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Unit B: Worksheet Title _______________________________
Date: _______________
Name: ________________________
Activity B2 – Part 1
At a picnic, the Hernandez family (wearing red shirts) has challenged the Park family (wearing blue
shirts) to a game of tug-of-war. Each family will pull on a rope attached to the handle of a cooler, and
the winning family gets all of the dessert inside. Each family has two children (each can pull with a force
of 50N), a parent (who can pull with a force of 100N), and an uncle (who can pull with a force of 150N)
who is willing to play the game.
Predictions:
For each scenario, predict which team will win (or if it will be a tie).
Net Force Calculation
Winning Team
Red team starts with a parent;
Blue team starts with one child and one parent.
Red team starts with two children;
Blue team starts with one parent.
Red team starts with one child;
Blue team also starts with one child, but this
child is distracted by a beetle and starts 3s after
the red team’s child has been pulling
Activity:
Now try the same scenarios and test your predictions using a simulation developed at the University of
Colorado at Boulder. The link has been posted on our Blackboard site. Here is the URL for your records:
http://phet.colorado.edu/en/simulation/forces-and-motion-basics
You may wish to select “sum of forces” and “values”, while un-selecting “sound”.
Net Force Calculation
Winning Team
Red team starts with a parent;
Blue team starts with one child and one parent.
Red team starts with two children;
Blue team starts with one parent.
Red team starts with one child;
Blue team also starts with one child, but this
child is distracted by a beetle and starts 3s after
the red team’s child has been pulling
Analysis:
1. Explain your observations in the Activity using the two Newton’s laws from your reading.
2. What can you conclude about the net force and the acceleration of an object?
3. What can you conclude about the net force and the direction of motion (velocity) of an object?
Unit B: Worksheet Title _______________________________
Date: _______________
Name: ________________________
Activity B2 – Part 2
Materials: force probe and adapter
1. Plug in the force probe with the adapter into your computer. Open up the program PASCO
Capstone, and choose a graphical display.
2. Choose your axes to display a force-vs.-time graph.
3. Note that the hook-end of the force probe is the actual probe location.
4. Be sure to ‘zero’ the force probe before each measurement.
5. Try various configurations, attaching objects to the hook, pulling, hanging, pushing, etc. Get a feeling
for the magnitude of the force that the probe measures. For example: How much force is required
to hold a book? Or, how much force do you exert pulling the cart on its track? (Note: The force
probe cannot exceed 50N.)
6. Get a feeling for the direction and corresponding sign of the measured force. In other words, when
is the force measured to be positive? Negative?
Activity B2 – Part 3
On your whiteboards, describe and sketch a realistic situation where each of the following is true.
Include real objects, real forces, and values.
Also state the direction of
i.
velocity,
ii.
acceleration, and
iii.
net force.
1. An object moves to the left with no forces acting on it.
2. An object moves to the left with one force acting on it from the left.
3. An object moves to the left with one force acting on it from the right.
4. An object moves to the left with equal forces acting on it from both left and right.
5. An object moves to the left with a greater force acting from the left than from the right.
6. An object moves to the left with a lesser force acting from the left than from the right.
7. An object is at rest with two forces acting on it.
8. An object is at rest with three forces acting on it.
Unit B: Worksheet Title _______________________________
Date: _______________
Name: ________________________
Activity B3- Part 1
Materials: force probe, set of masses and mass hanger
Don’t forget to zero the force probes before each measurement!
1. Hold the force probe so that its hook is facing downwards.
2. Place several masses on the probe, and record the force that each mass exerts on the probe. Be
sure to hold the probe steadily.
Force (N)
Mass (kg)
Force÷Mass
3. Divide the measured force by the mass, and add to the table above.
4. Hopefully all of the values from #3 are approximately the same. Does the value look familiar?
5. This is the gravitational force that is exerted near the Earth’s surface on a mass, and it is called
weight. Since we are always near the Earth’s surface (unless you are an astronaut), every object
feels this force.
6. In general: you can conclude that weight is given by ________________ multiplied by
____________________. In symbols: W = ______. (Note: Sometimes weight is given the symbol
FG.)
7. In your own words, state the main difference between mass and weight.
Unit B: Worksheet Title _______________________________
Date: _______________
Name: ________________________
Activity B3- Part 2
1. Imagine that you let go of your pen and it falls to the floor. Describe its motion. (Recall that motion
descriptions refer to position, velocity, acceleration, and time.)
2. Hold one of the masses above the floor and let it go. (Watch your toes!)
3. Did you see it accelerate? If so, then there must be a force acting on the mass. Why?
4. From Activity 1, what did we call this force?
5. Put one of the masses on the table. You should see that the mass is not accelerating. What does this
tell you about the result of all forces exerted on the mass? (In other words, what is the net force on
the mass?)
6. Hopefully you concluded that the force of gravity, weight, must be balanced by a force exerted on
the mass by the table. This force, that prevented the mass from falling through the solid table, is
called the normal force. It is often given the symbol n, N or FN.
7. What can you say about the magnitude of the normal force on the mass if the table were at an angle
(not horizontal or vertical), would it be greater than, same as, or less than the weight? Explain your
reasoning.
8. Instead of a table, imagine placing the mass ‘on’ a wall. When you let go of the mass, does the wall
exert a normal force on the mass? Explain.
9. The word ‘normal’ means perpendicular; the normal force is always perpendicular to the surface
with which the object is in contact.
Unit B: Worksheet Title _______________________________
Date: _______________
Name: ________________________
Activity B4
Materials:
two spring scales
Activity:
Attach two spring scales together. Make sure they remain parallel to each other. Do the following
activities and record the reading from each of the spring scales. Take turns pulling the scales and
recording data. Don’t forget units!
Activity
1. One person holds both scales and pulls equally on each.
Spring Scale A
2. One person holds scale A and another person holds scale B.
Each person pulls equally.
3. One person holds scale A and another person holds scale B.
Person A pulls but person B just holds their scale still.
4. One person holds scale A and another person holds scale B.
Person B pulls but person A just holds their scale still.
5. Attach scale A to an immovable object. One person pulls on
scale B.
Analysis:
1. What trend do you see in the readings on the two spring scales?
2. Do you think this trend is always true? Why or why not?
3. Do a few experiments to see if you can break the trend. Describe what you do here.
4. Put your ideas on a whiteboard and be prepared to discuss.
Spring Scale B
Unit B: Worksheet Title _______________________________
Date: _______________
Name: ________________________
Activity B5
1. Say that you are pushing your book continuously across the desk to your classmate. Draw the book
in the middle of the box below. (Remember that physicists are usually poor artists, so all objects are
circles, spheres, blocks, etc.) We will consider the instant in time when the book is increasing in
speed towards your classmate.
2. Indicate the direction of velocity of your book with a small, labelled arrow in one corner.
3. Use a different color to indicate the direction of acceleration of your book with another small,
labeled arrow.
4. If you were to represent the force of gravity acting on the book with an arrow, in what direction
would it point? In the box, and using a third color, draw and label this vector, starting the arrow on
the object itself (your book) and pointing away from it.
5. Draw and label the normal force acting on the book from the table as an arrow on the object, again
having the arrow begin on the object. Use the same color as you did for the force of gravity.
6. Using Newton’s Laws, describe the direction that the frictional force must act in order to slow down
a moving object. Draw and label this force as an arrow in your diagram, being sure to start the arrow
on the object, pointing away from it. Use the same color as you did for the other two forces.
7. You are pushing the book – this implies another force acting on the book. Draw this vector, starting
the arrow on the object and pointing away from it. Use the same colour as you used for the other
forces.
8. Based on your acceleration vector from #3, in what direction should the overall force be? Sketch this
force vector in your diagram, and label it Fnet. The overall force, or the total of all the forces acting
on an object, is called the net force.
9. Since forces are vectors, you need a coordinate system to distinguish between different dimensions.
Add this to your diagram.
10. This type of sketch, showing all forces acting on an object, is called a Free Body Diagram (FBD).
“Free” refers to the object being considered separate from all others. “Body” refers to a solid object.
FBD’s are used to help identify all forces acting on an object.
11. Make a list of the requirements, based on this worksheet, for a complete FBD.
Unit B: Worksheet Title _______________________________
Date: _______________
Name: ________________________
Day 8 – Group Practice
Free Body Diagrams
A. On your whiteboard, draw a free body diagram for each scenario described below. Have the
instructor check the diagram(s) for each number.
1. (a) A book resting on a box. Object is the book.
(b) A book resting on a box. Object is the box.
2. Holding a yo-yo by its string. Object is the yo-yo.
3. (a) Pulling a wagon at constant speed. Object is the wagon.
(b) Pulling a wagon, causing it to speed up. Object is the wagon.
4. The moon orbiting the Earth. Object is the moon.
5. A beach ball floating on a still lake surface. Object is the ball.
B. One your whiteboard, describe a scenario in which an object experiences at least 4 forces
simultaneously.
1. Draw the free body diagram for the object.
2. Trade your whiteboard with the group opposite yours, and critique their free body diagram.
3. Have your instructor check your critique.
C. Come up with a scenario, but do not put it on your whiteboard. Instead, only draw the free body
diagram for an object in the scenario.
1. Trade your whiteboard with the group opposite yours, and write a scenario that would fit the
given free body diagram.
2. Have your instructor check your scenario.
D. Draw a free body diagram for the book in each scenario described below. Keep all three scenarios
on your whiteboard so you can summarize.
1. A book is resting on a ramp.
2. You are pushing a book against the wall so that the book is stationary.
3. You are pushing a book against the ceiling.
4. In the three scenarios, note the direction of the normal force. What can you conclude about the
direction of the normal force? Is it related to the book’s weight?
Unit B: Worksheet Title _______________________________
Date: _______________
Name: ________________________
Activity B6
Answer these questions on your whiteboard using Newton’s Laws.
1.
If you were in a spaceship and fired a cannonball into space, how much force would have
to be exerted on the ball to keep it moving once it has left the spaceship?
2.
An object weighs 98 N on Earth. How much does it weigh on Planet X where the
acceleration due to gravity is 6 m/s2?
3.
The force of gravity is twice as great on a 2-kg rock as on a 1-kg rock. Why then does the
2-kg rock not fall with twice the acceleration?
4.
When a hammer exerts a force on a nail, how does the amount of force compare to that of
the nail on the hammer?
5.
Why does a cannon recoil when it fires a cannonball?
6.
If suddenly the force of gravity of the sun stopped acting on the planets, in what kind of
path would the planets move?
7.
Suppose you place a ball in the middle of a wagon, and then accelerate the wagon forward.
a. Describe the motion of the ball relative to the ground.
b. Describe its motion relative to the wagon.
8.
When you jump up, does the world recoil downward? Explain.
9.
A horse pulls a wagon with some force, causing it to accelerate.
Newton’s third law says that the wagon exerts an equal and opposite
reaction force on the horse. How can the wagon move?
10.
If an elephant were chasing you, its enormous mass would be most threatening. But if you
zigzagged, its mass would be to your advantage. Why?
11.
You are stranded in the middle of a frozen lake with rocks in your pockets. The ice istoo
slippery to walk on, nor can you walk on the rocks on the ice, because the rocks will slide
out from under your foot. Use Newton’s Laws to explain how you can get back to shore.
(Adapted from: http://student.pattersonandscience.com/ – retrieved 11 Sept 2014.)
Unit B: Worksheet Title _______________________________
Date: _______________
Name: ________________________
Activity B7
Example
A 6.5-cm-diameter tennis ball has a terminal speed of 26 m/s. What is the ball’s mass?
(Terminal velocity refers to the speed of a falling object when its weight is perfectly balanced by the drag
force acting on it.
STEP 1: Identify the object of interest.
Draw a free body diagram of the object.
Determine which dimension is the one of interest. Be sure to indicate it in your diagram with
your coordinate system.
STEP 2: Write down Newton’s Second Law (N2).
STEP 3: Apply N2 to the dimension of interest, including all forces in that dimension.
STEP 4: Solve your equation(s) from Step 2 to obtain your answer.
Unit B: Worksheet Title _______________________________
Date: _______________
Name: ________________________
Use the steps from the example above to solve the following problems on your whiteboard. Have one
member of your group do the writing on the board in each problem. Take turns writing.
6-22 An Airbus A320 jetliner has a takeoff mass of 75 000 kg. It reaches its takeoff speed of 82 m/s
(180 mph) in 35 s. What is the thrust of the engines? You can neglect air resistance but not rolling
friction.
6-40 A 2.0 kg steel block is at rest on a steel table. A horizontal string pulls on the block.
a. What is the minimum string tension need to move the block?
b. If the string tension is 20 N, what is the block’s speed after moving 1.0 m?
c. If the string tension is 20 N and the table is coated with oil, what is the block’s speed after moving
1.0 m?
6-38 A rifle with a barrel length of 60 cm fires a 10 g bullet with a horizontal speed of 400 m/s. The
bullet strikes a block of wood and penetrates to a depth of 12 cm.
a. What is the value of the resistive force (assumed to be constant) that the wood exerts on the bullet?
b. How long does it take the bullet to come to rest once it enters the wooden block?
c. Draw a velocity-versus time graph for the bullet in the wood.
d. What is the value of the thrust that the gun exerts onto the bullet when it is fired?
Additional Problems
A. The mass of the Mars Rover and its launcher is 5.31×105 kg. During the launch, the net force acting
on the launcher was 1.36×106 N. The launcher is approximately 5m in diameter and 58m tall.
Determine everything you can about the launch from this information.
B. The Rover itself has a mass of 900 kg. During landing, the Rover slowed down due to friction with
the atmosphere. When it reached a speed of 470 km/s, its parachute opened, causing a drag of
289×103 N. The parachute was open for a vertical descent of 8.2 km. Note that gravity of Mars is
only 38% of that of Earth. Determine everything you can about the parachute phase of the Rover’s
landing.
C. You are designing a lamp for the interior of a special executive express elevator in a new office
building. The lamp has two sections that hang one directly below the other. The bottom section is
attached to the top one by a single thin wire and the upper section is attached to the ceiling by
another single thin wire. Because the idea is to make each section appear to be floating without
support, you want to use the thinnest (and thus weakest) wire possible. You decide to calculate the
force each wire must exert on the lamp sections in case of an emergency stop. The elevator has all
the latest safety features and will stop with an acceleration of g/3 in any emergency. Each section of
the lamp weighs 7.0 N.
D. You are at a party with helium-filled balloons attached to strings on the tables. One balloon has
detached, and is touching the ceiling. A friend at the party knows you are taking physics and asks
you about the forces on the balloon that is touching the ceiling. You estimate each balloon to have a
mass of 10 g and holds 14 g of helium; knowing a little about electrostatics, you determine that the
balloon is attracted to the ceiling with 0.20-N of electrostatic force. Explain to your friend whether
there is a normal force on the balloon.
E. A 5-kg box sits on a 10-kg box in an elevator. Discuss the forces and accelerations of each box when
the elevator is (i) stationary, (ii) moving upwards with constant speed, (iii) speeding up while moving
upwards, (iv) slowing down while moving upwards.
…
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