Balanced Forces Review

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Balanced Forces Review
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Balanced Forces Review
For questions 1-6, draw a free-body (force) diagram for the situation.
1.
A boy pulls a box of mass 30 kg with a force of 25 N at an angle of 30 O above the ground, as
shown in the figure below, at constant speed. (a) What is the normal force exerted on the box
by the ground? (b) What is the frictional force between the box and the ground?
2.
A girl pushes a 25-kg lawn mower at constant speed with a force of 30 N at an angle of 37 O
below the horizontal, as shown in the figure below. (a) Calculate the frictional force acting on
the mower. (b) Calculate the normal force exerted on the mower by the lawn.
3.
A 1.40-kg bottle of vintage wine is lying horizontally in the rack shown in the drawing. The
two surfaces on which the bottle rests are 90O apart, and the right surface makes an angle of
45O with respect to the ground. Each surface exerts a force on the bottle that is perpendicular
to the surface. What is the magnitude of each of these forces?
4.
A picture is hanging from two ropes that join in the center of the top of the picture frame as
shown below. Rope 1 on the left has a tension of 200 N, and Rope 2 on the right has a tension
of 140 N. Find the weight of the picture.
55O
5.
35O
A sign that weighs 120 N is hanging by two wires. One wire is up and to the right on the sign
at an angle of 53O and has a tension of 150 N. The other wire is directly horizontal to the left
of the sign. Find the tension in this second wire.
53O
6.
A block of weight 130 N is hanging by two ropes. One rope is directed horizontally to the
right of the sign and has a tension of 75 N. The other rope is directed up and to the left of the
sign. Find the magnitude and direction of the tension in the second rope.
7.
The equilibrium rule ∑ 𝐹 = 0 applies to
[A] objects or systems at rest.
[B] objects or systems moving with constant speed in a straight line.
[C] both [A] and [B]
[D] neither [A] and [B]
8.
As a ball falls, consider one force to be the pull of the earth on the ball. What is the
other part of this interaction?
[A] air resistance acting against the ball.
[B] the pull of the ball on the earth.
[C] non-existent in this case.
[D] none of these.
9.
Suppose a particle is accelerated through space by a constant 10 N force. Suddenly
the force stops acting. The particle
[A] is brought to a rapid halt.
[B] gradually slows down to a halt.
[C] theoretically accelerates to speeds approaching the speed of light.
[D] continues at the speed it had when the force disappeared.
[E] none of the above
10.
A rock is thrown vertically into the air. At the very top of its trajectory the net force
on it is
[A] less than its weight.
[B] more than its weight.
[C] its weight.
11.
A person standing on a horizontal floor feels two forces: the downward pull of
gravity and the upward supporting force from the floor. These two
forces
[A] have equal magnitudes and form an interaction pair.
[B] have equal magnitudes but do not form an interaction pair.
[C] have unequal magnitudes and form an interaction pair.
[D] have unequal magnitudes and do not form an interaction pair.
[E] None of the above.
12.
A 2-kilogram block slides down a 30° incline as shown below.
Which of the following diagrams best represents the gravitational force W, the
frictional force f, and the normal force N that act on the block?
13.
If you exert a force of 237 N on an object at rest and it does not move, how great is
the frictional force acting on the object?
[A] more than 237 N
[B] 237 N
[C] less than 237 N
14.
Suppose a particle is accelerated through space by a constant 10 N force. Suddenly
the particle encounters another force of 10 N in the opposite direction of the
original force. The particle
[A] is brought to a rapid halt.
[B] gradually slows down to a halt.
[C] theoretically accelerates to speeds approaching the speed of light.
[D] continues at the speed it had when the second force appeared.
[E] none of the above
15.
An object following a straight-line path at constant speed
[A] has no forces acting on it.
[B] has a net force acting upon it in the direction of motion.
[C] has a net force of 0 acting upon it.
[D] none of these
****There were several multiple choice questions provided to you with the concept development
pages on Newton’s 1st and 3rd law. It is worth your time and effort to review those
questions…especially those that were incorrect the first time through!
Things you should be able to do:
 Identify forces acting on objects in a given situation
 Draw force diagrams to represent the forces acting on an object. Diagrams should identify
forces with labels, show proper direction, and show relative size of forces
 Write force equations for both the vertical and horizontal directions for a given situation
 Manipulate and solve the force equations for unknown quantities
 Find the vertical and horizontal components of a force whose direction is at an angle
 Apply the concept of equilibrium to analyze situations and determine unknown forces
 Explain the concept described by Newton’s first law and use it to analyze situations.
 Recognize that the first law is describing the condition of equilibrium. F = 0 (balanced
forces)
 Identify, analyze, and compare action/reaction pairs of forces in a given situation

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