Honors Physics
Chapter 4 Practice Problems (Part 1)
Note: The concepts and ideas presented in Chapter 4 are arguably some of the most
important in this course. It is EXTREMELY important that you master these
concepts as we progress through this course, because everything we do from this
point forward will rely on these skills!!! PLEASE try to solve all of these
problems, and DO NOT GIVE UP UNTIL YOU’VE CONQUERED THEM!!!
1) Four coplanar forces act on an object at point O, as shown in the diagram.
Find the resultant force vector (both magnitude and direction). Use two
significant figures.
2) Five coplanar forces act on an object. Find the resultant force vector.
Again, please use two significant figures.
3) A force of 100.0 N makes and angle of θ with the x-axis and has a scalar ycomponent of 30. N. Find both the scalar x-component of the force and the
angle θ. Don’t let the “scalar” word freak you out…all I am asking you to
do is find the x-component of the force vector and give the vector’s angle
with respect to the positive x-axis.
4) A child pulls on a rope attached to a sled with a force of 60. N. The rope
makes an angle of 40° to the ground.
a. Calculate the effective value of the pull that moves the sled along
the ground horizontally.
b. Calculate the force tending to lift the sled vertically
5) A car whose weight FW is on a ramp which makes an angle θ to the horizontal.
How large a perpendicular force must the ramp withstand if it is not to
break under the car’s weight? As a hit, I’d draw a picture and break the
weight force into its components!!
6) Find the weight on Earth (in Newtons) of an object whose mass is
a. 3.00 kg
b. 200. g
7) A 20.0kg object can move freely and is subjected to a resultant force of
45.0 N in the negative x-direction. What is the acceleration of the object?
8) An object that weighs 225 N hangs motionless from a ceiling suspended by a
cord. Draw a “Free Body Diagram” of the object and label all forces acting
on the object. What is the tension in the cord?
9) A 5.0kg object hanging vertically is given an upward acceleration of 0.30
m/s2 by a rope pulling straight upward on it. Draw a free-body diagram of
this situation. What must be the tension in the rope?
10)
A horizontal force of 140. N is needed to pull a 60.0 kg box across a
horizontal floor at constant speed. Draw a Free Body Diagram for the box.
What is the coefficient of friction between the floor and the box? Determine
this coefficient to THREE significant figures, even though that is quite
unrealistic in typical lab situations.
11)
The only force acting on an object with a mass of 5.0 kg has an xcomponent of 20.0 N and a y-component of 30. N. Find the acceleration of
the object.
12)
A 600.0 N object is to be given an acceleration of 0.70 m/s2. How large
of an unbalanced force must act upon the object?
13)
A constant force acts on a 5.0 kg object and reduces its velocity from
7.0 m/s to 3.0 m/s in a time of 3.0 seconds. Find the force.
14)
A 0.400kg block with an initial speed of 0.80 m/s slides along a
horizontal tabletop against a friction force of 0.70 N.
a. How far will it slide before stopping?
b. What is the coefficient of friction between the block and the
tabletop?
15)
A 600. Kg car is moving on a level road at 30.0 m/s.
a. How large of an opposing force (assumed to be a constant force) is
required to stop the car in over a distance of 70. meters?
b. What is the minimum coefficient of static friction between the tires
and the roadway if this is to be possible? Note that I asked for
STATIC friction, which is the frictional force that causes tires to
ROLL. Kinetic friction would apply if the car were skidding on ice
with the wheels locked, but it is STATIC friction that makes the
wheels roll!
16)
A 70. Kg box is slid along the floor by a 400.0N force as shown in the
diagram. The coefficient of friction between the box and the floor is 0.50
when the box is sliding. Find the acceleration of the box.
17)
Suppose, as shown in the diagram, that a 70.0kg box is pulled by a
400.0N force at an angle of 30.° to the horizontal. The coefficient of
kinetic friction is 0.50. Find the acceleration of the box.
18)
As shown in the figure below, a force of 400. N pushes on a 25kg box.
Starting from rest, the box achieves a velocity of 2.0 m/s in a time of 4.0
seconds. Find the coefficient of kinetic friction between the box and the
floor.
19)
A 200.0 N wagon is to be pulled up a 30.° incline at constant speed.
How large a force parallel to the incline is needed if friction can be
ignored? Draw a free body diagram of the situation!!!
20)
When a force of 500. N pushes on a 25kg box, as shown in the diagram,
the acceleration of the box up the incline is 0.75 m/s2. Find the
coefficient of kinetic friction between the box and the incline.
21)
A cord passing over an easily turning pulley (one that can be assumed
to be both massless and frictionless) has a 7.0 kg mass hanging from one end
and a 9.0 kg mass hanging from the other end, as shown in the diagram. Find
the acceleration of the masses AND the tension force in the cord connecting
the masses. I strongly recommend drawing a free body diagram for BOTH
masses, and remember that since they are connected, the magnitude of their
accelerations will be the same!
22)
In the diagram below, the coefficient of kinetic friction between
block A and the table is 0.20. Also, mass A is 25kg and mass B is 15kg. How
far will block B drop in the first 3.0 seconds after the system is released.
I’ve included the free body diagrams for each object for this problem,
although you should hopefully know how to draw them on your own by now!
23)
Consider the same situation as was provided in problem 22. How large
of a horizontal force in addition to FT must pull on block A to give it an
acceleration of 0.75 m/s2 TO THE LEFT? Note that a frictional force of 49N
will still be acting in this problem, only this time it will be acting in
the opposite direction since the motion is now toward the left!
24)
In the diagram below, the two boxes have identical masses of 40.0kg.
Both experience a sliding friction force with µK = 0.15. Find the
acceleration of the boxes and the tension in the tie cord.
25)
An elevator starts from rest with a constant upward acceleration. It
moves 2.0m in the first 0.60 seconds. A passenger in the elevator is holding
a 3.0kg package by a vertical string. What is the tension in the string
during the acceleration process?
26)
A 20. Kg wagon is pulled along the level ground by a rope inclined at
30.0° above the horizontal. A friction force of 30.0N opposes the motion of
the wagon. How large is the pulling force if the wagon is moving with
a. Constant velocity
b. An acceleration of 0.40 m/s2
27)
A 12.0kg box is released from the top of an incline that is 5.0m long
and makes an angle of 40° to the horizontal. A 60.0 N friction force impedes
the motion of the box.
a. What will be the acceleration of the box?
b. How long will it take to reach the bottom of the incline (hmm…a
kinematics equation, perhaps?)
28)
An inclined plane making an angle of 25° with the horizontal has a
pulley at its top. A 20.0kg block on the plane is connected to a freelyhanging 20.0kg block by means of a cord passing over the pulley. Compute the
distance the 20.0kg block will fall in 2.0s if it starts from rest. (Ignore
friction in this problem)
29)
Do #28 again, but now assume the coefficient of kinetic friction
between the block and incline is 0.20.
30)
Find the acceleration of the connected blocks in the figure if
friction can be ignored. What is the tension in the cord connecting the
blocks?
31)
Now repeat problem 30 if the kinetic friction between the blocks and
table is 0.30.