P10 - Force and Motion Flashcards
What important equation is known as Newton’s 2nd Law?
*Resultant Force = mass x acceleration
F = m x a
(N) = (kg) x (m/s^2)
*could be asked to calculate the mass or acceleration when 2 forces are given; you then have to find the resultant force value and use that.
What is Inertia?
An object’s tendency to stay at rest or continue at a constant speed.
How do we describe positive acceleration (speeding up)? What does this look like in real life?
The resultant force is in the same direction as the velocity.
E.g car speeding up - the resultant force is the thrust, and the car is moving forwards also.
How do we describe negative acceleration (slowing down)? What does this look like in real life?
The resultant force is in the opposite direction to its velocity (decelerating).
E.g slowing down a car - the resultant force is the braking force of the car, but the car still moves forwards.
What is the weight of an object?
The force acting on an object due to its mass and gravity.
Units: Newtons (N)
What is the mass of an object?
The quantity of matter an object has.
Units: kilograms (kg)
What is the earth’s gravitational field strength?
9.8 N/kg
What equation links weight, mass and gravity together?
Weight = mass x gravitational field strength
W = m x g
(N) = (kg) x (N/kg)
What pieces of equipment measure weight, mass and gravity?
Weight - newton-meter
Mass - balance
Gravity - gravimeter - (don’t need to know this)
What is the terminal velocity of an object?
The constant velocity an object eventually reaches as it falls.
When is an object said to have reached terminal velocity?
When the frictional force of air resistance is equal and opposite to the object’s weight.
What is the resultant force acting on an object at terminal velocity?
The resultant force is 0
Why is the resultant force 0 at terminal velocity?
The forces of weight and air resistance are equal and opposite.
They are balanced, so there is no sum effect.
What two frictional forces oppose the driving force of a vehicle?
1) Ground Friction
2) Air resistance
What two factors impact the braking force needed to stop a vehicle?
1) The speed of the vehicle when the brakes are applied.
2) The mass of the vehicle.
What is the stopping distance of a vehicle?
The distance a vehicle can safely stop in.
What is the thinking distance?
The distance a car travels before the brakes are applied.
What is the braking distance?
The distance a car travels whilst the brakes are applied.
What equation links stopping, braking and thinking distance?
Stopping Distance = thinking distance + braking distance
What factors affect thinking and braking distance? What factor affects both?
Thinking Distance:
- Speed
- Reaction time
- Alertness
- Drink / Drugs
Braking Distance:
- Speed
- Road conditions
- Tyres (traction)
- Mass of car
- Brake conditions
- Aerodynamics of car
Both:
-Speed - this is why speed is the most important factor in road safety.
What is momentum?
A measure of how difficult something is to stop moving.
It’s a vector.
What are the units for momentum?
kg m/s
What is the equation for momentum?
momentum = mass x velocity
p = m x v
kg m/s = kg x m/s
What does the law of the conservation of momentum state?
In a closed system, the *total momentum before and after an event are equal.
*total - this is the key
How can you calculate the masses and velocities before and after a collision?
1) Divide the page into before and after.
2) Record all the correct masses and velocities.
3) Calculate the various momentums.
4) Add them together to work out the total momentum on one side.
5) Equate the total momentum to the other side and work backwards from there.
What equation summarises the conservation of momentum?
(mass after event x velocity after event) + (mass before event x velocity before event) = 0 constant
Ma x Va + Mb x Vb = constant (0)
Essentially, total momentum, p before = total momentum, p after.
This equation can help us calculate the masses and velocities of vehicles before and after a collision.
What are the safety features of a car designed to do in an accident?
They are designed to lessen the force of impact.
What is the rule for reducing the force of impact in a car accident?
The longer the time of the impact, the lesser force of the impact.
What is the equation that links force, momentum and time?
Force = *Change in momentum / time
F = Δp / t
(N) = (Δkg m/s) / s
- final momentum - initial momentum
- You can see that the bigger t is, the bigger F is - whence why the time of impact is increased to lessen the force.
How can F = Δp / t be re-written if Δp is split into mass and velocity?
Force = mass x final velocity - mass x initial velocity / time
F = mv - mu / t
extra:
mv is essentially final momentum
mu is essentially initial momentum
What are some safety features of a car?
Headrest - reduce the risk of whiplash.
Airbags - spread force across the chest and increase impact time.
Seatbelts - spread force across the chest and icrease impact time.
Crumple Zone - absorb impact and protect survival cell.
Bumpers - absorb some kinetic energy from impact.
What do cycle helmets do for cyclists?
They absorb the impact - whence increasing the time of impact and protecting the head from major injury.
What safety features in a playground keep children safe?
Cushioned surfaces increase the time and reduce the force of impact if a child falls over or hits something.
When is an object referred to as elastically deformed?
Elastic deformation: When a force deforms (stretches) an object, but it returns to its original shape, after the force deforming it is removed.
Inelastic deformation: When a force deforms (stretches) an object, but it remains in its new shape after the force deforming it is removed.
What is a compressive force?
A force that has a shortening effect.
What is tensile force?
A force that has a stretching effect.
What is the spring constant of a spring/object? What is the symbol and unit?
The stiffness of a spring.
Symbol = k Unit = N/m
High spring constant = won’t compress or stretch easily.
What is the limit of proportionality?
The point where the extension of a spring is no longer directly proportional to the applied force.
The relationship becomes non-linear and the spring breaks.
What does Hooke’s Law state?
The extension of a spring is directly proportional to the force applied, as long as its limit of proportionality is not exceeded.
What is the Hooke’s Law equation?
Force applied = spring constant x extension
F = kx
(N) = (N/m) x (m)
What is the extension of a spring?
The extended length take away the original length.
How can the quantity of potential energy stored in a spring be calculated?
Energy = 1/2 x Sprint Constant x (Extension)^2
E = 1/2 x k x (x)^2
(J) = 1/2 x (N/m) x (m)^2
What 4 key terms are great for describing graphs?
1) Linear Relationship - the relationship between quantities where a linear trend is established.
2) Direct Proportion - the relationship between quantities whose ratio is constant.
3) Inverse Proportion - the relationship between 2 quantities where one increases at the same rate as the other decreases.
4) No correlation - 0 correlation between variables.
