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Motion and Forces (Stopping Distance (Stopping distance = Thinking…
Motion and Forces
Pairs of Forces
Objects can interact with each other:
- When they touch
- At a distance
(e.g. The gravitational attraction between the Earth and the Moon)
When objects interact, forces always occur in pairs. These are often called action - reaction forces.The two forces are always:
- The same size and in opposite directions
- The same type of force
The differences between action- reaction forces and balanced forces isAction - reaction Forces:
- Act on different objects
- Are always the same type of force
Balanced Forces:
- Act on the same object
- Not always the same type of force
Resultant force
A scalar quantity has magnitude but no particular direction.
A vector quantity has magnitude and acts in a particular direction.
Scalar quantity
Scalar quantity is a a quantity which direction is not important. All it matters is the magnitude (its size). For example, these quantities are scalar:
~ time
~ mass
Vector quantity
A vector quantity is a quantity which direction is important. For example, these quantites are vector:
~ force
~ acceleration
The differences between scalar and vectors
Speed is a scalar quantity - It is the rate of change in the distance travelled by an object
Velocity is a vector quantity - It is the speed of an object in a particular direction.
What is resultant force?
All objects have different forces acting on them which have different strengths and direction. The resultant force is a single force that has the same effect on the object as all the individual forces acting together.
When all the forces are balanced, the resultant force is zero. In this case:
- A stationary object remains stationary
- A moving object keeps on moving at the same speed in the same direction
For example, in the diagram of the weight lifter, the resultant force of the bar is 0, so the bar does not move. The weight of the weight lifter is acting downwards is balanced by the upwards force.
{When the arrow is longer the, the force is bigger}
In this diagram the arrows are the same length this means they are the same size.
When all the forces are not balanced, the resultant force is not zero. In this case:
- A stationary objects beings to move in the direction of the resultant force
- A moving object speeds, slows down or changes direction depending on the direction of the resultant force.
In the diagram, the resultant force is the upwards direction, so the bar moves upwards.
Newtons laws
What is Newtons first law?
"An object stays still or keeps moving in a straight line at the same velocity unless a resultant force makes it change."
What is Newtons second law?
"The acceleration in the direction of a resultant force depends upon the size of the force and the mass of the object."
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Equation of Newtons second law
Acceleration (m/s^2) = Force (F) / Mass (Kg)
Force (F) = Mass (Kg) x Acceleration (m/s^2)
Mass and weight
What is Mass?
Mass is the amount of matter in something, measure in kilograms.
What is Weight?
Weight is a force caused by gravity. The weight of the object is the gravitational force between the object and the earth. Measured in Newtons.
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Momentum
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Equation of Momentum
Momentum (Kgm/s) = Mass (Kg) x Velocity (m/s)
For example, what is the momentum of a 5kg object moving with a velocity of 2m/s
Equation of Momentum 2
Force (N) = Change in Momentum (Mv - Mu) / Time (S)
Mv - First Momentum
Mu - Initial Momentum
Direction
Momentum does not just depend on the objects mass and speed. Velocity is speed in a particular direction, so the momentum of an object also depends on the direction of travel. This means that the momentum of an object can change if:
- The object speeds up or slows down
- The object changes direction
Conservation of Momentum
The total momentum stays the same in explosions and collision.The momentum is conserved.
Momentum Calculations:
Two railway carriages collide and move off together. Carriage A has a mass of 12,000 kg and moves at 5 m/s before the collision. Carriage B has a mass of 8,000 kg and is stationary before the collision. What is the velocity of the two carriages after the collision?
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Momentum of carriage A before = 12,000 × 5 = 60,000 kg m/s
Momentum of carriage B before = 8,000 × 0 = 0 kg m/s
Total momentum before = 60,000 + 0 = 60,000 kg m/s
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Because momentum is conserved, total momentum afterwards = 60,000 kg m/s
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Total mass = mass of carriage A + mass of carriage B = 12,000 + 8,000 = 20,000 kg
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p = m × v, but we can rearrange this equation so that v = p ÷ m
Velocity (after the collision) = 60,000 ÷ 20,000 = 3 m/s
Stopping Distance
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Thinking Distance
It takes time for the driver to react to a situation and apply the breaks. The thinking distance is the distance travelled in this reaction time.The thinking time increases if the reaction time increases. This happens because the driver is:
- tired
- distracted
- under the influence of alcohol or drugs
The thinking distance also increases as the car's speed increases.
Braking Distance
The braking distance is the distance taken to stop once the brakes are applied.
The braking distance increases if:
- The cars brake or tyres are in poor condition
- There are poor road and weather condition (e.g icy or wet roads)
- the car has a larger mass (e.g. more people in it)
The braking distance also increases as the cars speed increases.
How are human reaction times measured?
Reaction time testing - Reaction time testing assess a persons quickness to react to a stimulus.
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Crash Hazards
Car safety feature
When there is a car crash, its contents and the passengers decelerate rapidly. They experience great forces because of the change in momentum which can cause injuries.Modern cars have safety features that absorb kinetic energy in collisions. These include:
- Seat belts
- Air bags
- Crumple zones
These features reduce injuries to the people in the car by absorbing energy when they change shape.
Other safety features
Designed to help you survive a crash:
Anti-lock braking system (ABS)
- prevents skidding, more friction between road and tyres
Traction control
- prevents skidding whilst accelerating so the car can quickly escape a dangerous situation.
Safety cage
- strengths the cabin section to protect people
Designed to help prevent accidents:
Electric Windows
- make it easier to open and close doors
Cruise Control
- helps reduce accidental speeding
Paddle shift Control
- allows the driver to keep both of their hands on the steering wheel while changing gear or radio stations.
Ajustable Seats
- comfortable for the driver
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