
get Roy moving. What is the coefficient of static friction between
Roy and the parking lot? (0.51)
b) Suppose the coefficient of kinetic friction is 0.40. How far would the police have to drag Robert with a force of 345
N to get him moving at 10 m/s? (Assume his laces are still at 40 o above the horizontal.) (69 m)
9) An 800 N crate is pushed across the floor with a force, F, at an angle of 30 o below the horizontal. The coefficients of
static and kinetic friction are 0.35 and 0.25, respectively. Describe the motion of the crate if:
a) F = 450 N (a = 1.6 m/s2)
b) F = 400 N (a = 0)
10) In a physics lab experiment, a 12.00 kg box is pushed, from rest, across a flat table by a horizontal force of 49 N which
causes the box to accelerate at 2.1 m/s2. The coefficient of static friction is 0.30.
a) Make a FBD and find the coefficient of kinetic friction. (0.20)
b) Suppose this force was exerted at an angle of 30o ATH. Find the box’s new acceleration. (2.0 m/s2)
c) Suppose this force was exerted at an angle of 30o BTH. Find the box’s new acceleration. ( 0 )
11) A box having a mass of 50 kg is pushed across a horizontal floor by means of a lawnmower-like handle attached to it.
The coefficient of kinetic friction between the box and the floor is 0.300. If the angle between the handle and the floor is
30o then what force must be exerted on the handle to move the box at a constant speed? This will require some
algebra!!! Do this on a separate sheet.
Inclined Planes - Honors Worksheet 4.2
1) A block weighs 80 N and rests on a ramp which is inclined at 27 o above the horizontal. The coefficients of
static and kinetic friction are 0.20 and 0.10, respectively.
a) What is the minimum force you would need to exert to prevent the block from sliding down the plane.
(22.3 N)
b) What force would you need, once the block was moving, to keep the block sliding down at a constant
velocity? (29.5 N)
c) Assuming the block began at rest, what is the minimum force you would need to start moving the block
up the plane? (50.6 N)
d) What force would you need to move the block up the ramp at a constant velocity? (Assume it is already
moving.) (43.4 N)
e) What force would you need to move the block up the ramp with an acceleration of 2.3 m/s2? (62.2 N)
f) What force would you need to move the block down the ramp with an acceleration of 0.4 m/s2? (25.9 N)
g) What force would you need to move the block down the ramp with an acceleration of 5.4 m/s2?(-14.9 N)
2) You place a box weighing 346 N on an inclined plane that makes a 35o angle with the horizontal.
a. Find the acceleration of the box if there is no friction. (-5.61 m/s2)
b. Find the coefficient of kinetic friction if the box slides down the ramp with an acceleration of 3.2 m/s 2.
What can we say about the coefficient of static friction? (0.300)
3) You slide a 460 N trunk up a 25o inclined plane with an acceleration of 2.3 m/s2 using a force of 375 N. Find
the coefficient of kinetic friction. (0.174)
4) A 46.0 kg block rests on an inclined plane which is inclined up at 39o above the horizontal. The coefficient of
static friction is 0.30 and the coefficient of kinetic friction is 0.25.
a) Make a FBD of the block and find its acceleration. (a = -4.3 m/s2 )
b) Edgar exerts 50 N against the block, parallel to the surface of the ramp, in an attempt to move it up the
plane. Does the block move? If so, in which direction, and with what acceleration? (-3.2 m/s2)
c) Edgar now increases his force to 500 N. Does the block move? If
so, in which direction, and with what acceleration? (2.7 m/s2)
5) What is the average population of one square mile in Wyoming? (Just
curious.)
6) A 3.4 kg block slides down a 56 degree inclined plane with a constant
acceleration. The block starts from rest at the top of the ramp. The
block takes 2.3 seconds to reach the bottom of the ramp that is 16.4
meters long. (Gosh, that’s a big ramp!)
a) What is the acceleration of the block? (-6.2 m/s2)
b) What is the coefficient of kinetic friction between the block and the
ramp? (0.35)
7) A box is placed on a frictionless inclined plane. It slides down the
plane with an acceleration of 7.4 m/s2. Find the angle of the plane.
Draw a free body diagram. (Notice…you are not given the mass of the
box here. You don’t need it to solve this. Why?) (49o)
8) Scooby-Doo has found a GIANT 48 kg box of Scooby Snacks. He tries
to push it up a ramp into the Mystery Machine. The ramp is angled at
34o and the top of the ramp is 1.20 m above the horizontal ground. The
coefficients of static and kinetic friction are 0.4 and 0.32 respectively.
If he pushes the box with a force parallel to the ramp and the box starts
at rest, then what force does Scooby need to get the box to the top in 0.8 seconds? (462 N)