Forces and motion
describe the factors affecting vehicle stopping distance
- eyesight - age - drugs+ alcohol - looking at phone - how fast the car goes
Explain how a graph may not obey hooke's law
- line is not straight - so force and extension cannot be directly proportional Hooke's law stops working when the force is great enough
describe the factors affecting vehicle braking distance
- speed the car is going - icy/wet -> less friction - age + condition of brakes - age + condition of tires - gradient of the route
How does shape and area affect terminal velocity?
1) On earth air resistance causes things to fall at different speeds, and terminal velocity of any object is determined by its drag compared to its weight. the drag depends on its shape and area.
Example of terminal velocity with human sky diver?
1) Without his parachute open he has quite a small area and a force equal to his weight pulling him down 2) He reaches a terminal velocity of about 120 mph 3) But with the parachute open, there's much more air resistance (at any given speed) and still only the same force pulling him down 4) This means his terminal velocity comes right down to about 15mph which is a safe speed to hit the ground at.
How to investigate Hooke's law with a spring
1) set up the apparatus - make sure you have plenty of extra masses, and measure the weight of each 2) measure the length of the spring with a ruler when no load is applied and measure the spring at eye level 3) add one mass at a time and allow the spring to come to rest, then measure the new length of the spring. The extension is the change in length from the original length 4) peat this process until have enough measurements 5) one you're done, repeat the experiment and calculate the average value for the length of the spring for each applied weight 6) plot your results on a show - show force on the y-axis and the total extension on the x-axis 7) you should find that the same increase in the weight on the end of the spring always leads to the same in crease in extension - this is Hooke's law in action
Forces acting on a object
A car for example ( look at diagram) 1) There are loads of forces acting on everything, but you won't know because they are all balanced. 2) Any object with a weight feels a reaction force back from the surface it is on. Otherwise it would keep falling 3) When an object moves in a fluid ( air, water etc) it feels drag in the opposite direction to its motion.
What is friction?
A force that opposes motion, slow things down To travel at a constant speed you need a driving force to balance with friction or it will slow down and stop Friction does this in three main ways: 1) Friction between solid surfaces which are gripping ( static friction) 2) Friction between solid surfaces which are sliding past each other 3) Resistance or drag from fluids ( liquid gases or air)
What is gravity?
A force that pulls objects toward each other, a force of attraction between all masses 1) on the surface of a planet, it makes all things accelerate towards the ground, the same acceleration (10 m/s2) 2) It gives everything a weight
What is force?
A push or a pull of an object
What is newtons 2nd Law?
A resultant force means acceleration if there is an unbalanced force, then the object will accelerate in that direction 1) the overall unbalanced forces are called resultant forces 2) An unbalanced force will always produce acceleration 3) This acceleration can be in the form of 5 thing: - Stopping - Starting - Speeding up - Slowing down - Changing direction
What is the equation for acceleration?
Acceleration = Change in Velocity/Change in Time a = △v/t unit: m/s2
how to find acceleration on a velocity-time graph
Acceleration = Gradient = Vertical -------- Horizontal Rise ----- Run The gradient Acceleration = Change in velocity --------------------- time
what is the equation for braking distance
Braking distance = stopping distance - thinking distance
a car mass of 1750kg has an engine which provides a resultant driving force of 5300N. Find the car's acceleration
F = ma a = f/m = 5200/ 1750 = 3.0 m/s2
What are the main forces?
Gravity or Weight - acts straight downwards Reaction Force - acts perpendicular to the surface, acts straight upwards Electrostatic Force- between charged objects. direction depending on the type of charge ( attract, repel etc) Drag or Air resistance or Friction - slowing things down Thrust- Speeding something up
What is Newton's third law?
If an object A exerts a force on an object B then object B exerts an equal and opposite force on object A
Practical: investigating motion
Investigate the acceleration of a trolley down a ramp There are different ways to investigate the acceleration of an object down a ramp. In this practical activity, it is important to: 1) make and record measurements of length and time accurately 2) use appropriate apparatus and methods to measure motion Aim of the experiment To investigate the acceleration of an object on an angled ramp. Method 1) Set up a ramp balanced on a wooden block at one end. Mark out 30 cm at the end of the ramp. Avoid making the ramp too steep, as this will cause the trolley to roll too quickly, which could make measuring difficult. 2) Practise recording the time it takes for the trolley to travel the length of the ramp. To do this, release the trolley from the top of the ramp, start the stop clock and record the time taken for the trolley to move the whole distance of the ramp. 3) Repeat this twice more and record all results in a table similar to the one below. Then calculate the mean time and record this also. Remember that these practise results. 4) Next, record the time it takes for the trolley to travel the final section of the ramp. To do this, release the trolley from the top of the ramp again but this time start the stop clock when the trolley reaches the last 30 cm of the ramp. 5) Record the time it takes for the trolley to travel the last 30 cm of the ramp in a table like the one shown below. These are your final results. 6) Repeat this twice more, and record a mean time for the trolley to travel the last 30 cm of the ramp. 7) Calculate the speed of the trolley when it was descending the last 30 cm of the ramp using the equation: speed (m/s) = distance (m) ÷ time (s). Remember to first convert 30 cm into metres by dividing it by 100 (there are 100 cm in 1 m).
What is the difference between mass and weight?
Mass is the amount of matter in an object measured in kg Weight is the measure of the pull of gravity on an object. Weight is measured in newtons
What is the equation for resultant force?
Resultant force = mass x acceleration F = m x a 1) the greater the force, the greater the acceleration or deceleration 2) The bigger the mass, the smaller the acceleration 3) To get a big mass accelerating as big as a small mass, you need a bigger force.
What does SUVAT stand for?
S- displacement (m) U- initial speed (m/s) V- final speed (m/s) A- acceleration (m/s2) T- time (s)
How to find the average speed on a distance-time graph
Speed = Distance/ time ( v=d/t) Speed= gradient = vertical ----------- horizontal You can find the average speed of an object over a period of time by dividing the total distance travelled by time it takes to travel that distance. For example ( whole journey = 500) ( Time us seconds is 80) 500/80 = 6.25 m/s
What is the difference between speed and velocity?
Speed is the rate at which an object is moving. ( how fast you're going) Velocity is the vector quantity that expresses both the rate and direction an object is moving (must have a direction)
what is the equation for stopping distance
Stopping distance = thinking distance + braking distance
How to find the distance on a velocity-time graph?
The area under the graph
State Hooke's law
The extension is directly proportional to the force applied
What is acceleration?
The rate at which velocity changes (how quickly velocity is changing)
What does the distance- time graphs show?
They tell you how far something has travelled. The gradient - at any point gives the speed of the object Flat -sections are where it's stops Steeper Gradient - Represents acceleration Curve getting steeper-means it is speeding up ( increasing gradient) A levelling off curve - means its slowing down the steeper the gradient, the faster the average speed
What is the difference between vector and scalar quantities
Vectors: directions Scalars: no direction Vectors: Have a direction Velocity Displacement Acceleration Deceleration Force Friction Scalars: don't have a direction Distance Speed Time Area Length Energy Mass
What are the two different graphs
Velocity- time graphs Distance - time graphs
What is the weight, in newtons of a 5 kg mass, both earth and on the moon
W = m x g Earth = 10 N/Kg Moon = 1.6 N/Kg On Earth: W = 5 x 10 = 50 N On the Moon: W = 5 x 1.6 = 8N
What is the equation for weight, mass and gravity
Weight = mass x gravitational field strength W = m x g
What does the Suvat equation give?
You can use it to get: S- displacement (m) U- initial speed (m/s) V- final speed (m/s) A- acceleration (m/s2) T- time (s) Which equation you use is based on the quesiton
A van travelling at 23 m/s starts decelerating uniformly at 2.0 m/s2 as it heads towards a built-up area 112m away. what will its speed be when it reaches the the built-up area?
a is negative because it is deceleration. v2 = u2 (2 x a x s) = 232 + (2 x -2.0 x 112) = 81 v = √81 = 9 m/s
Example of acceleration: A skulking cat accelerates from 2m/s to 6m/s in 5.6s. What is its acceleration?
a= (v-u) / t = (6-2) / 5.6 = 0.71 m/s2
What does a velocity- time graph show?
can have a postiste or negative gradient The gradient - acceleration Flat -constant speed, steady speed Steeper Gradient - the greater the acceleration or deceleration Uphill (/) - Are acceleration Downhill (/)- are deceleration Area underneath- the distance travelled in that time interval Curve- changing acceleration
Speed = v ( average speed)
distance/time d / t
What is Newtons 1st Law?
first law - balanced forces mean no change in velocity An object at rest remains at rest unless acted on and an object with constant velocity will keep having the constant velocity unless it is acted on. To keep going at a constant speed, there must be 0 resultant force, all forces must be balanced
What is deceleration?
negative acceleration, (slowing down)
What is terminal velocity?
of falling objects depends on their shape and area
What does terminal velocity depend on?
shape and area
Moving objects can reach a
terminal velocity