Mechanics and Materials (3): Newton's Laws of Motion Flashcards
State Newtons’ first law of motion
Object’s either stay at rest (acted on by no forces) or move with a constant velocity (forces acting on the object are in equilibrium) unless acted on by a force
What is the equation that relates force, mass and acceleration?
F = ma
What are the units for force?
Newtons, N
What are the SI units for force?
kgms^-2
What is Newtons second law of motion?
F = ma for a constant mass
What is the equation for the weight of an object?
W = mg
State Newtons’ third law of motion?
If object A exerts a force on object B, then object B will exert an equal and opposite force of the same type on object A
What is the definition of inertia?
The resistance of an object to changes in motion (affected by the mass of the object) e.g it requires a stronger force to give a heavier object a certain acceleration than to give an object with less mass that same acceleration
What is the resultant force on a car of mass, M, pulling a trailer with a mass, m, with tension in the tow bar being T?
F - T = Ma
What is the force on a trailer of mass, m, be pulled by a car with a mass, M, with tension in the tow bar being T?
T = ma
What is the equation for engine thrust, T of a rocket with a mass, m, and an acceleration upwards?
T - mg = ma
T = ma + mg
What is the general equation for tension, T, in a lift cable when taking: upwards as positive and m as the total mass of the lift and its occupants?
T - mg = ma
where m is the total mass of the lift and its occupants
What is the equation for tension, T in a lift when it is moving at a constant velocity?
T = mg
What is the equation for tension, T in a lift when it is moving up and accelerating?
T = mg + ma
ma > mg
What is the equation for tension, T in a lift when it is moving up and decelerating?
T = mg + ma
ma < mg
What is the equation for tension, T in a lift when it is moving down and accelerating?
T = mg + ma
(if taking downwards as negative)
What is the equation for tension, T in a lift when it is moving down and decelerating?
T = mg - ma
(of taking down as negative, therefore, a will be +ve)
The tension in a cable of a lift is less when…?
- The lift is moving up and decelerating
- The lift is moving down and accelerating
The tension is a cable of a lift is more when…?
- The lift is moving up and accelerating
- The life is moving down and decelerating
What force does any object moving in a fluid experience?
A drag force
What does the drag force acting on a object in a fluid depend on?
- The shape of the object
- The speed of the object
- The viscosity of the fluid (a measure of hos easily the fluid flows past a surface)
More viscous fluids flow…?
More slowly
Less viscous fluids flow…?
Less slowly
Give a standard response to a question about the terminal velocity of a falling
- Initially, only weight acts on the the object, therefore the object experiences a large resultant force acting downwards and so accelerates downwards
- As the velocity of the object increases the drag force on the object acting upwards increases
- This means that the resultant force downwards decreases and so the object acceleration decreases
- ## Terminal velocity is obtained when the drag force acting upwards on the object is equal and opposite to the force of weight acting downward on the object
What two factors does the top speed of a a road vehicle or aircraft depend on?
- Engine power
- Shape (whether the vehicle is more or less streamlined)
What is the driving force of a vehicle also known as?
Its motive force
What is the equation for the acceleration of a car with motive force Fe and resistive force Fr?
a = Fe - Fr/ m
When is the terminal speed/maximum speed of a vehicle reached?
When the resistive force becomes equal and opposite to the motive force
What is the definition of thinking distance?
The distance travelled by a vehicle in the time it takes a driver to react
What is the equation for thinking distance for a vehicle moving at a constant velocity v driven by a person with a reaction time, t0?
s1 = vt0
where s1 is the thinking distance
What is the definition of braking distance?
The distance travelled by a car in the time it takes to stop safely from when the brakes are first applied
What is the equation for the braking distance of a car assuming that its deceleration is constant and that it went from initial speed, u, to a final speed of 0?
v^2 = u^2 + 2as2
v = 0
a = -ve value
s2 = braking distance
2as2 = u^2
s2 = u^2/2a
What is the definition of stopping distance?
Thinking distance + stopping distance, the distance travelled by a vehicle from when the danger first appeared on the road to when the vehicle has reached 0 velocity
What is the equation for stopping distance?
ut0 + u^2/2a
where u is the speed before the brakes where applies
What is a demonstration you can do to measure the limiting friction between an object and a material?
- Place the object on the material you want to test
- Pull the object with a newton meter (increasing force) until it slides
- The limiting frictional force is equal to the pull force on the block until just before sliding occurs
What is the definition of the impact force experienced by a vehicle in a collision?
The force experienced due to the change in acceleration of the vehicle
What is the impact force of a vehicle in a collision related to?
Its deceleration/acceleration when it collides
What is the equation for the impact force experienced by a vehicle hitting a wall and experiencing an acceleration of -xms^-2?
In terms of g acceleration = -x/g
Impact force = mx/g
What is the definition of contact time?
The time two objects stay in contact when they collide - this is the same for both objects
Do two vehicles always have to separate after a collision?
No
How does contact time effect the impact force experienced by a collision of two vehicles?
Shorter contact time -> greater impact force
impact time: t = 2s/u+v (increase)
Acceleration: a = v-u/t (decrease)
Impact force: F = ma (decrease)
If two vehicles stick together after a collision, is their contact time the same as the duration of the impact force?
No
What is the equation for the impact time if you know one of the vehicles: distance moved during the impact, initial and final velocities?
t = 2s/u+v
What are the 5 car safety features we need to know about and how do they reduce the impact force experienced by the car and therefore, the drivers?
- Vehicle bumpers - Give way a little in low impact collisions, therefore, increasing the impact time -> reducing impact force
- Crumple zones - the engine compartment of car (the front part) is designed to give away in front-end impacts, this increases the impact time therefore reduces the impact force
- Seat belts - In a front-end impact a seat belt restrains the wearer from crashing into the vehicle frame after the vehicle suddenly stops. The restraining force on the wearer is much less than the impact force they would experience if they were to hit the vehicle frame. With the seat belt on the wearer is stopped more gradually.
- Collapsible steering wheel - In front-end impacts the seat belt restrains the driver, however, if the driver does end up hitting the steering wheel the impact force is lessened as a result of the wheel collapsing on impact
- Airbags - An airbag reduces the impact force on a person significantly as it acts as a cushion, increasing the impact time a lot. MORE SIGNIFICANTLY, the force experienced by the driver is spread over a large surface area so the pressure on the body is less than with a seat belt
What are some factors affecting thinking distance distance?
- Driver fatigue
- Alcohol consumption
- Speed of the car
- Distractions e.g other passengers and mobile phones
- Visibility
What are some factors affecting braking distance?
- Speed of the car
- Wet or icy roads
- Worn brakes or tires
- Mass of the car
