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Science of NHL Hockey: Force, Impulse, and Collisions
Subject Area: Physics
Grade Level: 9–12 (Physics)
Lesson Title: Force, Impulse, and Collisions
National Science Education Standards:
 Science as Inquiry: 9–12
 Motions and Forces: 9–12
 Conservation of Energy and Increase in Disorder: 9–12
Suggested Prior Knowledge: concepts of mass, velocity, momentum, acceleration, force, and
Newton’s second law of motion; lab techniques of measuring mass and velocity. Note that
students might more easily grasp the concepts in this video if they have already viewed the video
Newton’s Three Laws of Motion.
Purpose and Introduction: This video focuses on the interactions of the hockey stick and the
puck to advance the understanding of force, impulse, and collisions. The activity will help
students to understand the concept of impulse and to observe the difference between elastic and
inelastic collisions.
Key Vocabulary:
elastic collision—an impact of bodies resulting in a change in velocities and a transfer of
kinetic energy without loss.
force—an influence on an object that causes a change in velocity or shape.
impulse—a force multiplied by the amount of time during which it acts.
inelastic collision—an impact of bodies resulting in a change in velocities and a transfer of
kinetic energy with some loss.
Objectives:
1. Students will design and carry out a procedure to measure the acceleration of a body
produced by a known impulse.
2. Students will design and carry out an investigation to measure momentum and energy
transfer in an elastic collision.
3. Students will design and carry out an investigation to measure momentum and energy
transfer in an inelastic collision.
Materials:
- Safety goggles
- Skateboard
- Bricks
- Standard masses
- Fixed pulley
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Force, Impulse, Collisions (High School Lesson)
- String
- Stopwatch
- Tape
- Marker
- Meter stick
- Blocks of different masses
- Ring stands
- Modeling clay
Procedure:
1. After students review the video, discuss with students the fundamentals of collisions. Elicit
from volunteers examples and explanations of the concepts of impulse, momentum, and
energy. Ask students to critically analyze the video’s definition of force (the push or pull that
can accelerate an object by changing its velocity or changing its shape) and suggest how it
might be improved. Guide them to realize that the original definition implies that changing
the shape of an object is a way of accelerating it. Ask students to recast the definition so it
does not foster misconceptions, such as “a force is a push or pull that can change the shape of
an object or accelerate it, which means change its velocity.” Use questions such as these to
progress students’ discussion:
 How is impulse related to momentum?
 What quantity is always conserved in a collision?
 What quantity may not be conserved in a collision?
2. Lab protocols should be followed, incorporating safety equipment. Goggles must be worn at
all times.
3. Guide students to design an investigation that is valid and that allows them to observe the
conservation of momentum. The equipment will allow them to measure mass and velocity,
which should be familiar lab processes for most. Students may need a hint that producing a
constant horizontal force with standard masses requires using the fixed pulley. Some
questions to help focus students’ plans and guide them include the following:
 How can you measure impulse?
 How can you measure momentum?
 What investigation can you design to determine how impulse and momentum are related?
 What are the characteristics of an elastic collision?
 What are the characteristics of an inelastic collision?
 How can you measure momentum for a body on a string?
4. Remind students that the video treats impulse as the product of force and time, FΔt. The
video does not equate impulse to change in momentum, but it does state that a force changes
the motion of an object. So elicit from students that an impulse acting on an object must
therefore change its momentum. The standard masses are a convenient way to provide a
constant force. They can be suspended from a string that runs over a pulley at the edge of a
table. Make sure students arrange the pulley so that the string pulling the skateboard is
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Force, Impulse, Collisions (High School Lesson)
horizontal. Students should load the skateboard with enough bricks that it can be accelerated
on the level for several seconds using the available weights.
5. Students might develop a procedure for measuring momentum for horizontal motion such as
the following, where three students are needed to measure velocity. One measures the time
the skateboard accelerates, the second marks the point where the skateboard finishes
accelerating, and the third measures the time the skateboard takes to roll from that point to
the end of a predetermined course. The distance along the course and the mass of the loaded
skateboard can be measured either before or after the velocity measurement. Students should
find that the momentum they have measured for the skateboard is equal to the impulse they
have measured: FΔt = mv.
6. Remind students that, as discussed in the video, the force acting during a collision is an
impulse that lasts a very short time. Have students cite an example from the video that
explains why a collision between marbles is elastic (a collision in which very little energy is
lost). Then instruct students to develop a procedure for showing conservation of momentum
for an elastic collision involving objects suspended on strings. They might use the materials
to show the simplest case involving two blocks with the same mass. One block is drawn back
and strikes the other, stationary, block. Point out to students that it is not necessary to
measure velocity directly to show that energy and momentum are conserved for an elastic
collision. It is only necessary to show that m1v1i = m2v2f. Because the two masses are equal,
the distance of the initial swing should be equal to the recoil distance.
7. To show conservation of energy and momentum in the general case, students need to verify
the conservation equation m1v1i = m1v1f + m2v2f. If masses are unequal, the initial moving
block will either recoil or continue in the same direction after the collision, so v1f and v2f may
not be in the same direction. Again, students do not need direct measurements of initial and
final velocities. Because the arcs the blocks swing through are small, velocity is proportional
to the initial swing or the recoil distance. Regardless of the relative sizes of the two masses,
energy should be conserved: m1v1i2 = m1v1f2 + m2v2f2.
8. Review the portion of the video that shows a hockey puck striking a goalie’s pad as an
example of an inelastic collision (a collision in which kinetic energy is lost). Then instruct
students to develop a procedure for showing conservation of momentum for an inelastic
collision between objects suspended on strings. You might hint that the simplest case is the
same as that for an elastic collision, one involving two blocks with the same mass. To
produce an inelastic collision, students could attach a small piece of modeling clay to each
block. When the moving block strikes the stationary block, the two stick together. The
conservation of momentum requires that the final velocity be half the initial velocity.
Therefore the recoil distance will be half the initial swing distance. Students should be able to
explain that half of the initial kinetic energy is lost in this collision: m1v1i2 = 2(m1v1f2 +
m2v2f2). Point out that inelastic collisions with unequal masses are still subject to the
conservation of momentum. Again, absolute velocities are not needed to obtain correct
results, but the ratios of velocities are needed.
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Force, Impulse, Collisions (High School Lesson)
9. As a follow-up exercise, have students work in pairs to analyze a collision of two hockey
players. The two players both have a mass of 85 kg, and they have velocities of 4.5 m/s and
9.0 m/s, at right angles to each other. If one checks the other and both run into the boards
together, what is their velocity before they hit the boards? How much energy is lost when
they collide? How much energy is lost when they hit they boards? Consider varying the
values and have pairs share and compare their work. Or have small groups work at the board,
“racing” one another to the final answer. The winners are those who finish the quickest and
show the most complete, accurate steps to their answer.
Additional Resources:
 http://www.mme.wsu.edu/~ssl/pubs/imac2008c.pdf
 http://serc.carleton.edu/sp/compadre/demonstrations/examples/19119.html
 http://www.yenka.com/activities/Elastic_and_Inelastic_Collisions_-_Activity/
 http://www.education.com/science-fair/article/elastic-collisions/
 http://www.exploratorium.edu/hockey/shooting1.html
 http://www.physicsclassroom.com/class/momentum/u4l1b.cfm
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Force, Impulse, Collisions (High School Lesson)
Student Worksheet for Force, Impulse, and Collisions
Experiment Title: _____________________________Date: __________Name: _____________
Student Hypothesis: _____________________________________________________________
Materials:
- Safety goggles
- Skateboard
- Bricks
- Standard masses
- Fixed pulley
- String
- Stopwatch
- Tape
- Marker
- Meter stick
- Blocks of different masses
- Ring stands
- Modeling clay
Procedure:
Wear safety goggles for all lab work.
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Force, Impulse, Collisions (High School Lesson)
Data and Observations:
Analysis of Data: _______________________________________________________________
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Conclusion: ___________________________________________________________________
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Force, Impulse, Collisions (High School Lesson)