Force And Motion Flashcards
Why do objects recoil in explosions.
Objects needs to have equal momentum in the explosion e.g rowing boat recoils as it needs to have equal momentum to the Person jumping off it
What happens if impact time is made longer
Force and momentum decrease
What happens to a moving objects momentum if no forces act on it
It stays the same
What happens when an object is thrown in the air
Gravity decreases its momentum as it goes up
What is elastic collision
When no kinetic energy is lost it gained
What is a crumple zone
Front and end of a car to lessen impact force . Increase impact time
Equation for acceleration
Change in acceleration
——–/————
Time taken
Equation force mass acceleration
Force =mass x acceleration
Five momentum time
Force = change of momentum
——————-
Time taken
What happens in an impact when moving object is speeded up
Momentum increases
Momentum equation
Mass x velocity
What happens to momentum in ana explosion
Is conserved objects move off in different directions
Positive direction right
Negative direction left
Thinking distance
How far car moves when driver suddenly sees a new obstacle and takes time to react to new situation before taking action
How is thinking distance related to speed and reaction time
Longer reaction time longer thinking distance and greater speed longer thinking distance
Braking distance
How far car travels after driver pushes the brakes and brings it to a stop
Factors effecting thinking distance
Tiredness , influence of drugs and alcahol that slow reaction time
Factors effecting braking distance
Mass of vehicle , breaking force applied , velocity the vehicle travels at
Difference between scalar and vector
Vector is a quantity which has size and direction
Scalar has no direction and is only size
How does heavy rain affect cars stopping distance
Increased as car cannot break without skidding , lowers breaking force and increased braking distance
Inelastic collision
2 objects collide and stick together e.g two railway trucks
Elastic colisión example
Colission between molecules in a gas
When is momentum not conserved
In a collision when external forces act on the object
Kinetic energy
Kinetic energy = 0.5 mass x velocity squared
Unit for momentum
Kg m/s
Impulse
Force x time
What happens to momentum
Stays the same providing no external forces act
Partially elastic collision
Where object looses some kinetic energy
Elastic behaviour
Ability of a material to return to original shape after forces causing deformation have been removed
Hookes law
Extension of a spring is proportional to force up to the elastic limit
Why do rubber bands not demonstrate hooks law
At beginning the band is stiff and doesn’t stretch much but after it softens
Why do sky divers bend legs when landing
Landing requires rapid de acceleration so sky divers bend legs so landing takes longer and reduces the force that if the force was large it could break their bones . Bending legs makes landing take longer and reduces force
Stopping distance
Sum of thinking and breaking distance
Why does a trolley accelerate
It’s weight provides a force to accelerate it
What does friction do to speed
Friction increases with speed and leads to a smaller resultant force on the object and therefore a smaller acceleration
Instantaneous speed
Speed over a short amount of time
Distance covered
————–
Small time interval
Force
Mass x acceleration
What happens if balance forces act on an object
Moves at a constant speed or stays stationary
What is moment
Measuring of a forces turning effect
Moment equation
Moment = force x perpendicular distance to line of action of action
What must moment be for an object to be in equilibrium
Moment must be equal on both side
How to change an objects speed
Unbalanced force must act on it the greater the size the faster the object will speed up or slow down
What is a force
A push or a pull
What does the gravitation. Field do
Accelerates falling objects . Frictional forces increase until they balance gravitational force
Weight
= mass x gravitational field strength
Centre of gravity
Where the weight force acts
Elastic limit
The limit when the shape will change permanently and not return to original length
What is moment measured in
Newton meters
What is momentum
A force possessed by masses in motion, a measure of how difficult it is to stop something from moving
What is moment
Forces that make objects turn around a fixed point
Why do passengers wear seat belts
So the reduced deacceleration applies to the passengers body otherwise they will come into contact quickly with another person or part of the car causing a large force to act on them
How to find cemetery of gravity on a regular shape
Located where lines of symmetry cross
How to find centre of gravity on an irregular shape
The point of suspension is a pivot point . Shape will not come to rest until it is balance and the centre of gravity is just below the pivot . A plumb line is hung from the pivot point so a like can be drawn perpendicularly from the pivot . The centre of gravity is somewhere along this line . This procedure is repeated suspending the card from a different pivot point and the centre of gravity will be where the two lines cross
What affects an objects stability
Low centre of gravity and broad base
