Download Chapter 4: Forces and the Laws of Motion Name Use Chapter 4 in

Chapter 4: Forces and the Laws of Motion
Name ________________________
Use Chapter 4 in your book pages 119 – 144 and your notes to answer the following questions.
Introduction to Forces
1) Forces cause objects to accelerate.
2) What is the difference between contact forces and field forces?
Field force occurs between to objects that are not in contact.
3) Why is force a vector quantity?
Forces have a magnitude and direction
4) Draw a free body diagram for an apple hanging from a tree.
Fa (Branch pulls on apple) upward
Fg (Earth pulls on apple) downward
5) Draw a free body diagram and calculate the net force on a refrigerator that a person is sliding across the floor if
the person is pushing with a force of 32 N and force of friction opposing the motion is 12 N.
FN (Floor supports fridge) upward
Fnet = 20 N right
Ff = 12 N left
Fa = 32 N right
Fg (Earth pulls on fridge) downward
6) What is the affect of the forces on the refrigerator?
Fridge accelerates right
7) The object below is moving at a constant velocity to the right when the following forces are applied. What is the
net force acting on the object? Fnet = 0 N
2 N left
3 N left
8) What is the effect of the net force on the object in the above question?
No change in motion
5 N right
9) The object below is moving at constant velocity to the right when the following forces are applied. What is the
net force acting on the object? Fnet = 4 N right
2 N left
6 N right
10) What is the effect of the net force on the object in the question above?
Object accelerates to the right
11) What is the net force on an object in equilibrium?
Fnet = 0 N
12) Fill in the table below
Symbol
Definition
Fnet
The vector sum of all the forces acting on an object.
Fa
An applied force on an object like a push or pull.
Fg
The force due to gravity on an object.
Ff
The resistance to motion that occurs whenever 2 materials or media are in contact.
Ft
A tension force transmitted through a string or rope when it is pulled tight.
FN
The normal force (or support force) exerted upward by a surface and is perpendicular to the surface.
Force Pairs
13) If two objects are in contact how do you compare the forces between the 2 objects?
The 2 forces are equal in magnitude and opposite in direction.
14) Why don’t these forces cancel each other out?
These forces act on 2 different objects.
15) Draw and label the force pairs on the picture below. The man is dropping a ball off the Leaning Tower of Pisa.
Earth pulls on ball downward
Ball pulls on Earth upward
16) Why does the ball accelerate and the Earth does not accelerate? The Earth is much more massive.
17) Draw and label the 2 force pairs on the picture below. The weight is hanging from the ceiling on a rope.
Ft Rope pulls weight, up
Draw the free body diagram for the
weight.
Ft
Fa Weight pulls rope, down
Fg Earth pulls weight, down
Fg
Fg Weight pulls Earth, up
If the rope is cut, why does the
weight accelerate? There is an
unbalance force on the weight.
18) To reach the finish line, a runner sprints from 5.2 m/s to 6.0 m/s in 2 seconds. Her mass is 50 kg. What is the
runner’s average acceleration?
a = 0.4 m/s2
19) What is the average backward force that the runner exerts on the ground?
F = 20 N
20) What is average force that the ground exerts on the runner? F = 20 N in the opposite direction
21) Why don’t these two forces cancel out?
The forces are acting on 2 different objects.
Mass versus Weight
22) What is the difference between mass and weight?
Mass is the amount of matter (“stuff”) in an object. Weight is the force of gravity on an object.
23) What is the weight of a 300 kg mass on Earth? Fg = 2940 N, downward
24) The acceleration due to gravity on the moon is 1.6 m/s2. What is the weight of a 300 kg mass on the Moon?
Fg = 480 N, downward
25) The mass of a book is 54 g. What is the mass of this book on the Moon? Explain your answer.
54 g, mass doesn’t change with location
26) You have a 20 N bar of gold on the Earth. You also have another 20 N bar of gold on the moon. Which bar of gold
has a greater mass? Why?
The bar of gold on the moon has a greater mass. For a constant force, if the acceleration is greater the mass is smaller.
Net Force, Acceleration, and Mass
27) The acceleration of a sliding block is 2 m/s2. If the net force acting on a sliding block is tripled, what is the
acceleration of the block?
6 m/s2
28) The acceleration of a sliding block is 1.8 m/s2. If the mass of the sliding block is tripled while a constant force is
applied, what is the acceleration of the block?
0.6 m/s2
29) If the mass of a sliding block is tripled while the net force on it is tripled, how the does resulting acceleration
compare to the original acceleration?
The accelerations are the same. The ratio of F/m is the same.
30) Calculate the acceleration of a 40 kg crate of softball gear when pulled sideways with a net force of 200 N.
5 m/s2
31) A 2.0 kg coconut falls on Gilligan’s Island. Ignoring air friction, what was the net force on the nut as it falls?
Fg = 19.6 N, downward
32) A snowball of mass 0.1 kg leaves little Joey’s hand at a speed of 15 m/s and comes to rest 0.15 seconds later.
Calculate the force on the snowball from Joey’ hand. (Calculate the acceleration first.)
F = -10 N
33) A soccer ball kicked with a force of 13.5 N accelerates at 6.5 m/s2 to the right. What is the mass of the ball?
2.1 kg
34) Look at the free body diagram below.
A book is pushed along a horizontal surface.
FN = 49 N, upward
Fa = 30 N right
Ff = 5 N left
Fg = 49 N, downward
m = 5 kg
35) Fnet = 25 N, right
36) a = 5 m/s2, right
Force of Friction
37) List the four types of friction that we talked about in class from the type that opposes motion the most to the
type that opposed motion the least.
Static, sliding, rolling, fluid
38) If you push horizontally with a force of 50 N on a crate and make it slide at constant velocity, what is the force of
friction that acts on the crate?
50 N in the opposite direction
39) In the above case, if you increase your force, will the crate accelerate? Explain.
Yes, the applied force is greater that the sliding (kinetic) friction force.
40) How does the force of static friction compare to the applied force on a huge box that you cannot move?
It is equal in magnitude and opposite in direction.
41) Explain what the coefficient of friction means?
The coefficient of friction shows how dependant the frictional forces are on the materials/media in contact.
42) A 95 kg clock at rest on a horizontal floor requires a 650 N force to set it in motion, After the clock is in motion, a
horizontal force of 560 N keeps it moving with a constant velocity, Find µs,mas and µk.
µs,max = 0.7
µk = 0.6
43) Look at the free body diagram below.
Fill in the blanks.
FN = 78.4 N, upward
Fk = 31.4 N, left
Fa = 31.4 N, right
µk = 0.4
m = 8 kg
a = 0 m/s2
Fg = 78.4 N, downward
Fnet = 0 N, balanced forces
What is the motion of the object? How do you know?
The object is moving a constant velocity to the right. If the acceleration is zero, the net force is zero. It must be
moving for there to be a kinetic friction force.
Terminal Velocity
44) What is meant by free fall?
There is no air resistance. The object is acted on only by gravity.
45) In free fall, why doesn’t a more massive object accelerate more than a less massive object?
The ratio of F/m stays the same.
46) What is the net force (magnitude and direction) that acts on a falling object with a weight of 10 N when it
encounters 4 N of air resistance? Draw the free body diagram.
Fnet = 0 N
R = 4 N upward
Fg = 10 N, downward
47) What is the net force (magnitude and direction) that acts on a falling object with a weight of 10 N when it
encounters 10 N of air resistance?
Fnet = 0 N
48) Explain how the cross sectional area of an object and the speed of an object affect the force of air resistance on
an object if it is falling in air. As both the speed increases and the cross sectional area increases, the object
moves through more air molecules and increases the air resistance.
49) What is the acceleration of a falling object that has reached terminal velocity?
There is no acceleration.
50) If you drop a pair of tennis balls simultaneously from the top of a building, they will strike the ground at the
same time. If you fill one of the balls with lead pellets and then drop then together, which one will hit the
ground first? Why? (Do not ignore air resistance)
The one filled with lead pellets has a greater mass and therefore more weight. It will reach a greater terminal velocity.
51) A skydiver with a mass of 100 kg experiences air resistance of 700 N. What is the skydiver’s acceleration? (Find
the weight of the skydiver first.)
a = 2.8 m/s2, downward
Centripetal Acceleration and Centripetal Force
52) Why is an object moving in a circle with constant speed accelerating?
The direction of the velocity is changing.
53) What is the direction of the acceleration?
The direction is towards the center of the circle.
54) A race car is moving along a circular track with a speed of 30.0 m/s. The distance from the car to the center of
the track is 58.4 m. What is the centripetal acceleration of the car?
ac = 15.4 m/s2 , inward
55) Why is the centripetal acceleration directed inward?
The force is directed inward to the acceleration is also.
56) Explain what makes a passenger in a turning car slide toward the door of the car?
When the car turns, the inertia of the passenger keeps the passenger moving the direction that the car was originally
moving.
57) A Physics student twirls a set of keys with a mass of 0.5 kg in a circle at the end of a cord. If the keys have a
speed of 7.0 m/s and the cord has a length of 0.34 m, what is the centripetal force on the keys?
Fc = 72 N, inward
58) A 40 kg child is riding the amusement park ride called the Rotor. After the ride gets up to the correct speed the
floor drops. If the centripetal force on the child is 533 N, determine the speed of the child if the radius of the
ride is 2.90 meters.
V = 6.2 m/s
Inerita
59) What is inertia?
Inertia is the tendency of an object to resist a change in motion.
60) In space which would be harder to move, a semi truck or Toyota Rav? Why?
The semi truck because it has more mass so it has more inertia.