Forces

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  • Created by: GCSE2020
  • Created on: 11-01-19 18:09

Mass and Weight

Mass is the amount of matter in an object. The mass of an object is the same on the Moon and the Earth. It is measure in kilograms.

Weight is the force of gravity on an object. The weight of an object on the Moon is about 1/6 that is on Earth. Weight is measure in Newtons.

The acceleration due to gravity (g) varies with plant, moon and star. It also depends on the height of the object. The gravtational field strength on Earth= 10N/Kg

Calculating Weight:

Weight= mass x gravitational field strength

(N)          (Kg)         (N/Kg)

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Changing Shape

  • Force can change the shape of an object. A stretching force puts an object such as wire or a spring under tension. A squashing force puts an object under compression.
  • Brittle materials such as glass, don't change shape easily. They break before noticeably stretching.
  • Resilent materials don't break easily, they bend.
  • You can have Brittle Resilent materials.
  • Elastic materials return back to their orginal shape when the forces on them are removed.
  • Plastic materials retain their new shape.
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Stretching Springs - Experiemental procedure

1. Place the weight holder only on the spring and note the position of the pin again on the metal rule.

2. Add 1N (100g) to the holder and note the new position of the pin against the metal rule. Then calculate the extension of the spring.

Repeat these steps until the spring is permanently deformed.

Formula:

Force = Spring Constant x extension

 (N)              (N/m)                 (m)

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Elastic Potential Energy

Elastic Potential Energy= 1/2 x spring constant x extension squared#

Hooke's Law:

Hooke's law states that the extension of a spring is directly proportional to the force used to stretch the spring. Proportional means that if the force is doubled then the extension also doubles.

Elastic Limit:

Up to a certain extension, if the force is removed, the spring will return to its original length. The spring is behaving elastically. If this critical extension is exceeded, the spring will be permanently stretched. This is known as the elastic limit. 

The elastic limit can be shown on a graph as the point the line starts to curve.

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The Moment of a force

  • The moment of the force is also known as the turning effect

moment = force x perpendicular distance from the pivot

 (N/m)       (N)                 (m)

  • Moments can be clockwise and anti-clockwise

Principle of moments:

  • When an object is not turning, (e.g. balanced): The total clockwise moment is equal to the total anticlockwise moment.
  • When clockwise moment= anti-clockwise moment: Force (1) x Distance (1) = Force (2) x Distance (2)
  • Distance means from the pivot, force is the weight on the object (i.e. a balanced ruler)
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Gears, Levers and Perpendicular Distance

How do gears work?

If the smaller wheel is turned, it turns the larger wheel slower but with more force. I.e. The distance to the pivot has increased.

How do lever work?

The difference between the effort and the pivot is much larger than the distance between the load and the pivot. Larger distance= Less force

What is the meaning of Perpendicular distance?

Turning moments work best when applied perpendicular ( at a right angle) to the pivot. I.e. following the tangent of the cirle around the pivot. You can use trigonometry  to work out the perpendicular angle of a spanner.

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Centre of Gravity

  • The centre of gravity of a body is that point of which the weight of the body acts.
  • The centre of gravity of a symmetrical body is along the axis of symmetry. 
  • The centre of gravity is also known as the centre of mass.
  • If a shape is suspended, it will come to rest with it's centre of gravity directly below the point of suspension.

Why don't buildings fall over?

The weight is acting through the base so the force to the pivot is 0 so there is no turning moment and the building won't fall over.

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Centre of Gravity

  • The centre of gravity of a body is that point of which the weight of the body acts.
  • The centre of gravity of a symmetrical body is along the axis of symmetry. 
  • The centre of gravity is also known as the centre of mass.
  • If a shape is suspended, it will come to rest with it's centre of gravity directly below the point of suspension.

Why don't buildings fall over?

The weight is acting through the base so the force to the pivot is 0 so there is no turning moment and the building won't fall over.

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Pressure in Liquids and Gases

  • Particles in a fluid move around randomly. Everytime the particles hit the side of the container, the partcles exert a force at right angles on the container. This is called pressure
  • The pressure in a fluid (liquid or gas) at a particular point acts equally in all directions. The pressure is constant.

Pressure = density of liquid x height of container x gravitational field strength

  • In the atmosphere, density decreases as height increases
  • A metal block will sink in water because it is more dense,
  • A wooden block will float because it is less dense than water.
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Upthrust

Upthrust is caused by a difference in pressure at the top and bottom pf an object.

  • At the top of an object pressure acts downwards,
  • At the bottom of an object, pressure acts upwards.
  • As the top and bottom of an object are at different positions, there will be a pressure difference whcih will cause an upwards force.

As : Pressure = Force x Area

  • Calculating the pressure at the top of the block using area, gives us the force downwards.
  • Calculating the pressure at the bottom of the block using the area, gives us the force upwards.
  • The difference in force= upthrust
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Distance vs Displacement

Distance is how far you have gone, whereas displacement is how far you are from a point and it can be either positive or negative.

Example question:

A man walks 10km North and then 10km West.

a) What is the distance he has covered

10km + 10km = 20km

b) What angle would his displacement be?

(To work this out use pythagoras)

a^2+b^2=c^2

100+100 =200

(square root of 200) = 14.14km

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Vector vs Scalar

Vector quantities have both the same size and direction

Scalar quantities have size ('magnitude') only and NO DIRECTION.

Examples of Vector quantities:

  • Acceleration,
  • Velocity,
  • Force

Examples of Scalar quantities:

  • speed,
  • mass,
  • distance,
  • energy,
  • time,
  • power
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Average speed & Speed of Sound

Distance= speed x time

Speed of Sound

The speed of sound in air is around 330m/s. The denser the material, the faster sound travels through it. Ice is less dense than water so the sound will travel through it slower.

Distance-time graphs

On distance-time graphs, the gradient of the line increases with speed.

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Velocity, Acceleration & Circular Motion

  • Velocity of a body is its speed in a given direction. E.g. An airplain may loop at a constant speed, but its velocity changes as its direction of motion changes.
  • Acceleration is the rate of change in velocity
  • Acceleration= change in velocity/time

or: a= (v-u)/t      where v= final velocity, u= intial veloctiy, t= time, a = acceleration  (SUVAT)

Circular motion at a constant speed:

Acceleration is only happening because the direction of motion is continually changing and hence so is velocity. Acceleration ia towards the centre of the circular motion.

  • Change in velocity= final velocity - intial velocity
  • Deceleration = speed decreases with time.
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