Mechanics 2 Flashcards
Newton's Laws Freefall Momentum Energy Efficiency
What does freefall mean?
Freefall is when the only force acting on an object is gravity.
In which direction does g act?
Vertically down
At what acceleration do objects in freefall accelerate towards the ground at?
g (9.81m s-2)
What’s freefall motion called when the object is given an initial velocity?
Projectile motion
What should you do if you need to use the SUVAT equations on an object that has an initial velocity at an angle to the horizontal?
Resolve the velocity into its vertical and horizontal components. Using the vertical velocity for the calculation with the SUVAT equations, to calculate how long it’s in the air. Then use the horizontal component of velocity along with the time calculated from the vertical component to calculate the distatance the projectile travelled.
What is the only force present in free fall motion?
Weight
Describe an experiment you could do to obtain data that you could use to determine the value of g.
Using light gates or an electomagnetic timer. Time the time taken for a ball bearing the fall through a certain distance. Take readings for each height and obtain an average.
Explain how you can plot the data obtained from the measuring g practial on a graph to obtain the value for g.
Plotting height against time squared and drawing a line of best fit. Then find the gradient of that line. 1/2 the gradient will give you the acceleration due to gravity.
Ignoring air resistance, which ball will accelerate fastest. One with a mass of 5kg or one with a mass of 50kg?
They will both accelerate at the same rate, g.
What is the effect of air resistance on the trajectory of a projectile?
It will reduced the distance travelled by the projectile. If the projectile starts with vertical velocity, it will reduce the height the projectlie reaches.
What is meant by friction?
A force which opposes the motion of an object.
What is meant by drag?
A force which opposes the motion of an object in a fluid.
In what direction does a frictional force act?
In the opposite direction to the motion of the object.
In what direction does lift act, relative to a flowing fluid?
At right angles to the flowing fluid.
What can you say about the frictional forces and the driving forces acting on an object when it reaches terminal velocity?
They are equal and opposite (balanced)
Sketch a velocity-time graph for an object falling through the air and reaching terminal velocity.
Initially there is some uniform acceleration, the rate of acceleration slows as the the air resistance increases, eventually leading to a constant velocity.
How can a skydiver reduce her terminal velocity?
Open her parachute to increase the drag force acting on her.
How does friction change the energy store of an object?
Converts kinetic energy into thermal energy.
Explain how air resistance affects the maximum speed of a car.
As the speed of the car increases, the air resistance acting on the car also increases, until these forces are balanced and the car is travelling at it’s top speed. The force of air resistance limits the maximum speed of the car. A larger air resistance will mean a slower top speed, a smaller force of air resistance means a higher top speed.
Other than increasing the power of the engine, how could the maximum speed of the car be increased?
Make the body of the car more aerodynamic to reduce air resistance.
What affect does speed have on air resistance?
Air resistance increases as speed increases.
How can frictional forces acting on an object be reduced?
By making the object more streamlined.
Which of these graphs shows the velocity-time graph of an object falling through air and reaching terminal velocity?
Teacher to draw
What does the acceleration-time graph look like for a parachutest falling and reaching terminal velocity?
Acceleration starts at 9.81 ms-2 and reduces in a non-uniform way until reaching 0 ms-2 where the parachutest reaches terminal velocity.
What is the initial rate of acceleration on earth? Why does it decrease as the object falls?
9.81 ms-2. Air resistance increases reducing the resultant force.
Why does a kicked football move in a parabolic curve/projectile motion
Because the horizontal velocity remains constant, while the vertical velocity is affected by acceleration downwards due to gravity.
When looking at projectile motion questions, how should you treat the horizonal and vertical components of motion?
Horizontal and vertical components of motion should be treated independently.
Define terminal velocity
The maximum velcoity an object will fall at when the force of weight is balanced by the force of air resistance
State Newton’s first law of motion
If there is no resultant force acting on an object it will either stay at rest or move with a constant speed in the same direction.
State Newtons second law of motion
If there is a resultant force acting on an object it will accelerate in the direction of the resultant force. The acceleration of an object is proportional to the resultant force acting on it.
State Newtons third law
If object A exerts a force on object B, object B will exert and equal force on A but in the opposite direction.
Explain with reference to Newton’s third law how we are able to walk across the floor.
In order to walk across the floor, you must push back on the floor with your foot; then, according to Newton’s Third Law, the floor pushes forward on your foot, which propels you forward. This, of course, requires friction to work.
Explain why the equal and opposite force of a books weight on the table is not the reaction force of the table acting on the book.
The forces are acting on the same object, the forces are of different types.
Name the equal and opposite force of a book’s reaction force on a table according to Newton’s third law.
The reaction force of the table acting on the book.
How much resultant force is needed to accelerate a 24.1 kg mass by 3.5 ms-2?
84 N (2.s.f)
A resultant force of 18N is applied to a toy car with a mass of 0.61 kg. What will the magnitude of the car’s acceleration be?
30 ms-2 (2.s.f)