8177567
Forces and Motion
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Slide Set by Sifat Symum, updated more than 1 year ago
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Created by Sifat Symum
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Resource summary
Slide 1
Forces
Slide 2
Contact and Non-contact Forces
Force is a vector quantity - has magnitude (size) and direction: e.g. Force (N), Velocity (s/t) - displacement/time, Displacement
A scalar quantity is a quantity that has only magnitude: e.g. Time (s), Mass (kg), Speed (m/s)
Vector arrows are drawn from the centre of an object towards its direction. The length represents the magnitude
A force is either contact or non-contact
A contact force is a force resulted in the interaction of objects. The objects must touch: e.g. Friction, Air Resistance
A non-contact force is when objects don't need to touch for a force to be produced: e.g. Magnetic Force, Gravitational Force
When two objects interact there is a force upon both objects
A resultant force is the overall force exerted upon an object as a result of one ore more forces
Slide 3
Weight, Mass and Gravity
Gravity is a force that gives every object with mass a weight. It can be felt upon planets such as the Earth with a gravitational field strength of 9.8 N/kg.
Gravitational field strength varies according to location. The further away from a planet an object is, the less gravity has an effect
Mass is simply the amount of particles contained within an object. Mass is a constant and remains the same regardless of gravity.
Mass is measured using a mass balance
Weight is the force acting upon an object due to gravity. The weight varies according to location and gravity.
Weight is measured using a newtonmeterEquation: W = MG
Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg)Weight is proportional to mass
Slide 4
Forces and Elasticity: Transfer of Energy
Applying a force may cause an object to: Stretch, Compress or Bend. This however only happens when two or more forces act on an object
An object that has been Elastically Deformed can return to its original shape and length. These are referred to as Elastic Objects: e..g. Spring
An object that has been Inelastically Deformed can't return to original shape and length
Work is done when an object has been stretched or compressed. If the object is elastically deformed then All enegy is transferred into the object's Elastic Potential Energy Store
Equation: E = 1/2ke^2Energy (E) = 1/2 x Spring Constant (k) x Extension (e)^2
Slide 5
Forces and Elasticity: Spring Constant
A Spring is an Elastic Object. The Extension of a spring is directly proportional to the Load or Force applied: F ∝ E.
Equation: F = keForce (N) = Spring Constant (N/m) x Extension (m)
The Spring Constant depends upon the Material being stretched. The stiffer the material the greater the spring constant.
This equation can also be used for compression where e is the difference between natural and compressed lengths
Caption: : This graphs above shows the stretching of a spring. P is the limit of proportionality where the spring is inelastically deformed.
Slide 6
Moments
A Moment is a turning effect of a force
Equation M = FdMoment (Nm) = Force (N) x Diestance (m)
The distance is the perpendicular distance from the pivot point such as on a spanner. The longer the spanner (more distance) the less force is required for the same moment as with a shorter spanner.
If the total clockwise moment is equal to the total anticlockwise moment about the pivot point the object is balanced. In this case the object (like a spanner) won't turn.
A lever increases the distance from the pivot. This means less force is required to get the same moment and so it is easier to do work.
.Gears are circular discs with teeth around the edges which interlock. The turning of one gear causes the next gear to turn in the opposite direction.
Gears of different sizes can be used to change the moment of a force.
A large gear can be used to cause a larger moment due to the greater distance from the pivotal point. The large gear will turn slower than the small gear.
Slide 7
Pressure: Fluids
Pressure is the force per unit area. A fluid is a substance that can flow: liquid or gas.
Particles within the fluid move around and collide with other particles and the container. Particles exert a force on collision and so particles provide a force per unit area: Pressure.
The pressure of a fluid means a force is exerted normal (at right angles) to any surface in contact with the fluid.
Equation: p = F / APressure (Pa) = Force (N) / Area (m^2)
Slide 8
Pressure: Liquids
Pressure in a liquid depends on depth and density. The density of a liquid is uniform throughout meaning that it's the same throughout
The denser the liquid the more particles there are within a certain volume and so more particles collide at a greater rate, This increases pressure.
As depth increases there are more particles above and the weight of this adds to the pressure felt at that point.
Equation: p = hρgPressure (Pa) = height (m) x density (Kg / m^3) x gravitational field strength (N/Kg)
Caption: : As force is applied the pressure increases on the other side
Slide 9
Upthrust
Objects in fluids experience upthrust
The pressure of the fluid exerts a force upon the object in all directions. The force acting upon the bottom of the object is greater than the top because pressure within fluids increases with depth.
This causes the resultant force known as upthrust. The upthrust is equal to the weight of the fluid displaced.
If an object floats then weight = upthrust
The forces balance in this case and so the object floats. If an object's weight is greater than the upthrust then the object sinks.
Density is the main factor of upthrust. An object that is less dense than the fluid weighs less than the equivalent volume of fluid. As a result, the object displaces a volume of fluid equal to its weight before being fully submerged.
A denser object won't displace enough fluid equal to its weight meaning weight is always greater than upthrust so it sinks.
Slide 10
Atmospheric Pressure
Atmospheric pressure decreases with height
The atmosphere is a layer of air surrounding the Earth. It is created on the surface by air molecules colliding with the surface.
As you go up the atmosphere becomes less dense so there are fewer air molecules able to collide with the surface. The weight of the air above therefore exerts less pressure
Slide 11
Distance, Displacement, Speed, Velocity
Distance is a scalar quantity. It is simply how far an object has moved without direction involved
Displacement is a vector quantity. It measures distance and direction in a straight line from the starting point to the finishing point.
If you walk 10 metres North then 10 metres South then distance travelled is 20m.
The displacement however is 0m as you have finished at the same starting point.
Speed and Velocity both measure how fast an object is going but speed is scalar and velocity is a vector. Velocity is speed in a direction.
Speed can be constant while velocity is changing e.g. moving at the same speed in a circle (changing direction)
Equation: v = d / t and v = s / tspeed (m/s) = distance (m) / time (s) and velocity (m/s) = displacement (m in a direction) / time (s)Typical Everyday Speeds:Person walking - 1.5 m/s A car - 25 m/s
Person running - 3 m/s A train - 55 m/s Speed of Sound - 330 m/sPerson cycling - 6 m/s A plane - 250 m/s
Slide 12
Acceleration
Uniform acceleration is speeding up or slowing down at a constant rate. Acceleration due to gravity is uniform for objects to free-fall. It is around 9.8 m/s^2
Acceleration is the overall change in velocity over a certain amount of time. Deceleration is negative acceleration.
Equation: a = Δv / tacceleration (m/s^2) = change in velocity (m/s) / time (s)Equation: v^2 - u^2 = 2asfinal velocity (m/s) - initial velocity (m/s) = 2 x acceleration (m/s^2) x distance (m)
Slide 13
Distance / Velocity - Time Graphs
Caption: : A velocity-time graph - the gradient represents the ACCELERATION
Caption: : A distance-time graph - The gradient represents SPEED
Slide 14
Terminal Velocity
If an object has no force propelling it along then it will eventually stop due to friction (unless in space). Friction will always act in the opposite direction to movement.
Friction is caused by contact between two surfaces (or within a fluid). In order to travel at a steady or constant speed the driving force will need to balance the frictional forces.
Air resistance (drag) is the resistance you will feel within a fluid. Air resistance increases as speed increases and so a car moving at 30 mph will require less work from the engine than a car travelling at 70 mph in order to remain at a constant speed.
Terminal velocity is the maximum velocity that an object can travel at within a certain medium. It is when a falling object's overall resultant force is equal to zero.
An object will accelerate initially due to gravity but the resultant force ends at zero due to air resistance.
An object's shape and area affect terminal velocity. If there was no air resistance then all objects would fall at the same rate.
After a falling skydiver reaches terminal velocity and opens his parachute the air resistance increases significantly compared to the force (weight) acting in the opposite direction. As a result, the skydiver decelerates to a much lower terminal velocity.
Slide 15
Terminal Velocity
Caption: : This diagram shows how terminal velocity is reached
Slide 16
Caption: : Velocity-time graph of a parachutist
Terminal Velocity
Slide 17
Newtons Laws: I (Inertia)
A resultant force is needed upon an object for it to start moving, speed up or slow down
A non-zero resultant force will always result in acceleration
A zero resultant force upon a stationary object will cause no movement
A zero resultant force upon a moving object will mean constant speed
Acceleration can take different forms: starting, stopping, speeding up, slowing down or changing direction
Slide 18
Newton's Laws: II
The larger a resultant force on an object the more the object accelerates. Force and acceleration are directly propotional: F ∝ a
Acceleration is inversely proportional to mass: a ∝ 1/m
Equation: F = maForce (N) = mass (kg) x acceleration (m/s^2)
Slide 19
Newton's Laws: III
" For every action there is an opposite and equal reaction "
If you push an object, the object will also push back at you with an equal force in the opposite direction
Equilibrium is the state in which the overall resultant force acting upon an object is equal to zero.
Slide 20
Required Practical: Motion
Slide 21
Reaction Times
Reaction time can be affected by: tiredness, drugs, alcohol and distractions
Reaction time can be tested using a computer or a ruler drop test. A computer is much more accurate whereas a ruler drop test will need a lot more repeats and calculate an average
Caption: : A ruler drop test
Slide 22
You catch a ruler at 20cm.v^2 - u^2 = 2asu = 0a = 9.8 m/s^2 (gravitational field strength)s = 0.2 mv = Square_Root(2 x 9.8 x 0.2 + 0) = 1.97 m/sa = Δv / tt = 1.97 / 9.8= 0.2 seconds (1sf)
Example Calculation: Reaction Time
Slide 23
Stopping Distances
Slide 24
Momentum
Momentum is a property that all moving objects have. It is a vector quantity (magnitude and direction)
The greater the mass or velocity of an object, the greater the momentum
Equation: p = mvmomentum (kg m/s) = mass (kg) x velocity (m/s)If the momentum before an event is equal to the momentum after an event: Conservation of Momentum e.g Explosion - the momentum before is zero. Then the pieces fly outwards in different directions - total momentum cancels to zero.When there is a car crash, everything decelerate rapidly. A great force is felt due to the great change in momentum. If the time taken for the change in momentum on the body is increased, the forces on the body are reduced too. Seat belts and crumple zones are designed to reduce the forces on the body if there is a collision.
Caption: : An example of a momentum question
Slide 25
Changes in Momentum
When a non-zero resultant force acts upon a moving object, velocity changes and so momentum also changes.
F = ma and a = Δv / t. Therefore F = Δv / t
The force causing the change is equal to the rate of change of momentum. A larger force means a faster change of momentum
This means that if the change in momentum is very quick (like in a car crash) the overall force acting on the body is greater. This means that a person is more likely to get injured
Equation: F = mΔv / tForce (N) = change in momentum (kg m/s) / change in time (s)
Slide 26
Momentum: Safety Features
CarsCrumple zone - these are designed to crumple on impact to increase stopping distance and timeSeatbelts - these are designed to stretch slightly to increse time taken to stop for wearerAir bags - these inflate upon impact before you hit the dashboard. There is a hole in the air bag to allow air to be released. As you hit the air bag the air is compressed to slow the person down gradually.Bike HelmetsThese contain a layer of foam that can be crushed. This is there to increase the time taken for your head to fully stop after a crash. This reduces the overall impact to the brain.Crash Mats and Playground FlooringThese increase the time taken to fully stop if you fall on them as they are made of soft, compressible materials.
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