Mechanics Flashcards
When does an object accelerate?
An object accelerates when its velocity changes
- When velocity is positive > the object is speeding up > positive acceleration
- When velocity is negative > the object is slowing down > negative acceleration > deceleration
Terminal Velocity Question
1 • At the instant the diver leaves the plane, air resistance = 0N and the acceleration will be 10ms^-2 (acceleration of gravity).
2 • The weight of the diver remains constant, but the opposing force (air resistance) will increase as the diver falls. There is a resultant force acting downwards because the air resistance is less than the weight force of the diver. This means the forces are unbalanced and the diver will accelerate.
3 • The velocity of the diver will increase and they will continue to accelerate, but at a smaller rate (due to increased air resistance). This means the resultant force acting on the diver will decrease and the acceleration of the diver will decrease, until the resultant force = 0N, where weight = air resistance. At this point, the change in velocity will be 0ms^-1.
4 • This means the diver no longer accelerates (the velocity does not increase). This corresponds to its maximum velocity, called ‘terminal velocity’. Terminal velocity occurs when the resultant force acting on an object is 0.
Force and pressure question
(If the two objects being compared have the same weight, but exert a different pressure)
1 • The pressure exerted by the object with the smaller surface area is greater than the pressure exerted by the object with the larger surface area, because although the weight force exerted by the object remains the same, the surface area for which that force is applied is smaller for the object with the smaller surface area.
2 • As P = F/A, (and the weight force remains constant) pressure is inversely proportional to surface area. Therefore, when surface area decreases, pressure increases.
3 • The object with greater pressure will have increased friction, as it will sink into the ground more and enable a better grip and have more stability.
4 • The object with the lower pressure will have decreased friction, as it will not be able to sink into the ground. And therefore, it will likely slip and lose traction.
Gravitational potential energy question
1 • Work done is equal to the amount of energy an object possesses. Therefore, the work done = the gravitational potential energy gained.
2 • There is an energy difference because some of the energy used is converted to heat and sound due to the friction between the moving parts of the object.
3 • This means the gravitational potential energy the object gains is less than the total energy it uses.
Units for power
Watts (W)
Formula for area of a trapezium
Sum of parallel sides / 2
X the perpendicular height
Units for pressure
Pascals (P)
Units for work and energy
Joules (J)
Distance over time graph question
/ - ) (
/ A. The object moves at a constant speed of ___ for the first ___ seconds.
• The gradient is constant > The rate of change of distance is constant > The velocity is constant > The object is traveling at a constant speed
(This means the forces acting on the object are balanced, they are equal and opposite. The net force is 0.)
- B. The object is stationary at ___m for the next ___ seconds.
• The gradient is 0 > The rate of change of distance is 0 > The velocity is 0 > The object stops moving
) C. The object travels ___m in the next ___ seconds at an increasing speed.
• The gradient is increasing > The rate of change of distance is increasing > The velocity is increasing > The object is speeding up (experiencing a positive acceleration)
( D. The object travels ___m in the next ___ seconds at a decreasing speed.
• The gradient is decreasing > The rate of change of distance is decreasing > The velocity is decreasing > The object is slowing down (experiencing a negative acceleration)
Velocity over time graph question
(When line on graph has an increasing gradient / )
A. The object accelerates at a constant ___ for the first __ seconds.
• The gradient is constant (positive) > The rate of change of distance is increasing > The velocity is increasing > The object is speeding up (experiencing a positive acceleration)
(When line on graph has a gradient of 0 - )
B. The object remains at a constant velocity of ___ for the next ___ seconds.
• The gradient is 0 > The rate of change of distance is constant > The velocity is constant > The object is traveling at a constant velocity, with no acceleration
(When line on graph has a decreasing gradient \ )
C. The object negatively accelerates at a constant ___ for the final ___ seconds.
• The gradient is constant (negative) > The rate of change of distance is decreasing > The velocity is decreasing > The object is slowing down (experiencing a negative acceleration)
Dropping objects question
(work, power and energy)
PRESSURE
How far the object sinks into the ground is determined by the pressure they exert on the surface when they land.
- P = F/A. As the (object 1) has a greater mass than the (object 2), its weight force (F = mg) will be higher.
- As pressure is directly proportional to force, as the weight force increases, the pressure increases. Therefore, the (object 1) will exert more pressure on the ground and therefore sink more into the flour than the (object 2).
Dropping objects question
(work, power and energy)
ENERGY
Both objects are at the same height, but the (object 1) has a bigger mass and therefore greater gravitational potential (GPE = mgh) energy than the (object 2).
- When the objects fall to the ground, all the gravitational potential energy is converted into kinetic energy. Therefore assuming the conservation of energy, initial GPE = final Ek.
- Because the (object 1) has a larger initial gravitational potential energy and therefore larger final kinetic energy when it hits the ground, it has a greater impact and causes a deeper crater to be created.
Conservation of energy
Total energy is conserved through being transformed into other form(s).
Eg. Kinetic energy is converted into thermal energy (heat) or friction force that does work on an object.
Define mass vs. weight
Mass is the fundamental measurement of matter which an object has
(constant)
Weight is the downward force that an object experiences due to gravity
(not constant because it is influenced by location, as to how much gravity there is)
How power is affected by speed and time
If an object is lifted at twice the speed, the power needed to lift it will increase. This is because if the speed is doubled, the time taken to lift the load is halved.
As P = W/t, (power is a measure of the amount of work done per second) and since the amount work done does not change, if the time is halved the power is doubled.
What does a constant speed indicate?
A constant speed means that the forces acting upon the object are balanced and that they are equal and opposite.
(there is no resultant force)
Velocity over time graph
What does the gradient of the graph and area under the graph indicate?
- The gradient of a velocity over time graph indicates the acceleration of an object. A steeper slope indicates a greater acceleration.
- The area under a velocity over time graph indicates the distance traveled by an object.
The formula for the area of a trapezium
(sum of parallel sides / 2) x perpendicular height
Speed and velocity of a bouncing ball when it is dropped onto the ground
SPEED
(From when the ball leaves the hand of the person dropping it )
• The ball is speeding up and experiencing a positive acceleration, due to the influence of gravity applying 10ms-2 to the ball in a downwards direction, aiding the movement of it. ˅ ˅
(From when the ball is bouncing back from the ground into the air)
• The ball is slowing down and experiencing a negative acceleration, due to the influence of gravity applying 10ms-2 to the ball in a downwards direction, opposing the movement of it. ˅ ^
VELOCITY
(The instant the ball is in contact with the ground)
• The ball experiences maximum velocity on impact, and changes direction.
Distance and Displacement
• Distance is how far has been traveled in total by the object
(Area A + Area B)
• Displacement is how far has been traveled from the start to the endpoint of an object
(Area A = Area B)
Average speed formula
Distance / time
Vector quantities and the resultant vector
A vector quantity has both size and direction
(Eg. displacement, velocity, acceleration)
- Vectors quantities of the same quantity type (displacement + displacement etc.) can be added by placing the tail of the next vector on the head of the first vector
- The resultant vector is then represented by the arrow joining the tail of the first vector to the head of the last vector. This is equivalent to the vector sum of the forces.
Force, mass and acceleration
F = ma
Acceleration is directly related to force and indirectly related to mass. Therefore, as force increases, acceleration increases and as mass increases, acceleration decreases.
• The acceleration of an object is directly proportional to the resultant force and inversely proportional to its mass.
(See terminal velocity question)
Define net force
The net force is that single force that has the same effect as the sum of all force added head to tail.
If the forces are acting in the same direction, they add to give a larger net force.
If the forces are in the opposite direction, they subtract to give a smaller net force.
Net forces determine whether an object is experiencing a positive acceleration, negative acceleration or maintaining a constant speed.
If the net force is pointing in the same direction as the direction of motion, the object positively accelerates.
If the net force is pointing in the opposite direction to the direction of motion, the object negatively accelerates.
If the net force is 0, the object is either maintaining a constant speed or is stationary.
Final velocity formula (without time)
Vf = Vi + 2ad
• Can be used to find acceleration or distance if initial and final velocity is given, through substitution
Distance formula
(Vi + Vf / 2) t