Impulse and momentum dodgeball example | Physics | Khan Academy
TLDRIn this educational video, the instructor uses the scenario of a person getting hit by a dodgeball to explore the principles of impulse, momentum, force, and time. By assigning values to the mass of the ball, its incoming and outgoing speeds, and the contact time with the person's head, the instructor calculates the impulse exerted by the person's face on the ball. He explains the concept of impulse as the product of force and time, and how it relates to the change in momentum. The video simplifies complex physics concepts with a mnemonic, 'Jape Fat', to help remember the relationship between these variables. It also touches on Newton's third law, illustrating that the force exerted by the ball on the person's face is equal and opposite to the force exerted by the face on the ball.
Takeaways
- π The script discusses a scenario where a person gets hit by a dodgeball, providing a real-world context for physics concepts.
- π It introduces the concept of impulse (referred to humorously as 'jimpulse') as the product of force and the time over which it acts.
- π’ The instructor provides specific values for the mass of the ball (0.2 kg), its incoming speed (10 m/s), and outgoing speed (5 m/s) to illustrate calculations.
- β±οΈ The time of contact between the ball and the person's face is estimated to be 0.02 seconds, which is a crucial parameter for the calculations.
- π Impulse is also defined as the change in momentum of an object, which is the final momentum minus the initial momentum.
- π The change in momentum is calculated by considering the mass, initial velocity (negative, as it's to the left), and final velocity (positive, as it's to the right).
- π§ A mnemonic 'Jape Fat' is introduced to help remember the relationship between impulse, change in momentum, force, and time.
- π« The instructor clarifies that gravity can typically be ignored in such collision problems due to the short duration of contact and the small mass of the ball.
- π The net impulse in the horizontal direction is found by considering only the horizontal velocities, as gravity acts vertically.
- π The impulse from the person's face on the ball is determined to be 3 kgΒ·m/s to the right, indicating the direction of the force exerted.
- βοΈ Using the impulse and the time of contact, the average force exerted on the ball by the person's face is calculated to be 150 Newtons.
- π Newton's third law is highlighted to explain that the force on the face from the ball is equal and opposite to the force on the ball from the face.
Q & A
What is the scenario described in the script?
-The script describes a situation where a person is playing dodgeball and gets hit in the head by the ball, which bounces off.
Why is this scenario a good opportunity for a scientific discussion?
-This scenario is a good opportunity for a scientific discussion because it allows for an exploration of the concepts of impulse, momentum, force, and time.
What is the mass of the ball used in the example?
-The mass of the ball used in the example is 0.2 kilograms.
What are the initial and final velocities of the ball given in the script?
-The initial velocity of the ball is 10 meters per second, and the final velocity after bouncing off the person's head is 5 meters per second.
What is the duration of the contact between the ball and the person's face in the example?
-The duration of the contact between the ball and the person's face is approximately 0.02 seconds, or 20 milliseconds.
What is the definition of impulse mentioned in the script?
-Impulse, represented by the letter J, is defined as the force acting on an object multiplied by the time duration during which that force is acting.
What is the alternate formula for impulse mentioned in the script?
-The alternate formula for impulse is that the net impulse on an object should equal the change in momentum of that object.
What is the mnemonic device 'Jape Fat' used for in the script?
-The mnemonic device 'Jape Fat' is used to help remember the relationship between impulse, change in momentum, force, and time.
How is the change in momentum calculated in the script?
-The change in momentum is calculated by taking the final momentum (mass times final velocity) and subtracting the initial momentum (mass times initial velocity).
What is the calculated impulse on the ball from the person's face?
-The calculated impulse on the ball from the person's face is 3 kilogram meters per second.
Why can the force of gravity be ignored in this scenario?
-The force of gravity can be ignored in this scenario because the time period of the collision is very small and the weight of the ball is much smaller than the force exerted by the person's face.
What is the average force on the person's face from the ball calculated to be?
-The average force on the person's face from the ball is calculated to be 150 Newtons, directed to the left.
How does Newton's third law relate to the forces in this scenario?
-According to Newton's third law, the force exerted by the ball on the person's face is equal and opposite to the force exerted by the person's face on the ball.
Outlines
πΎ Physics of Dodgeball Impact
The instructor uses the scenario of a person getting hit by a dodgeball to introduce the concepts of impulse, momentum, force, and time relationship. They assign hypothetical values to the mass of the ball (0.2 kg), its incoming speed (10 m/s), and outgoing speed (5 m/s) after bouncing off the person's head. The contact time between the ball and the person's face is estimated to be 0.02 seconds. The impulse, represented humorously as 'jimpulse', is explained as the product of force and time duration. Since the force exerted by the person's face on the ball is unknown, the instructor uses the change in momentum formula to calculate the net impulse. The final and initial momenta are calculated using the mass and velocities, taking into account the directionality of the velocities. The calculated impulse is found to be 3 kilogram meters per second, indicating the ball was acted upon by a force to the right.
π Analyzing the Impulse and Force in Dodgeball Collision
The instructor clarifies that the calculated impulse represents the net impulse on the ball from all forces, but effectively, it's the impulse from the person's face since gravity's influence in the horizontal direction is negligible due to the short contact time and the small mass of the ball. The focus remains on the horizontal components of velocity. The instructor proceeds to calculate the average force exerted on the person's face by the ball using the net impulse and the contact time, resulting in a force of 150 Newtons to the right. By Newton's third law, the force on the face from the ball is equal and opposite, which means it's 150 Newtons to the left, or negative 150 Newtons when considering directionality.
π§ Memorizing the Relationship Between Impulse, Momentum, and Force
The instructor concludes by emphasizing the mnemonic device 'Jape Fat' to remember the relationship between net impulse, change in momentum, and net force multiplied by the time interval. They reiterate that during collisions, there is always an equal and opposite force exerted on the two objects involved, which is a fundamental principle in understanding the dynamics of collisions.
Mindmap
Keywords
π‘Dodgeball
π‘Impulse
π‘Momentum
π‘Force
π‘Time
π‘Mass
π‘Velocity
π‘Change in Momentum
π‘Net Impulse
π‘Newton's Third Law
π‘Jape Fat
Highlights
The instructor uses a dodgeball scenario to introduce the concept of impulse, momentum, force, and time relationship.
A .2 kilogram ball is used as an example to explain the physics concepts.
Ball's incoming speed is given as 10 meters per second, and the outgoing speed as 5 meters per second.
The time of contact between the ball and the person's face is estimated to be about 0.02 seconds.
Impulse is humorously referred to as 'jimpulse' to help remember the letter J is used.
Impulse is defined as force multiplied by the time duration of the force's action.
An alternate formula for impulse is the change in momentum of an object.
The mnemonic 'Jape Fat' is introduced to remember the relationship between impulse, change in momentum, force, and time.
The change in momentum is calculated using the mass and velocities before and after the collision.
The importance of considering direction when calculating momentum is emphasized.
The calculated impulse on the ball is found to be 3 kilogram meters per second.
The direction of the impulse is the same as the direction of the force applied.
The force of gravity is considered negligible in the context of this collision due to the short contact time and small mass of the ball.
The net impulse in the X direction is found to be the impulse from the person's face on the ball, ignoring gravity.
The average force on the person's face from the ball is calculated to be 150 Newtons.
Newton's third law is applied to determine the force on the face from the ball is equal and opposite to the force on the ball from the face.
The mnemonic 'Jape Fat' is reiterated as a tool to remember the relationship between the physical concepts discussed.
The importance of recognizing equal and opposite forces during collisions is highlighted.
Transcripts
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