Mechanics Flashcards
What is the difference between uniform and non- uniform motion?
Uniform motion – acceleration is constant (including an acceleration of 0m/s2)
Non-uniform motion – acceleration is not constant
What do the slopes and areas under the graphs of the graphs below tell you:
Displacement-time
Velocity-time
Acceleration-time
Displacement-time graph:
Gradient = velocity
Area = nothing
Velocity-time graph:
Gradient = acceleration
Area = displacement
Acceleration-time graph:
Gradient = nothing
Area = change in velocity
Define distance.
How far something travels.
Define displacement.
How far an object travels from its starting point in a given direction.
Define speed.
The distance travelled by the object in a given time.
Define average speed.
the total distance travelled by the object divided by the total time.
Define instantaneous speed.
The speed of an object at a given time.
Define velocity.
The rate of change of displacement.
Define average velocity.
The total displacement divided by the total time.
Define instantaneous velocity.
The velocity at a given time.
How could you find the instantaneous velocity and average velocity from a displacement-time graph?
Instantaneous velocity – draw a tangent at a given point, and determine the gradient (using as big a triangle as possible)
Average velocity – total distance/displacement divided by time
Describe how the bouncing ball graphs would be similar/different for a softer/less inflated ball?
The
The time taken for the balls to hit the ground will be the same
The maximum displacement of each bounce will decrease as kinetic energy is lost due to friction between the ground and the ball.
The maximum velocity would decrease due to the reduction in kinetic energy.
The gradient of the velocity-time graph will be the same as gravity (the force only acting on the ball) is always 9.81ms^2.
The gradient of the velocity-time graph where the ball makes contact with the ground will be less steep as the time taken for the ball to change directions is longer.
What is the difference between scalar and vector quantities?
Scalar quantities only have magnitude
Vector quantities have magnitude and direction
List common scalars.
Temperature, energy, power, work done, current.
List common vectors.
Force, velocity, acceleration, displacement, moment/torque.
For a projectile moving under gravity, describe the horizontal motion.
Constant
For a projectile moving under gravity, describe the vertical motion.
-9.81ms^2 as the only force acting is gravity.
Define Newton’s first law.
The velocity of an object will not change/remain at rest/move in a straight line with constant speed, unless there is a resultant force acting on it.
Define Newton’s second law.
The acceleration of a body is proportional to the resultant force.
F = ma
Define Newton’s third law.
If an object A exerts a force on object B, then object B must exert a force of equal magnitude and opposite direction back on object A.
What properties must be true for Newton’s third law pairs of forces?
The pair of forces must be the same type of force acting in opposite directions and in the same plane.
What does a free-body force diagram show?
A free-body force diagram shows all the forces acting on a single body.
It should not include forces exerted on the environment around the body, or the forces acting on a second object.
What does it mean to resolve forces/vectors?
Resolving a force is splitting a single force into its components.
What does it mean to add forces/vectors?
If two or more forces acting on an object, the resultant force can be found by adding the components together.
Define Newton’s second law in terms of an equation. What must you be careful of?
F=ma
F is the resultant force; this may need to be found first, or it may not be the force you have been asked for.
What are the two main types of friction?
Friction between solid surfaces
Friction in a fluid (aka drag). Air resistance is an example of fluid resistance. Fluid resistance is affected by viscosity.
What do frictional forces always cause?
A force that is opposite to the direction of motion. Kinetic energy is converted to heat and sound.
Describe what causes an object to reach terminal velocity.
Initially, there will be a resultant force.
This resultant force causes the object to accelerate.
As the velocity increases, the drag increases.
This reduces the resultant force, therefore reduces the acceleration
Eventually, the fluid resistance will be equal to the driving force or weight.
There is no resultant force, therefore no acceleration.
The object is moving at a constant speed, called terminal velocity).
What factors affect the terminal velocity of a car?
Size of the driving force.
Size of the frictional force.
What equation is used to calculate the weight of something?
W = mg
Define momentum.
Momentum is the product of the mass of an object and its velocity.
Define the principle of conservation of linear momentum.
The total momentum before a collision or explosion is equal to the total momentum after providing no external forces act.
Define Newton’s laws in terms of momentum.
Newton’s First Law: The momentum of a body remains constant, unless there is a resultant force.
Newton’s Second Law: If the is a resultant force, the rate of change of momentum is directly proportional to the resultant force, and in the same direction.
Newton’s Third Law: If two objects collide, they exert an equal and opposite force on each for the same time. So momentum is conserved.
What is Newton’s second law, in terms of moment as a formula?
F = mv / t
Ft = mv
Define a moment.
Force multiplied by the perpendicular distance from the line of action of the force and the axis of rotation.
Define centre of gravity.
The single point of an object that you can consider its whole weight to act through.
State the conditions needed for something to be in equilibrium.
Resultant force in any direction is zero.
Sum of the moments about any point is zero.
If there are 3 forces acting, you should be able to close a triangle
Define work done.
Work is done when a force is used to move something a distance in the same direction as the force is acting. It is usually the same as energy transferred.
Work is usually done against another force (e.g. work done against gravity, work done against friction).
W = Fs
What is the equation for kinetic energy?
(1/2)mv^2
What is the equation for gravitational potential energy?
mgh
What is the equation for power?
P = W / t
What is the equation for efficiency?
Useful (energy/power out / total energy input) x 100
What is power?
The rate of doing work/how much energy is transferred per second.
State the principle of conversation of energy.
Energy cannot be created or destroyed. Energy can be transferred from one form to another but the total amount of energy in a closed system will not change.
Why does a bouncing object never return to its original height
Work is done against friction of the ball and the ground or air resistance.
Some energy is converted to thermal energy therefore less energy is available as gravitational potential energy.
Why will a rollercoaster never return to its original height?
Work is done against friction of the wheels and the track or air resistance.
Some energy is converted to thermal energy therefore less energy is available as gravitational potential energy.
Define a newton.
1 Newton is force needed to give a mass of 1kg an acceleration of 1 ms^2
Define a joule.
1 Joule is the work done by a force of 1 newton when its point of application moves 1 metre in the direction of action of the force
Define a watt.
1 Watt is when 1 joule of energy is transferred per second
Describe the “CPAC 1: Determine the acceleration of a freely-falling object.” experiment.
1) Clamp an electromagnet to a stand with a trapdoor a set distance (measured with a metre ruler) below the electromagnet.
2) The electromagnet and trapdoor are attached to a timer where when the switch for the electromagnet is turned off, the timer starts and is stopped when the object passes through trap doors.
3) Turn the switch on and attach a small, heavy metal ball bearing to the electromagnet (to assume air resistance is small and therefore negligible).
4) Turn the switch off to release the ball and start the timer and when the ball passes through the trapdoor the timer will stop. Record the time.
5) Repeat this experiment 3 times and take an average of the three times.
6) Repeat this experiment for different distances.
7) Use these results to plot a graph of height over (time taken to fall)^2. This is done by rearranging the suvat equation (s = ut + (1/2)at^2) to get “a”: (1/2)a = s / t^2.
8) The gradient of the line will determine the acceleration halved so multiply the gradient by 2 to get the acceleration.
9) Calculate a percentage difference from your answer compared to gravity (the actual deceleration) to evaluate the accuracy of your answer.
