Impulse and Momentum
When objects interact, like forces are exchanged and momentum is transferred.
◊Each student will determine the momentum of an object.
◊ Each student will determine a change in momentum experienced by an accelerated object.
◊ Each student will determine the change in momentum experienced by an object given a force vs. time graph for that object. / DO NOT WRITE ON THIS HANDOUT
Engage
The teacher will show the bobsled.mov quicktime movie. / Examine the picture on the left.
1. What’s happening? What are these pictures about?
2. What are the two people doing to the bobsled?
3. Draw a free-body diagram for the bobsled showing the net force exerted on the sled.
4. What kind of motion will the sled experience while being pushed?
5. Sketch the appearance of a position vs. time graph and a ticker tape for this kind of motion.
We are introduced to a new term: Momentum or “mass in motion”.
Just like the volume, mass, or color are used to describe an object, momentum gives us information about an object.
Momentum is the product of its mass and velocity.
Momentum / = / mass / • / velocity
p / = / m / • / v
kg• m/s / = / kg / • / m/s
5b. Why would it be useful to know an object’s momentum
6. Predict what happens to the momentum of the bobsled as the force is exerted for a period of time. Explain your answer.
Explore I
You will now make some predictions and measurements about the change in momentum experienced by a cart when a force is exerted on the cart for different amounts of time.
Consider this set-up.
7. What exerts the force on the cart in this set up?
8. What will happen to the momentum of the cart as the force is exerted?
9. If the drop distance is increased, what will happen to the amount of time the force is exerted?
10. Suppose you conducted this experiment for a drop distance of 20 cm and for a drop distance of 40 cm. Which drop distance would produce a greater change in momentum for the cart?
11. Prepare the table below.
12. Check your work with your teacher before proceeding to the equipment .
Drop Distance(cm) / Time Cart Traveled while hanger dropped
(from Graph)
(sec) / Average Speed
/ Instantaneous Speed
Instantaneous speed = 2 x average speed / Change in Cart Momentum
Change in momentum = Change in speed x mass / Force
(weight of the hanger) / Impulse
Impulse= Force x time
20
30
40
50
60
How to read the Datastudio graph and complete the data table.
distance traveled=0.76m - 0.21m = 0.55m
time=2.5 sec – 0.5 sec = 2.0 sec
average speed = 0.55 m / 2.0 sec = 0.28 m/s
instantaneous speed = 2 x 0.28 m/s = 0.56 m/s
change in momentum = 0.56m/s • 0.25 kg = 0.14 kg•m/s
Explain I
13. What happens to the change in momentum experienced by the cart as the time a force is exerted increases?
The relationship between a change in momentum and the force exerted on an object is known as the
IMPULSE MOMENTUM THEOREM
It can be summarized in the following equation:
Change in Momentum = Force • Time
This means that the longer a net force is exerted, the greater the change in momentum will be observed.
13.b Using your data table from page 2, do your data support the impulse momentum theorem? If you found differences, what might they be contributed to?
Practice Problems
14. A net force of 15 Newtons is exerted on a cart for 3 seconds. What is the change in momentum experienced by the cart?
15. A net force of 5 Newtons is exerted on a cart for 5 seconds. What is the change in momentum experienced by the cart?
Use this graph for question 16-17.
16. What is the change in momentum experienced by this object after 2 seconds?
17. What is the change in momentum experienced by this object after 7 seconds?
18. Why wouldn’t the bobsled riders push the sled all the way down the track?
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