Learning Outcomes. Define:

Transcription

1 Dr Pusey

2 Learning Outcomes Define: Momentum Impulse Give the SI units for momentum and impulse Define the direction of momentum using positive and negative signs Analyse real world situations in terms of momentum and impulse (e.g. car safety designs) Use and rearrange the equations for momentum to solve problems Create and analyse a force-time graph to calculate impulse

3 If an object is moving it has momentum Vector quantity needs a direction The amount of momentum that an object has is dependent upon two variables: how much stuff is moving and how fast the stuff is moving. Think of it as then tendency of an object to keep moving with the same speed, in the same direction Simply put, Momentum is the product of an objects Mass and it s Velocity

4 Momentum (kg.m/s) Velocity (m/s) p = m v Mass (kg)

5 An object at rest has NO momentum p = mv = 0 Momentum is not the same thing as inertia Inertia Momentum Mass Mass Velocity

6 Example 2 Aaron throws a water balloon with a mass of 152 g and a velocity of 11 m/s at poor unsuspecting Zach. How much momentum did the water balloon have? p = m v = kg x 11 m/s = kg.m.s -1 Therefore, p = 1.7 kg.m.s -1 towards unsuspecting Zach

7 Another way to consider problems is to analyse the change in momentum, by looking at the change in velocity We can only get a change in momentum if there is a net force acting on an object p = p f p i = m v u = m v

8 Example 2 Rachel passes a netball with a velocity of 3 m/s and a mass of 2 kg to Caris on the other side of the court. What is the change of the netballs momentum: a) as Rachel throws it? b) As Caris catches it?

9 Example a) As Rachel throws it?

10 Example a) As Caris catches it?

11 Example 3 A car accelerates at the rate of 0.5 m/s 2 over 5 minutes. If the car has a mass of 300 kg find the change in momentum of the car as it speeds up.

12 Example 3 A car accelerates at the rate of 0.5 m/s 2 over 5 minutes. If the car has a mass of 300 kg find the change in momentum of the car as it speeds up.

13 Impulse = change in momentum Impulse has the same units as momentum kg.m A force acting for a given amount of time will change an object's momentum. s Faster the change in motion, the larger the force required

14 Impulse (kg.m/s) Change in time (s) I = F t Force (N)

15 Why do boxers wear boxing gloves? (think in terms of impulse)

16 Why do boxers wear boxing gloves? (think in terms of impulse) Change of momentum/impulse = same! Change in velocity of hand will go from moving to being still Gloves are padded, increases contact time of a punch Increased time means less force applied, for same change in momentum

17 The impulse experienced by an object is the force time. I = F. t The momentum change of an object is the mass velocity change. p = m. v The impulse equals the momentum change. I = p F. t = m. v

18 Example A 0.5 kg cart (#1) is pulled with a 1 N force for 1 s; another 0.5 kg cart (#2) is pulled with a 2 N force for 0.5 s. Which cart (#1 or #2) has the greatest impulse?

19 Example A 0.5 kg cart (#1) is pulled with a 1 N force for 1 s; another 0.5 kg cart (#2) is pulled with a 2 N force for 0.5 s. Which cart (#1 or #2) has the greatest impulse?

20 Example Dr Pusey jumps from the top of the Gymnasium (51 m) but forgot his parachute. Dr Pusey is a little on the heavy side (101 kg) How much momentum will he have at the bottom of his 51 m fall? If he falls onto super-duper-hard concrete which slows his body in 1 ms. What force will he experience? If he falls on mega-soft marshmallows which take 1 second to bring him to a stop, what is the average force he ll experience?

21 How much momentum will he have at the bottom of his 51 m fall? v 2 = u 2 + 2as = 2*9.8*51 = 999.6m/s v = 31.6 m/s = 32 m/s p = mv = 101 * 32 = 3190 kg.m/s = 3200 kg.m/s

22 If he falls onto super-duper-hard concrete which slows his body in 1 ms. What force will he experience? I = F. t = p F = p t =3200kg.m/s s = N

23 If he falls on mega-soft marshmallows which take 1 second to bring him to a stop, what is the average force he ll experience? I = F. t = p F = p t =3200kg.m/s 1 s = N That s 1000 times less! Airbags in vehicles use the principal of marshmallows to help save lives! Even seatbelts are designed to stretch a little bit to enlarge the time taken for the body to stop.

24 Example If he falls onto super-duper-hard concrete which slows his body in 1 ms. What force will he experience? I = F.t = p F = p/t = 3200kgm/s 0.001

25 If we graph F vs t

26 Example Find the impulse (or change in momentum) for this situation.

27 Example

28 How did you go? Define: Momentum Impulse Give the SI units for momentum and impulse Analyse real world situations in terms of momentum and impulse (e.g. car safety designs) Use and rearrange the equations for momentum to solve problems Create and analyse a force-time graph to calculate impulse