Impulse

Impulse is really just a fancy word, for a relatively simple concept. When we talk about impulse, we are really just talking about the change in momentum of an object. You have probably heard Captain Kirk from Star Trek order the crew to give "impulse 9". All he is talking about is changing the momentum of the ship! It all seems so clear to you now :)

Impulse can be described as the change in momentum of an object, that is the difference between the final and initial moment of an object. Also, the rate of change of momentum, is equivalent to the force exerted on the object.

Chester hits a 10g banana with a tennis Racket, which travels initially at 10m/s and reaches a final velocity of 30m/s.

a) Work out the initial momentum of the banana.

b) Calculate the final momentum.

c) What is the impulse of the banana?

d) If the ball is in contact with racket for 0.2s, what is the average value of the force exerted on the Banana.

e) Draw a diagram showing what the Banana will look like after the experiment.

SOLUTION :

a ) Initial momentum = mu = 0.01 x 10 = 0.1 Ns

b) Final momentum = mv = 0.01 x 30 = 0.3 Ns

c) Impulse = mv - mu = 0.3 - 0.1 = 0.2Ns

d) Force = Impulse/time = 0.2 / 0.2 = 1N

e) Too messy.

Well, thats another two formulae to jot down and remember. They will be very valuable for you, whenever you are working on problems involving momentum. The above example covers the work you have learnt in the past two lessons, and hopefully you had no trouble following it. The secret is remembering these three formulae, and being able to substitute values into them.

Now it's time for your first test on this topic. Attempt the 5 questions below, and see how many you can get right. Once that's done, you will be ready to go to the next topic, which is about the law of conservation of momentum.

Questions

1) The momentum of an object will:

a) Increase with a decrease in velocity and mass
b) Increase with in increase in mass and velocity
c) Not change when mass decreases
d) Will never change

2) A 10kg bowling ball travelling at 2m/s will have a momentum of:

a) 5Ns
b) 12Ns
c) 10Ns
d) 20Ns

3) Captain Kirk orders the crew of the Starship Enterprise to give 'impulse 9', by which he means to increase the momentum 9 times (multiply by 9). Initially it is travelling at 30,000,000m/s and weighs 3,000,000,000kg. Find the NEW momentum.

4) A ball weighing 100g travelling east at 50m/s bounces off a wall and now travels west at 50m/s find the impulse. (in m/s - let;East be the positive direction.

5) The Starship Enterprise in Question 3 changed from 9000000000000000Ns to 810000000000000000Ns. It achieved this in 30 seconds. Find the Force applied by the thrusters. (In Newtons)

Answers

1) B
2) D
3) 810000000000000000
4) -10
5) 24000000000000000