GIÁO TRÌNH

College Physics

Science and Technology

Impulse

Tác giả: OpenStaxCollege

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 Δp size 12{Δp} {}.

By rearranging the equation Fnet=ΔpΔt to be

Δp=FnetΔt, 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 FnetΔt size 12{F rSub { size 8{"net"} } Δt} {} is given the name impulse. Impulse is the same as the change in momentum.

Calculating Magnitudes of Impulses: Two Billiard Balls Striking a Rigid Wall

Two identical billiard balls strike a rigid wall with the same speed, and are reflected without any change of speed. The first ball strikes perpendicular to the wall. The second ball strikes the wall at an angle of 30º size 12{"30"°} {} from the perpendicular, and bounces off at an angle of 30º size 12{"30"°} {} from perpendicular to the wall.

(a) Determine the direction of the force on the wall due to each ball.

(b) Calculate the ratio of the magnitudes of impulses on the two balls by the wall.

Strategy for (a)

In order to determine the force on the wall, consider the force on the ball due to the wall using Newton’s second law and then apply Newton’s third law to determine the direction. Assume the x size 12{x} {}-axis to be normal to the wall and to be positive in the initial direction of motion. Choose the y size 12{y} {}-axis to be along the wall in the plane of the second ball’s motion. The momentum direction and the velocity direction are the same.

Solution for (a)

The first ball bounces directly into the wall and exerts a force on it in the +x size 12{+x} {} direction. Therefore the wall exerts a force on the ball in the x size 12{ - x} {} direction. The second ball continues with the same momentum component in the y size 12{y} {} direction, but reverses its x size 12{x} {}-component of momentum, as seen by sketching a diagram of the angles involved and keeping in mind the proportionality between velocity and momentum.

These changes mean the change in momentum for both balls is in the x size 12{ - x} {} direction, so the force of the wall on each ball is along the x size 12{ - x} {} direction.

Strategy for (b)

Calculate the change in momentum for each ball, which is equal to the impulse imparted to the ball.

Solution for (b)

Let u size 12{u} {} be the speed of each ball before and after collision with the wall, and m size 12{m} {} the mass of each ball. Choose the x size 12{x} {}-axis and y size 12{y} {}-axis as previously described, and consider the change in momentum of the first ball which strikes perpendicular to the wall.

p xi = mu ; p yi = 0 size 12{p rSub { size 8{"xi"} } = ital "mu""; "p rSub { size 8{"yi"} } =0} {}
p xf = mu ; p yf = 0 size 12{p rSub { size 8{"xf"} } = - ital "mu""; "p rSub { size 8{"yf"} } =0} {}

Impulse is the change in momentum vector. Therefore the x-component of impulse is equal to 2mu and the y size 12{y} {}-component of impulse is equal to zero.

Now consider the change in momentum of the second ball.

p xi = mu cos 30º ; p yi = –mu sin 30º size 12{p rSub { size 8{"xi"} } = ital "mu""cos 30"°"; "p rSub { size 8{"yi"} } = - ital "mu""sin 30"°} {}
p xf = mu cos 30º ; p yf = mu sin 30º size 12{p rSub { size 8{"xf"} } = - ital "mu""cos 30"°"; "p rSub { size 8{"yf"} } = - ital "mu""sin 30"°} {}

It should be noted here that while px size 12{p rSub { size 8{x} } } {} changes sign after the collision, py size 12{p rSub { size 8{y} } } {} does not. Therefore the x size 12{x} {}-component of impulse is equal to 2mucos 30º size 12{ - 2 ital "mu""cos""30"°} {} and the y size 12{y} {}-component of impulse is equal to zero.

The ratio of the magnitudes of the impulse imparted to the balls is

2mu2mucos 30º=23=1.155. size 12{ { {2 ital "mu"} over {2 ital "mu""cos""30" rSup { size 8{ circ } } } } = { {2} over { sqrt {3} } } =1 "." "155"} {}

Discussion

The direction of impulse and force is the same as in the case of (a); it is normal to the wall and along the negative x size 12{x} {}-direction. Making use of Newton’s third law, the force on the wall due to each ball is normal to the wall along the positive x size 12{x} {} -direction.

Our definition of impulse includes an assumption that the force is constant over the time interval Δt size 12{Δt} {}. Forces are usually not constant. Forces vary considerably even during the brief time intervals considered. It is, however, possible to find an average effective force Feff that produces the same result as the corresponding time-varying force. [link] shows a graph of what an actual force looks like as a function of time for a ball bouncing off the floor. The area under the curve has units of momentum and is equal to the impulse or change in momentum between times t1 and t2 size 12{t rSub { size 8{2} } } {}. That area is equal to the area inside the rectangle bounded by Feff, t1, and t2. Thus the impulses and their effects are the same for both the actual and effective forces.

A graph of force versus time with time along the x size 12{x} {}-axis and force along the y size 12{y} {}-axis for an actual force and an equivalent effective force. The areas under the two curves are equal.

Section Summary

  • Impulse, or change in momentum, equals the average net external force multiplied by the time this force acts:
    Δp=FnetΔt.
  • Forces are usually not constant over a period of time.

Conceptual Questions

Professional Application

Explain in terms of impulse how padding reduces forces in a collision. State this in terms of a real example, such as the advantages of a carpeted vs. tile floor for a day care center.

While jumping on a trampoline, sometimes you land on your back and other times on your feet. In which case can you reach a greater height and why?

Professional Application

Tennis racquets have “sweet spots.” If the ball hits a sweet spot then the player's arm is not jarred as much as it would be otherwise. Explain why this is the case.

Problems & Exercises

A bullet is accelerated down the barrel of a gun by hot gases produced in the combustion of gun powder. What is the average force exerted on a 0.0300-kg bullet to accelerate it to a speed of 600 m/s in a time of 2.00 ms (milliseconds)?

9 . 00 × 10 3 N size 12{9 "." "00" times "10" rSup { size 8{3} } `N} {}

Professional Application

A car moving at 10 m/s crashes into a tree and stops in 0.26 s. Calculate the force the seat belt exerts on a passenger in the car to bring him to a halt. The mass of the passenger is 70 kg.

A person slaps her leg with her hand, bringing her hand to rest in 2.50 milliseconds from an initial speed of 4.00 m/s. (a) What is the average force exerted on the leg, taking the effective mass of the hand and forearm to be 1.50 kg? (b) Would the force be any different if the woman clapped her hands together at the same speed and brought them to rest in the same time? Explain why or why not.

a) 2.40×103 N size 12{2 "." "40" times "10" rSup { size 8{3} } " N"} {} toward the leg

b) The force on each hand would have the same magnitude as that found in part (a) (but in opposite directions by Newton’s third law) because the change in momentum and the time interval are the same.

Professional Application

A professional boxer hits his opponent with a 1000-N horizontal blow that lasts for 0.150 s. (a) Calculate the impulse imparted by this blow. (b) What is the opponent’s final velocity, if his mass is 105 kg and he is motionless in midair when struck near his center of mass? (c) Calculate the recoil velocity of the opponent’s 10.0-kg head if hit in this manner, assuming the head does not initially transfer significant momentum to the boxer’s body. (d) Discuss the implications of your answers for parts (b) and (c).

Professional Application

Suppose a child drives a bumper car head on into the side rail, which exerts a force of 4000 N on the car for 0.200 s. (a) What impulse is imparted by this force? (b) Find the final velocity of the bumper car if its initial velocity was 2.80 m/s and the car plus driver have a mass of 200 kg. You may neglect friction between the car and floor.

a) 800 kgm/s size 12{"800"`"kg" cdot "m/s"} {} away from the wall

b) 1.20 m/s size 12{1 "." "20"`"m/s"} {} away from the wall

Professional Application

One hazard of space travel is debris left by previous missions. There are several thousand objects orbiting Earth that are large enough to be detected by radar, but there are far greater numbers of very small objects, such as flakes of paint. Calculate the force exerted by a 0.100-mg chip of paint that strikes a spacecraft window at a relative speed of 4.00×103m/s size 12{4 "." "00" times "10" rSup { size 8{3} } "m/s"} {}, given the collision lasts 6.00×108s.

Professional Application

A 75.0-kg person is riding in a car moving at 20.0 m/s when the car runs into a bridge abutment. (a) Calculate the average force on the person if he is stopped by a padded dashboard that compresses an average of 1.00 cm. (b) Calculate the average force on the person if he is stopped by an air bag that compresses an average of 15.0 cm.

(a) 1.50×106N size 12{ - 1 "." "50" times "10" rSup { size 8{6} } N} {} away from the dashboard

(b) 1.00×105N size 12{ - 1 "." "00" times "10" rSup { size 8{5} } N} {} away from the dashboard

Professional Application

Military rifles have a mechanism for reducing the recoil forces of the gun on the person firing it. An internal part recoils over a relatively large distance and is stopped by damping mechanisms in the gun. The larger distance reduces the average force needed to stop the internal part. (a) Calculate the recoil velocity of a 1.00-kg plunger that directly interacts with a 0.0200-kg bullet fired at 600 m/s from the gun. (b) If this part is stopped over a distance of 20.0 cm, what average force is exerted upon it by the gun? (c) Compare this to the force exerted on the gun if the bullet is accelerated to its velocity in 10.0 ms (milliseconds).

A cruise ship with a mass of 1.00×107 kg size 12{1 "." "00" times "10" rSup { size 8{7} } " kg"} {} strikes a pier at a speed of 0.750 m/s. It comes to rest 6.00 m later, damaging the ship, the pier, and the tugboat captain’s finances. Calculate the average force exerted on the pier using the concept of impulse. (Hint: First calculate the time it took to bring the ship to rest.)

4 . 69 × 10 5 N size 12{4 "." "69" times "10" rSup { size 8{5} } " N"} {} in the boat’s original direction of motion

Calculate the final speed of a 110-kg rugby player who is initially running at 8.00 m/s but collides head-on with a padded goalpost and experiences a backward force of 1.76×104 N size 12{1 "." "76" times "10" rSup { size 8{4} } " N"} {} for 5.50×10–2 s size 12{5 "." "50" times "10" rSup { size 8{"-2"} } " s"} {}.

Water from a fire hose is directed horizontally against a wall at a rate of 50.0 kg/s and a speed of 42.0 m/s. Calculate the force exerted on the wall, assuming the water’s horizontal momentum is reduced to zero.

2 . 10 × 10 3 N size 12{2 "." "10" times "10" rSup { size 8{3} } `N} {} away from the wall

A 0.450-kg hammer is moving horizontally at 7.00 m/s when it strikes a nail and comes to rest after driving the nail 1.00 cm into a board. (a) Calculate the duration of the impact. (b) What was the average force exerted on the nail?

Starting with the definitions of momentum and kinetic energy, derive an equation for the kinetic energy of a particle expressed as a function of its momentum.

p = mv p 2 = m 2 v 2 p 2 m = mv 2 p 2 2m = 1 2 mv 2 = KE KE = p 2 2m alignl { stack { size 12{p=mv drarrow p rSup { size 8{2} } =m rSup { size 8{2} } v rSup { size 8{2} } drarrow { {p rSup { size 8{2} } } over {m} } =mv rSup { size 8{2} } } {} # drarrow { {p rSup { size 8{2} } } over {2m} } = { {1} over {2} } mv rSup { size 8{2} } = ital "KE" {} # {underline { ital "KE"= { {p rSup { size 8{2} } } over {2m} } }} {} } } {}

A ball with an initial velocity of 10 m/s moves at an angle 60º above the +x size 12{+x} {}-direction. The ball hits a vertical wall and bounces off so that it is moving 60º above the x size 12{ - x} {}-direction with the same speed. What is the impulse delivered by the wall?

When serving a tennis ball, a player hits the ball when its velocity is zero (at the highest point of a vertical toss). The racquet exerts a force of 540 N on the ball for 5.00 ms, giving it a final velocity of 45.0 m/s. Using these data, find the mass of the ball.

60.0 g

A punter drops a ball from rest vertically 1 meter down onto his foot. The ball leaves the foot with a speed of 18 m/s at an angle 55º size 12{"55"°} {} above the horizontal. What is the impulse delivered by the foot (magnitude and direction)?

 
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