physics notes#2

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PHYSICS: Velocity and Distance time graphs

Distance-time graphs show where the object is heading so that could be forward,stopped or backwards. We can tell this by the graph itself. If the line is going up that must mean it's going forwards, when the line is flat it has stopped and when the line goes down it's going backwards. The whole area underneath the main line is the distance travelled from the object.

We can work out the gradient of the graph by drawing a triangle so it has a tangent with the line and then we can work out the area of the triangle by the units on the x and y-axis.

Velocity-time graphs show whether the object is accelerating, decelerating or staying at a constant speed. We can tell this by the line. If the line is going up the object is accelerating, if the line is flat the object is staying at a constant speed and if the line is pitched downwards this means that it is decelerating.

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PHYSICS: Newton's laws of motion

1. An object's velocity will not change unless a resultant force is acting

2. The acceleration is proportional to the resultant force

F=ma

3. Every force has an equal and opposite resultant force

Page 3

PHYSICS: The resultant force

The resultant force is the force moving when all of the forces acting on a body are added up allowing for the fact that they may be opposing each other.

For example, a car will experience gravity, this is normally cancelled out by the resultant force at each wheel so that there is a resultant vertical force.

The engine provides a driving force which is opposed by the air resistance and rolling resistance (friction). This may also be the wind that could be helping or hindering the cars motion.

These forces may not cancel out meaning that there could be a resultant horizontal force and acceleration or deacceleration.

example question- we assume no vertical resultant, but horizontally:

5000 forwards

-4200 backwards = 800 forwards

f=ma

a= f/m a= 800/6000 = 0.13 m/s squared

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PHYSICS: The force of gravity

The equation for the force due to gravity close to the surface of the earth is f=mg where f is weight in N, m is mass in kg and g is gravitational strength in N/kg.

If we compose this equation with f = ma it is clear that g is also an acceleration the acceleration due to gravity is 10m/s squared

For most things falling small distance air resistance is not significant and as the weight as the resultant force.

Example question- two cyclists are evenly matched both can produce 3000 N at peak output. In a sprint, the first cyclist is traveling at his maximum velocity, the second is slipstreaming which means that the air resistance is reduced by 15% if the second rider and bike have a mass of 105kg how fast can he accelerate?

15% of 3000 = 450

f = ma

450 = 105 x a

a = 450/125 = 4.2 m/s squared