SP2- Motion And Forces Flashcards
What is the difference between a vector quantity and a scalar quantity
vector quantities has direction and magnitude
Scalar quantities only have magnitude
What’s acceleration
Is a change in a vector quantity (velocity) acceleration is also a vector
What type of quantity is a force
A vector
How is the size of the force represented in a diagram
The length of the arrows
What is the resultant force
The overall force of an object
How do yu work out the resultant of 2 forces
If the forces are acting in the same direction add them
If they are acting in opposite directions subtract them
What happens if the resultant forces of an object is 0
The forces are balanced
If there is a non zero resultant force on an object the forces are unbalanced
What’s Newton’s first law of motion
A moving object will continue to move at the same speed and direction unless an external force
Acts on it
A stationary object will remain at rest unless an external force acts on it
How could forces change the speed and/or direction of an object
If there are balanced forces it will not change
If there are unbalanced forced the speed and/ or direction of an object will change
What’s the centripetal force
The resultant force that causes the change in direction
Eg an object moving in a circle has changing velocity even though it’s speed remains the same
Tell me about the centripetal force
Acts towards the centre of a circle
Other types of force that can make objects move in circular paths include friction and gravity
What is mass
The quantity of matter there is in an object
It only changes if the object itself changes
Units are kilograms
What is weight
A measure of pull of gravity of an object and depends on the strength of gravity
Units are newtons
How can weight be measured
Using a force meter which has a scale in newtons
Many contain a spring which stretches as the force on it increases allowing her weight to be read of the scale
What’s the earths gravitational field strength
10 N/kg
Every kg is pulled down with a force of 10 newtons
The field strength is different on other planets and moons
Tell me the triangle that links weight, mass and gravitational field strength
W on the top (weight) and m(mass) X gravitational field strength on bottom
How does air resistance increase
With speed
What is newtons Second law of motion
The acceleration in the direction of a resultant force depends on
The size of the force (for same mass, bigger force bigger acceleration)
The mass of an object (for the same force, the more massive the object the smaller the acceleration)
How do you calculate force
Force (n) = mass (kg) X acceleration (m/s^2)
What’s the inertial mass
The inertial mass of an object is the force on it divided by the acceleration that force produces
The more massive an object is - the more force is need to change its velocity.
Tell me about Newton’s third law
It’s about the forces on two different objects when they interact with eachother
This interaction can happen;
When objects touch- such as when you sit on a chair
At a distance, such as the gravitational attraction between the earth and the moon
What are action-reaction forces
There is a pair of forces acting on the two interactive objects
The two forces are Always the same size and in opposite directions, they are also the same type of force
What’s a equilibrium situation
When nothing is moving eg when a there is a tug of war
How are action-reaction forces different to balanced forces
In both cases the sizes of the forces are equal and act in opposite direction but:
Action-reaction forces act on different objects
Balanced forces all act on the same object
Tell me about collisions
The action and reaction forces that occur during the collision are the same size but don’t necessarily have the same effects on the 2 objects Becuase the objects have different masses
What is momentum
A measure of the tendency of an object to keep moving or of how hard it is to stop it moving
The momentum of an object depends on its mass and its velocity
Momentum depends on a vector quantity (velocity) and Momentum is also a vector
How is momentum calculated
P (momentum) (kg m/s) on top of triangle and m (mass) (kg) X v (velocity) (m/s) on bottom
Tell me the 3 ways to work out force
F = m(v-u) divided by t
Or f = mv - mu divided by t
Or f = m X a
How do you work out change in velocity and time
A = v-u divided by t
What’s the conservation of momentum
When moving objects collide the total momentum of both objects is the same before the collision as it is after the collision - as long as there are no external forces acting
How do we show momentum
If the objects are moving in opposite directions we give the momentum of one object a positive sign and the other a negative
What’s the thinking distance
When a driver sees a problem ahead - their vehicle will travel some distance while the driver reacts to the situation
What’s the braking distance
The vehicle will then go some distance further while the brakes are working to bring it to a halt
What is the stopping distance
The sum of the thinking and braking distances
Stopping distance = thinking distance + braking distance
What is the reaction time
The time between a person detecting a stimulus and their response
How can response times be measured
Using computers or electric circuits that measure the time between a stimulus and a response
What’s a typical reaction time to a visual stimulus
0.25 seconds
Why could a reaction time be longer
If the person is tired, Ill, or been taking drugs as alcohol
Distractions such as mobile phones can also increase reaction times
How do cars break
Uses friction so slow car down
How can the braking distance be increased
If the brakes are worn - they create less friction and don’t slow the vehicle effectively
Friction between the tyres and road is also important
If the road is wet or has loose gravel on it - or if the tyres are worn there is less friction
A heavier vehicle needs what to decelerate
More force - so if the same amount of friction is used to syllable a vehicle the heavier vehicle will travel further than a lighter one
When does think distance and braking distance increase
As the speed increases
What is the work done
The energy transferred by a force acting over a distance
How do you calculate work done
Work done (J) = force (N) X distance moved in the direction of the force (m)
What is kinetic energy
The Energy stored in a moving object
How do we calculate kinetic energy
Kinetic energy (J) = 1/2 X mass (kg) X speed^2 (m/s^2)
What happens to kinetic energy when it stops
The kinetic energy is transferred to other energy strokes by the braking force
Tell me the work done, force and distance triangle
Work done on top and force X distance on bottom
The braking distance of a car depends on its kinetic energy and so…
It depends on the square of its velocity
So if the velocity doubles - the braking distance is multiplied by 2^2 which is 4
What does the force need for acceleration depend on
Th size of acceleration and on the mass of the object
What are crumple zones
Built into the front and sometimes the back of cars - if the car hits something it takes a little time for this crumpling to happen
So the deceleration of the car is less and the force on the car is also less than if it had a solid front
The effect of the crumple zone reduces the forces in the passengers as well as on the car
Airbags increase the time it takes a persons head to stop a collision
What does the force in a road collision depend on
The change of momentum as the car comes to a stop
