P5d: Action and Reaction

Coming to a sudden stop is far more painful and dangerous than stopping gently. Seatbelts and crumple zones in cars are designed to bring people and moving objects to rest slowly and safely. People falling from a burning building are caught in a ‘Fireman’s Blanket’ for the same reasons. Even objects with a small mass can have a lot of momentum when struck hard and given a high velocity, and even individual atoms can contribute momentum to launch a powerful rocket, if there are a large enough number of atoms involved.

What does every action have? (in terms of reaction)
An equal and opposite reaction
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In an action, what is the is significant about opposite reactions in a parallel collision?
They have parallel velocities.
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Explain the collision between a car and a tree.
The car and tree exert equal and opposite forces on each other in the collision.
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Explain how gas causes pressure on a wall.
The particles in a gas are able to move quickly and randomly in any direction. When they hit the wall of their container, they exert a force on the wall. This causes pressure on the wall.
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Talk about the equal force in a rocket.
the force pushing the particles backwards equals the force pushing the rocket forwards.
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What is Newton's third law of motion?
when an object collides with another object or two bodies interact, the two objects exert an equal and opposite force on each other
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Describe the situation of force -with a book on a table
The book is pulled down by the Earth’s gravity, and this force is balanced by the reaction force from the table top.
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What do equal but opposite forces act as?
A collision
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What is recoil?
Moving backwards
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What happens to a gun when it's fired? Why?
It recoils because the bullet leaves the front of the gun and this is balanced by the gun moving backwards.
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Give an example of an object that recoils like a gun?
if you try to step off a boat – the boat moves away from you as you reach for the side.
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What does pressure equal?
force/area
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Complete the sentence: For a given _____ of a container wall, the greater the _____ exerted, the _______ the gas pressure.
For a given area of a container wall, the greater the force exerted, the greater the gas pressure.
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What will a change in gas volume cause?
A change in pressure.
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Give an example of changes gas volume and pressure in an everyday situation.
the pressure increases as you pump up a bike tyre. More gas particles get squashed into the tyre, so more of them collide with the walls of the tyre each second.
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Where does a similar situation to pumping up a bike tyre happen?
In a piston.
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Describe the changes of gas volume and pressure in a piston.
When the piston is pushed downwards, the volume inside decreases. The same number of gas particles are squashed into a smaller volume. This means that the gas particles collide more often with the walls of the piston, so the pressure is increased.
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What will a change in temperature cause?
A change in pressure
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What happens as the temperature increases? How does this increase pressure?
the gas particles move faster. They hit the walls of their container more often, so increasing the pressure.
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'As the temperature increases, the gas particles move faster. They hit the walls of their container more often, so increasing the pressure.' What does this mean drivers should do?
This is why car drivers should check the pressure of their tyres when they are found to be cold.
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Why shouldn't drivers check the pressure of their tyres after a long journey?
the pressure in the tyres would be higher than the manufacturer’s specifications, as the tyres would be warm. The car driver might respond by letting some air out - but then the pressure would be too low when the tyres were cold again.
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What is produced when the fuel burns in a rocket? How to these particles move? What does this cause?
Hot exhaust gases - these particles move very quickly. They collide with the walls of the rocket engine, creating pressure. Since one end of the engine is open, the hot gases are pushed outwards and away from the rocket at high speed.
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Because of Newton’s Third Law of Motion, where can rockets work? Why?
in space, as well as in the atmosphere - they don’t have to push against the ground to keep moving.
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How are satellites launched into space? What is needed to do this?
Using rockets. A lot of force is needed to lift a large rocket into space.
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What two things are needed to produce enough force to launch a satellite into space? How are these achieved?
a large number of exhaust gas particles are needed and the exhaust gas particles must be moving at high speed. This is achieved by burning huge amounts of fuel very rapidly.
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Momentum is a property that is....
Conserved
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The total momentum before a collision is the same as what?
The total momentum afterwards.
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What three things does the conservation of momentum explain?
Recoils, Explosions and Rocket Propulsion.
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What is Rocket Propulsion?
The act of generating thrust entirely from chemicals that are stored on board. Rockets can therefore be used in space because they require no atmospheric oxygen.
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In recoil, the momentum of the bullet moving forwards is the same as the momentum of the gun moving backwards.
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In an explosion of a static object, such as a bomb, the total momentum is zero before the explosion. It is also zero during the explosion because the particles are moving away in different directions.
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Talk about momentum in rocket propulsion.
In rocket propulsion, the total momentum of the exhaust gas particles moving backwards is the same as the momentum of the rocket moving forwards.
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What is the equation for applying the principle of conservation of momentum to collisions between two objects moving in the same direction, which join together in the collision.
m˅1 u˅1 + m˅2 u˅2 = (m˅1 + m˅2)v
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What is m˅1 in the equation m˅1 u˅1 + m˅2 u˅2 = (m˅1 + m˅2)v?
mass of object 1 in kg
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What is u˅1 in the equation m˅1 u˅1 + m˅2 u˅2 = (m˅1 + m˅2)v?
initial velocity of object 1 in m/s
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What is m˅2 in the equation m˅1 u˅1 + m˅2 u˅2 = (m˅1 + m˅2)v?
mass of object 2 in kg
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What is u˅2 in the equation m˅1 u˅1 + m˅2 u˅2 = (m˅1 + m˅2)v?
initial velocity of object 2 in m/s
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What is v in the equation m˅1 u˅1 + m˅2 u˅2 = (m˅1 + m˅2)v?
the common velocity after the collision in m/s
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Other cards in this set

Card 2

Front

In an action, what is the is significant about opposite reactions in a parallel collision?

Back

They have parallel velocities.

Card 3

Front

Explain the collision between a car and a tree.

Card 4

Front

Explain how gas causes pressure on a wall.

Card 5

Front

Talk about the equal force in a rocket.