P8,9,10 Forces Flashcards
What is a force
A push or pull on an object that which is caused by it interacting with something.
What is a contact force?
Give examples.
When 2 objects have to be touching for a force to act.
Eg Friction and air resiatance.
What is a non contact force?
Give examples.
When 2 objects dont need to be touching for a force to act
Eg Gravitatinal force
What is a interaction pair?
A pair of forces that are equal and opposite and act on 2 interacting objects.
What is gravitatinal force?
The force of attraction between masses. This effects all masses but you only notice it on large scales.
Eg Planets and the Sun
2 important effects of gravity are…
It makes all things on planets fall to the ground.
It gives everything a weight.
What is mass?
How would your mass change in space?
The amount of “stuff” in a object. Measured in KG by a scale
Your mass will never change on the Moon or in space…
What is weight?
How would weight change in space?
The force acting on an object due to gravity. Measured in N by a newtonmeter.
Your weight would change in space because there less/no gravity.
When is gravitatinal field strength the strongest?
When you are the closest to the mass which causes the field.
Eg in earth it would be the core.
What is the gravitainal field strength of…
The Earth?
The Moon?
Mars?
- 8N
- 6N
4N
What is the center of mass?
Where the whole mass is concentrated.
What is the equasion for Weight?
W = M x G
Weight (N) = Mass (KG) x Gravitatinal Field Strength (N/kg)
What would the weight be of an object of a mass of 10kg on earth?
What would the mass of an object be with a weight of 100N on earth?
What would the gravitatinal field strength be of a planet where someone has a weight of 200N and a mass of 50KG?
Earths Gravitatinal field strength is 9.8
98 N
0.098 KG
4g
Weight and mass are?
Directly proportinal.
How do you show all of the forces acting on an object?
A free body diagram.
What forces act on a skydiver?
Weight (Going down)
Drag/Air resistance (Going up)
What do the size and direction of the arrows on a free body diagram show us?
The size of the arows show the reletive magnatude (amount of force)
The direction of the arow shows which way the force is acting.
What is resultant force?
The overall force acting on an object. This is a vector, so will have a direction.
What must a resultant force represent?
If there are two forces on a object. The resultant force must have the same effect on the object as the other forces would of.
If 2 forces are parrallel then you need to?
Of 2 forces are going in opposite directions then you need to?
Add them.
Subtract them.
If a resultant force moves an object then what is done?
Work is done.
Energy is transphered (likely through friction)
Force needs a?
Sorce.
Eg fuel or food.
When work is done what happens?
Energy is moved from 1 energy store too another.
Work is normally done when there is an action.
Eg holding a box isn’t doing work but lifting up a box is.
Are you doing work when you push something through a carpet?
You do work against frictional forces.
Energy is transferred to the kinetic store when it moves and a bit to thermal store.
Therefore work is done.
What is the equasion for Work Done
W = F x S
Work done (J) = Force (N) x Distance (M)
What is the conversion from J to Nm
1 J = 1 Nm
1 Joule = 1 Newton meter
How do you work out resultant forces which arn’t opposite pr parrallel
You draw a right angle triangle of the forces.
The corner of the right angle is the center of mass (where all the forces originate from)
Lable each side of the triangle with the line going horizontal being the force acting on the left or right side. And the vertical line being the force acting up or down.
You use trigonomotry or pythagorus to work out the missing angle and side.
The hypotinues will be the resultant force The angle (barring) will be the directio. (Make sure to include a direction like left,right,up or down)
What is equilibium?
A state where all of the forces are equal to 0.
If you are told to find the force acting on the object which combined with other forces make the object be in equelibirum. What do you do?
You draw the triangle and fill in your know values and make sure the triangle finishes where it started.
Then work out the unknown value.
If a force is at an akward angel then you can split it into it’s?
Horisontal and vertical componants
To work out the componants you can draw in the right angle triangle (to scale) and measure the other 2 sides because it is to scale theses sides will be the correct values.
Note: You could always half the size of each angle. But make sure to double the final result.
What happens when you apply forces to an object?
The object might…
Stretch, compress or bend
For an object to strech it need’s how many forces?
You need 2 or more forces for an object to strech
Eg a pull and a resistance force.
What does elasticly deformed mean?
If an object has been eleasticly deformed then it can go back to it’s original shape after the force has been removed.
An object that can be elasticly deformed is considered to be?
Elastic
What is inelasticly deformed?
When an object doesn’t go back to it’s original shape when the force is removed. This happens when an object has passed it’s elastic limit.
If an object elasticly deformed then what happens the the energy?
The elastic energy is transfered into elastic potential enery.
What is the link of extention and force?
It is directly proportinal.
How do you work out force?
F = K x E
Force (N) = Spring Constant (N/m) x Extention (m)
The spring constant of a material will increase as a material becomes…
More stiffer.
Equasion for compression?
C = K x E
Compression = Spring Constant (N/m) x change in compression
When will the material become inelastic?
A material becomes inelastic when the material passes it’s elastic limit.
What is elastic limit?
The most force an object can hold untill it becomes inelastic.
What is an extention-force graph?
What features does it have?
It is a graph that plots the extention and force. Once the firce reaches the elastic limit then the graph starts to curve upwards.
How can you investigate the link between force and extention (RP)
You will need: a spring, ruler, clamp, masses, marker (to mark where the mass started) and a weighted stand.
1) Measure the natual length of the spring (with no weight). Make sure it is accurate.
2) Add a mass to the spring and allow it to come to a stop, you then want to record the new length, then work out the extention (new length - original length)
3) Repeat step 2 untill you have a sutiable amount of results (more than 6)
4) Plot a force-extention graph of your results.
The spring constant is equal to the?
Gradient of the line of best fit (the straight bit)
What is the relationship of extention and force when the line begins the curve?
The relationship is now non linear.
How do you work out Elastic Potential Energy?
Ee = 0.5 x K x E^2
Elastic Potential Energy (J) = 0.5 x Spring Constant(N/m) x Extention (m)
Is distance scalar or a vector?
Scalar.
Is displacement scalar or a vector?
What is displacement?
Vector. The dorection could be an angle, direction or eg towards school.
Displacement measures the direction and distance from the starting point of an object.
If you move 5 miles south and then 6miles north. What is your displacement?
What is your distance?
Displacement is 1 mile north.
Distance is 11 miles.
Is speed a vector or scalar?
Scalar.
Is velocity scalar or a vector
Vector.
If you have 2 cars with the same velocity. Both cars continue at the same speed but 1 of the car’s velocity has changed. What happened?
1 car’s direction changed.
How do you calucate speed?
S = D ÷ T
Speed (M/s) = Distance (M) ÷ Time (s)
How would you find out how fast something is going if it has a constant velocity?
1) Use a stopwatch and ruler to measure how long it takes to travel a certian distance.
2) Calcuate the speed using S = D ÷ T.
How do you work out the speed of something that changes it’s speed?
1) Use a stopwatch and ruler to measure how long it takes to travel a certian distance.
2) Calcuate the speed using S = D ÷ T.
THIS WILL GIVE AN AVARAGE THROUGHT THE WHOLE JURNEY.
What is the avarage speed of…
Someone walking?
Someone running?
Someone cycling?
A car?
A train?
A plane?
1.5 m/s
3 m/s
6 m/s
25 m/s
30 m/s
250 m/s
What can effect speed?
Atributes?
Distance, age, gender, fitness, terrain…
How is the speed of sound effected in different materials?
Normally 340 m/s sound can slow down or become faster when it travels through different mediums.
How can the speed of wind be effected?
Tempreture, atmospheric pressure, building/forests.
What is acceleration?
How do you calcuate it?
Acceleration is the change in velocity/speed.
A = ΔV ÷ T
Acceleraion (m/s^2) = Change in Velocity (m/s) ÷ Time
What is deceleration?
Negative acceleration.
If a question says what is the acceleration they say -x
If a question asks what the the deacceleration then say x
How do you estimate acceleration?
Example: A car hit a tree and stopped. What was the deceleration?
1) Estimate a speed for the car (25m/s)
2) Estimate a time for how long it would take the car to stop (1s)
3) Put these values into the the acceleration equasion
A = ΔV ÷ T
A = -25 ÷ 1
A = -25
The question asks for deceleration so the answer is 25 m/s^2
Constant acceleration is sometimes known as?
Uniform acceleration. Acceleration due to gravity is uniform.
How do you calcuate uniform/ constant acceleration?
v^2 - u^2 = 2as
Final velocity (m/s) - initial velocity (m/s) = 2 x Acceleration (m/s^2) x distance (m)
In a distance time graph.
What does a flat line equal to?
What does the gradient equal to?
When the line is steeper is the object going faster or slower?
What do all straight lines represent
What do curves (which are rissing out) represent?
How do you find the speed the object is going at any point whilst it is accelerating/decellerating?
The object is stationary.
The speed (because distance ÷ time)
The steeper the line is the faster it is going.
The shallower angle the line is going in the slower it is going.
All straight lines represent a constant speed.
A curve that is flattening out represents a deceleration.
A curve that is rising represents acceleration.
To find the speed duran acceleration you need to draw a tangent to the point you want to work out the speed of, then find the gradient of that line.
On a velocity time graph what does the
Gradient equal to? Flat sections? Diagonal lines represent? A uphill curve represents? The area under the graph represents?
The gradient is equal to the acceleration.
Flat sections represent a steady speed.
Diagonal lines represent a constant acceleration/deceleration.
An uphill curve represents acceleration.
A downhill curve represents deceleration.
The are under the graph represents the distance. If you need to you can use trapesium to work out areas with curves.
What will happen to an object that has no forces pushing it which is traveling at 5 m/s?
Where would this not happen?
The object will eventually stop as there is friction acting on the object. This will happen everwhere except space.
To drive/fall… at a steady speed what does the resultan force need to equal.
What if you wanted to accelerate
The resultant forces need to be at 0.
To accelerate the forces need to be a positive number in the way you are moving.
How could you reduce friction between surfaces?
By using lubricant’s.
As speed increases…
Drag will increase.
What is drag?
The resistance you get in liquids or gas.
Eg Air resistance.
Why are some objects are designed to be areodynamic?
Some objects are designed to be areodynamic so that they are faster and more efficent.
Some objects like parahoots work in the opposite way, to slow down objects.
Why is it harder to travel faster?
It is harder to travel faster because as your speed increases drag increases. Therefore it is harder to get faster because you need to generate more force.
What are the stages of a skydive?
1) The skydiver jumps and the force of gravity is greater than the air resistance. Therefore the skydiver ACCELERATES.
2) As the speed increases, air resistance increases
3) Eventually the force of air resistance os equal to the force of gravity and the skydiver is at terminal velocity (the resultant force is 0) He doesn’t accelerate or decelerate. He will now fall at a steady speed.
4) The skydiver opens his parashoot and his air resistance increases rapidly but the gravity stays the same. This means that the skydiver will decrease in speed.
5) The air resistance starts to decrease as the speed decreases. Eventually the skydiver reaches the second terminal velocity as the gravity is equal to the air resistance.
6) After a while the skydiver will reach the ground.
What is terminal velocity?
Terminal velocity is when the resultant forces are equal to 0. The object is going at the fastest speed which it can go.
What can effect terminal velocity?
Shape and area.
The less streamlined/areodynamic an object is the lower it’s terminal velocity is.
If an object has a large surface area than it will also have a lower terminal velocity.
This is because they are more air resistante and therefore spend less time accelerating and don’t speed up as much.
What is Newtons first law?
What does this tell us?
A force is needed to change motion.
If a resultant force is not 0, there will be acceleration/deceleration.
What are the 5 forms of acceleration?
Starting. Stopping. Speeding up. Slowing down. Changing direction.
If a free body diagram is showing acceleration or decelleration. The arrows ill be…
Unequal.
What is newtons second law?
Acceleraion is proportinal to the resultant force.
How does an onject accelerate faster?
It needs a higher resultant force.
Mass is inversly proportinal to…
What does this mean?
Acceleration
This means that as mass increases acceleration will decrease at the same rate.
How do you calcuate force?
F = M x A
Force (N) = Mass (kg) x Acceleration (m/s^2)
What is inertia?
The tendency for motion to remain unchanged.
This includes an object which is not moving or and object which is moving at a steady speed.
What does inertial mass measure?
How difficult it is to change it’s velocity.
How can you calcuate mass?
F = M x A
Force (N) = Mass (kg) x Acceleration (m/s^2)
(You need to rearange it)
M = F ÷ A
What is Newons third law?
When two object interact, the forces that they exert on each other are equal and opposite.
(If you push something, it will push back just as hard. When you stop pushing the trolly stops pushing)
What is an good example of Newtons third law?
A man pushing against a brick wall.
The man exerts a force of 10N and the wall exerts a force of 10N back.
Is a book resting on the floor an example of Newton’s third law?
Yes
How do you investigate hoe mass and force affect acceleration?
How would increasing the mass change this?
You will need: a horizontal board, trolly of known mass, weights, light gate, string, a pully and a peace of string
You need to conect the trolly to the peace of string and then the string to the pully and the masses.
You then let the trolly go at a set starting point. Make sure that the string is tight and not loose (to keep the variables the same) the light gate will measure the acceleration of the trolly.
Repete these steps and find the avarage.
If you increase the mass then force decreases
If you have done this right then then you can work out the force of the pully by using F = M x A.
How do you work out stopping distances?
Stopping distance = Thinking Distance + Braking Distance.
What can affect your total stopping distance?
Thinking Distance.
Braking Distance.
What is stopping distance?
The total time it takes a car to go from moving to a complete stop.
What is thinking distance?
How far the car travels before the driver reacts to the problem (starts breaking)
What is braking distance?
The distance it takes to stop under a breaking force.
You can estimate stopping distances using typical values.
What are some cars typical braking distances?
30mph?
60mph?
70mph?
14m.
55m.
75m.
What is thinking distance affected by?
Your speed.
Your reaction times.
Any distractions Eg drunk driving and mobile phones.
What is braking distance affected by?
Your speed.
The weather/road surface. Eg Wing, rain
Condition of your tyres. If raining you want your tyres to have grooves on them.
Condition of your breaks.
Why are speed limits important?
Speed dramaticly increases breaking distance.
How do cars stop?
The stop pedal causes friction between the wheel and braking plates. This causes kinetic energy to transffer to thermal energy but this increases the tempretue of brakes.
How does speed affect brakes?
The faster you are going the more kinetic energy there is therefore the higher braking force is needed to stop the car or a longer distance.
Why are higher braking forces dangerous?
Because a large deceleration can cause the brakes to overheat and then they won’t work as well.
What is the avarage reaction time?
0.2 - 0.9 seconds.
What effects reaction time?
Tiredness.
Drugs.
Alcohol.
Other distractions.
What is the ruler drop test?
This requires 2 people.
You start with your fingers inbetween the 0cm mark.
The second person then drops tye ruler withought warning.
You need to catch the ruler.
You can then calcuate how long it took you to catch the ruler because gravity is constant. You can use V^2 - U^2 = 2as.
This test is very inconsistant so you will want to multiple retests.
You could also introduce a distraction such as music.
How do we caluate momentum?
P = M x V
Momentom (kg m/s) = Mass (kg) x Velocity (m/s)
What is the conservation of momentum?
The total momentum before will equal the total momentum after.
Eg. Of the momentum before the event was 0 kg m/sthen the momentum after the event will also be 0 kg m/s. In an explosion bits of it will fly out in different directions which cancel out the initial explosion to 0 kg m/s.
