Starter Momentum Watch https www youtube comwatch vOi

Starter Momentum

Watch: https: //www. youtube. com/watch? v=Oi. Ti. KOy 59 o 4 What happenend with Sandra Bullock? What is the action force? What is the reaction force? Do the action force and the reaction force act on the same object? What can we say about the size of the forces? What does Newton’s third law state?

Watch https: //www. youtube. com/watch? v=CCt. N 9 jwhcyg What happenend with the two cars? Is Newton’s third law valid? Why not? Do both cars have a momentum before the collision? Why? Why not? Have they got the same momentum?

What is the equation for momentum? P Momentum = Mass x Velocity (in kgms-1) (in kg) (in ms-1) M What can you tell me about the momentum after the collision? How do we calculate the change in momentum? V

Conservation of Momentum

Starter • A car crash is an example of a serious collision. • Can you think of some less serious “everyday” examples?

Learning Objectives • Know how to reduce injuries in a collision. • Know what conservation of momentum is

Success criteria • To be able to explain why a collision cause injury. (Grade C) • To be able to explain how to reduce injury in a collision. (Grade C) • To be able to carry out calculations related to conservation of momentum (Grade A)

Watch the video: Why would this have happened? ←

Force = change in momentum time Therefore. . . • Short impact time = large force • Large impact time = small force So, how can we avoid or reduce injuries in collisions?

P 3 revision Safety features in/on a car Worksheet – stopping more safely

Decreasing the Force • We need to decrease the force experienced by passengers in a crash. • If: Force = change in momentum time . . . Then the force can be decreased if the time it takes for the vehicle to stop is increased. • For example, crumple zones cause energy to be absorbed over an extended period, decreasing velocity more gradually. This channels energy away from, and reduces the force on the people inside the car.

• Foundation: w/s P 5 d 4 – old course • Higher tier: work on conservation of momentum

Conservation of Momentum In any collision or explosion momentum is conserved (provided that no external forces have an effect). Example question: Two cars are racing around the M 25. Car A collides with the back of car B and the cars stick together. What speed do they move at after the collision? Speed = 50 ms-1 Mass = 1000 kg Speed = 20 ms-1 Mass = 800 kg Speed = ? ? ms-1 Mass = 1800 kg Momentum before = momentum after… …so 1000 x 50 + 800 x 20 = 1800 x V… …V = 36. 7 ms-1

Momentum in different directions What happens if the bodies are moving in opposite directions? Speed = 50 ms 1 Speed = 20 ms 1 Mass = 1000 kg Mass = 800 kg Momentum is a VECTOR quantity, so the momentum of the second car is negative… Total momentum = 1000 x 50 – 800 x 20 = 34000 kgms -1 Speed after collision = 34000 kgms-1 / 1800 = 18. 9 ms-1

Now do worksheet P 5 d 6 – old course

Answers 1. 2. 3. 4. 5. 6. Very rapid acceleration of parts of the body To extend the time of any collision, reducing the rate of change of momentum, and so the force on the motorcyclist’s head The front crumples in an accident, prolonging the time taken to stop; this reduces the rate of change of momentum, and so the force on the people in the car; the passenger compartment needs to be strong so that it does not collapse, injuring the occupants To prevent the person wearing it stopping too quickly in a collision; this reduces the rate of change of momentum, and so the force on the person wearing it, reducing injury An air bag inflates and squashes; it slows the driver down more slowly as well as protecting them from protruding objects; this protects both the head and thorax (chest) of the driver Gymnasts bend their knees on landing so that they stop more slowly, reducing the rate of change of momentum, and so the force on their knees Higher 1. 3 m/s 2. 20 Ns (or kg m/s) 3. 2. 2 m/s 4. 0. 0016 s 5. Total momentum = 0 before firing pellet, so total momentum = 0 after firing pellet as momentum is conserved; this means ‘momentum of pellet forwards = momentum of rifle backwards’ – – – 0. 4 kg m/s 0. 2 m/s

Explosions • An explosion is the opposite of a collision – objects move apart. However, momentum is still conserved. For example. . .

• Before firing, total momentum is zero. • After firing, total momentum must also be zero • As the bullet moves forward, the gun must move backwards (it recoils)

Learning Objectives • Know how to reduce injuries in a collision. • Know what conservation of momentum is

Success criteria • To be able to explain why a collision cause injury. (Grade C) • To be able to explain how to reduce injury in a collision. (Grade C) • To be able to carry out calculations related to conservation of momentum (Grade A)

Plenary • Watch the video: • The car is designed to break up. Why?