Momentum & Impulse

Momentum Momentum is the amount of ‘oomph’ a moving object has. When an object has momentum, the object tends to carry on moving. The more the momentum, the harder it is to stop or slow down. It is NOT kinetic energy

Question: “Why do we wear seat belts?”

Even with the best protection

“Who has more momentum?”

p = mv Momentum is a VECTOR quantity. p = momentum (unit: ________) m = mass v = velocity Momentum is a VECTOR quantity.

“I’ve got a lot of oomph!”

Jonah Lomu

“Momentum is a conserved quantity” Any moving object will maintain its momentum (i.e. the momentum will be conserved) unless there is an external force applied to it.

The more the momentum the harder it is to stop or slow down. (i. e The more the momentum the harder it is to stop or slow down. (i.e. more force is required)

p = mv Momentum is a VECTOR quantity. p = momentum (unit: ________) m = mass v = velocity Momentum is a VECTOR quantity.

Example: In a cricket match, the ball (500 g) is bowled and it travels straight towards the batter’s face at 31 ms-1. The batter blocks the ball, then it travels straight towards the bowler’s face at 29 ms-1. Calculate the ball’s initial momentum Calculate the ball’s final momentum Calculate the ball’s change in momentum

Another Example: A bird is flying East and it slows down from 12 ms-1 to 5.0 ms-1. The mass of the bird is 320 g. Calculate: pi pf Δp

Change in Momentum & Impulse

Two kids, Andy (120 kg) and Bobby (130 kg), are running towards each other at 9.00 ms-1. After they collide, they bounce off each other and: Andy bounces back at 8.32 ms-1 Bobby bounces back at 6.98 ms-1 Calculate the change in momentum of: Andy Bobby

Momentum is a conserved quantity Any moving object will maintain its momentum unless there is an external force applied to it

To change the momentum, an unbalanced force (Fnet) must be applied. This is because: Unbalanced force  Acceleration  Change in velocity  Change in momentum  Newton’s Laws!! 

change in momentum (Δp) is also called “Impulse” Δp = F Δt Δp = impulse (unit: ______) F = force Δt = the time the force is applied for “Change in momentum is a result of force!!”

Impulse, Δp = F Δt

Δp = F Δt Example: To stop a 4500 kg truck travelling at 25 ms-1 in exactly 5 seconds, how much net force is required? How much net force would be required to stop the truck in just 2 seconds instead?

Another Example: A braking force of 25N is applied on a toy car (mass = 35 kg) travelling at 4.17 ms-1. How long would it take for the car to stop? How long would it take to stop the car if a net force of 100 N is applied instead?

Extra Example: A driver tries to stop a runaway car by pushing it. The car’s mass is 2100 kg and its initial velocity is 3.0 ms-1. Will the driver manage to stop the car if he produces the resultant force of 600 N steadily for 9.0 seconds?

Question: “How do air bags work?”

“Airbag or no airbag – what’s the difference?” Δp = F Δt

“Gloves or no gloves?”

“Crumple Zone”

Textbook Activity 10A skip question 4

Deriving the impulse formula: Δp = pf – pi = mvf – mvi = m (vf – vi) = m Δv = m a Δt = F Δt F = ma Δv a = Δt

Explain why a flower vase falling on carpet is less likely to shatter than if it were to fall on a hard wooden floor.

Achieved Achieved with Merit “The wooden floor exerts a greater force on the vase” “The vase stops more suddenly on the wooden floor” Achieved with Merit “The wooden floor exerts a greater force on the vase BECAUSE it stops more suddenly” “The carpet exerts less force on the vase BECAUSE the cushioning means it takes longer to stop”

Achieved with Excellence For both floor surfaces, the vase would experience the same change in momentum (Δp) BECAUSE they hit at the same speed and come to a stop. HOWEVER, the vase hitting the carpet will stop over a longer period of time, BECAUSE of the cushioning. SINCE Δp = F Δt , a longer stopping time means a smaller force is exerted on the vase to give the same change in momentum. The vase is less likely to break BECAUSE a smaller force is exerted by the carpet.

How to write answers for merits and excellences Make links by using conjunctions “because, therefore, as a result of, however, since, which means, whereas, although, etc.” Use or mention the relevant formula Answer the original question in the context, e.g. “the vase is less likely to shatter because a smaller force is applied by the carpet”

Question How do air bags work?

With or without the airbag, the person would experience the same change in momentum However, with the presence of the airbag, the person will stop over a longer period of time, because of the cushioning. Since Δp = F Δt, a longer stopping time means a smaller force is applied on the person, to give the same change in momentum. Without the airbag, the person will stop over a very short period of time (almost instantaneously). Since Δp = F Δt, short stopping time means a much greater force is applied, to give the same change in momentum. A greater force means the person is more likely to suffer greater injuries.