Mechanics: Newton's Laws of Motion Flashcards
(40 cards)
Newtons first law of motion.
Objects either stay at rest or move with constant velocity unless acted on by a resultant force.
Newton’s second law of motion.
Condition required?
By defining force (N) as resultant force that will give an object of mass 1kg an acceleration of 1 m/s^2 , F = ma.
Only true for constant mass.
Acceleration direction?
Acceleration is always in the same direction as the resultant force, no matter the direction of motion.
negative acceleration = positive deceleration
When object in eqm… S and W?
Support force on it is equal and opposite to its weight.
What is inertia?
The mass of an object is a measure of its inertia, which is its resistance to change of motion.
F = ma problems coming up!
Rocket problems, lift problems, pulley problems, sliding down slope problems.
Rocket:
If T is the thrust of the rocket engine when its mass is m, and the rocket is moving upwards, its acceleration a is ?
T is ?
Condition for takeoff?
a = T - mg = ma
Rocket thrust, T = mg + ma
Therefore rocket thrust must overcome the weight of the rocket for the rocket to take off.
Lift:
What is resultant force in lift, if T is tension in lift cable and m is total mass?
If lift moving up and accelerating vs down and dec vs .. vs.. then tension eq where ma </> mg.
T - mg = ma
T = mg + ma
Eg up + acce T = mg + ma > mg
Tension in cable less than weight if:
- up + dece (v > 0 and a < 0)
- down + acce (v < 0 and a < 0)
Tension in cable greater than weight if:
- up + acce (v > 0 and a > 0)
-down and dece (v< 0 and a > 0)
Pulley:
2 masses M and m attached to either side of string on pulley.
When released, M accelerated down and m accelerates up
M: Mg - T = Ma
m: T - mg = ma
Therefore Mg - mg = (M+m) a
Sliding down slope:
Block sliding down slope resultant F?
Resultant force on block as it accelerates down against resistance = mg sinθ - F0
Drag force (when moving through fluid) depends on…
-Shape of object (streamline vs not)
-Its speed,
-The viscosity of the fluid which is a measure of how easily the fluid flows past a surface.
Explain terminal velocity. (6)
-Speed of object as it falls through fluid increases as it falls;
-So drag force increases;
-Resultant force is difference between weight and drag force.
-As drag force increases, resultant force decreases, therefore acceleration decreases.
-If continues to fall, attains terminal velocity;
-which is when D = - W. a = 0 and speed is constant.
Acceleration eq at any instant during this process?
At any instance F = mg - D
so a = mg - D / m = g - D/m
Initial acceleration in this process?
Initial acceleration = g because speed = 0 so D = 0.
At terminal speed, what’s the energy transfer that occurred?
At terminal speed, potential energy of object is transferred, as it falls, into internal energy of fluid by the drag force.
Cars:
What is motive force?
What is resistive force?
What is resultant force, and a?
Motive force, Fe = driving force provided by the engine.
Resistive force, Fr = sum of drag forces acting on the vehicle.
Resultant force, F = Fe - Fr
So acceleration = Fe - Fr / m
If you reach terminal velocity freefalling, the open parachute, what happens?
Drag force increases therefore speed drops until drag force = weight again. New lower terminal velocity reached.
What is thinking distance, S1?
(assume constant speed)
The distance travelled by a vehicle in the time it takes the driver to react. If speed is constant, S1 = speed x reaction time = u x t0
What is braking distance, S2?
(assume constant deceleration)
The distance travelled by a car in the time it takes to stop safely from when the brakes are first applied. Assuming constant deceleration, a, to zero speed from speed u;
S2 = u^2 / 2a
What does braking distance depend on?
-Speed of vehicle
-Condition of vehicle tyres
-Road conditions eg if road is icy, skidding is more likely because limiting friction is reduced from dry value.
What is stopping distance?
Thinking distance + braking distance.
ut0 + u^2/2a
True or false: a car can only move forward by pushing backwards on the road?
True!
Skidding - friction between tyres and road prevent wheel spin (slipping) so the driving wheels therefore roll along the road.
If driver tries to accelerate too fast, the wheels skid. This is because there’s an upper limit to the amount of friction between the tyres and the road.
Skidding - when the brakes are applied, wheels slowed so vehicle slows, provided wheels don’t skid.
If braking force increased, the friction force between the tyres and the rod increase. However, if upper limit of friction/limiting frictional force reached, wheels skid. When this happens, the brakes lock and the vehicle slides uncontrollably forward.
