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8.2 Impulse

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Learning objectives

By the end of this section, you will be able to:

  • Define impulse.
  • Describe effects of impulses in everyday life.
  • Determine the average effective force using graphical representation.
  • Calculate average force and impulse given mass, velocity, and time.

The information presented in this section supports the following AP ® learning objectives and science practices:

  • 3.D.2.1 The student is able to justify the selection of routines for the calculation of the relationships between changes in momentum of an object, average force, impulse, and time of interaction. (S.P. 2.1)
  • 3.D.2.2 The student is able to predict the change in momentum of an object from the average force exerted on the object and the interval of time during which the force is exerted. (S.P. 6.4)
  • 3.D.2.3 The student is able to analyze data to characterize the change in momentum of an object from the average force exerted on the object and the interval of time during which the force is exerted. (S.P. 5.1)
  • 3.D.2.4 The student is able to design a plan for collecting data to investigate the relationship between changes in momentum and the average force exerted on an object over time. (S.P. 4.1)
  • 4.B.2.1 The student is able to apply mathematical routines to calculate the change in momentum of a system by analyzing the average force exerted over a certain time on the system. (S.P. 2.2)
  • 4.B.2.2 The student is able to perform analysis on data presented as a force-time graph and predict the change in momentum of a system. (S.P. 5.1)

The effect of a force on an object depends on how long it acts, as well as how great the force is. In [link] , a very large force acting for a short time had a great effect on the momentum of the tennis ball. A small force could cause the same change in momentum    , but it would have to act for a much longer time. For example, if the ball were thrown upward, the gravitational force (which is much smaller than the tennis racquet’s force) would eventually reverse the momentum of the ball. Quantitatively, the effect we are talking about is the change in momentum Δ p size 12{Δp} {} .

By rearranging the equation F net = Δ p Δ t to be

Δ p = F net Δ t , size 12{Δp= F rSub { size 8{"net"} } Δt} {}

we can see how the change in momentum equals the average net external force multiplied by the time this force acts. The quantity F net Δ t size 12{F rSub { size 8{"net"} } Δt} {} is given the name impulse    . Impulse is the same as the change in momentum.

Impulse: change in momentum

Change in momentum equals the average net external force multiplied by the time this force acts.

Δ p = F net Δ t

The quantity F net Δ t size 12{F rSub { size 8{"net"} } Δt} {} is given the name impulse.

There are many ways in which an understanding of impulse can save lives, or at least limbs. The dashboard padding in a car, and certainly the airbags, allow the net force on the occupants in the car to act over a much longer time when there is a sudden stop. The momentum change is the same for an occupant, whether an air bag is deployed or not, but the force (to bring the occupant to a stop) will be much less if it acts over a larger time. Cars today have many plastic components. One advantage of plastics is their lighter weight, which results in better gas mileage. Another advantage is that a car will crumple in a collision, especially in the event of a head-on collision. A longer collision time means the force on the car will be less. Deaths during car races decreased dramatically when the rigid frames of racing cars were replaced with parts that could crumple or collapse in the event of an accident.

Bones in a body will fracture if the force on them is too large. If you jump onto the floor from a table, the force on your legs can be immense if you land stiff-legged on a hard surface. Rolling on the ground after jumping from the table, or landing with a parachute, extends the time over which the force (on you from the ground) acts.

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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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