Motion 3 Forces In Action Flashcards
What 3 things can cause acceleration?
Change in speed, change in direction and a combination of both
What forces should and should not a force body diagram contain?
It should have all the forces acting on the body
It should not have forces exerted by the body.
Define Normal contact force.
It is the force equal and opposite to the force applied on a surface by an object. It is perpendicular to the surface.
Define Tension.
A pulling force exerted by a string, rope or cable on the objects, pulling on the string, rope or cable, at either end
Define upthrust and friction.
Upthrust is a force acting on an object moving through a fluid, it is equal to the weight of fluid displaced and acts vertically upwards.
Friction is a resistive force that opposes motion at the point of contact and acts in opposite direction to the motion. It is independent of velocity
What 3 things should you consider while making a free body diagram?
. Label everything
. Only consider forces acting on the object and disregard forces exerted by the object
. Mention the type, size and direction of all forces.
Define drag and state the difference between it and friction.
Drag is the frictional force experienced by an object while moving through a fluid.
Drag Force is directly proportional to velocity squared, whereas friction is independent of velocity
What factors affect drag?
Cross sectional area of body,
It’s Velocity,
The density of the fluid,
Co-efficient of drag.
Describe the motion of a parachutist as he jumps from a plane.
At the beginning of his decent, acceleration due to gravity would be at it’s highest and the drag and upthrust would be zero.
As he accelerates downwards, drag would increase and his acceleration downwards would start to decrease until the upward resistive forces equal the downward weight.
The parachutist will now fall at terminal velocity with no resultant force acting upon him. Then when he opens his parachute, the drag will increase exponentially and his velocity downwards would decrease exponentially, as resultant force is upwards and parachutist has negative acceleration.
As the velocity decreases the drag will also decrease until it again equals the force of weight, after which the parachutist falls at a new terminal velocity.
Define an experiment to calculate the terminal velocity of an object in a fluid.
Take a measuring cylinder and fill it with the fluid, take a ruler, stopwatch, retort stand, magnet, ball bearing and a video camera.
Set the ruler beside the measuring cylinder using the retort stand and place the ball just above the fluid. Roll the video camera. Drop the ball and start the stopwatch, at the same time. After the ball has reached the bottom of the cylinder, use video analysis to work out the distance travelled by the ball at each second, by looking at the ruler.
Repeat the experiment and plot a graph of mean velocity per second against time and draw a lobf. The point where the gradient dips to zero is where terminal velocity begins.
Define moment of force and principle of equilibrium
Moment of a force is the product of the magnitude of a force applied and the perpendicular distance between the line of action of the force and the pivot.
The principle of moments states that for an object to be in rotational equilibrium, the sum of all clockwise moments must equal the sum of all anti-clockwise moments.
What is a couple and what is the formula used to calculate it’s torque?
A couple occurs when two forces of the same magnitude act in an anti parallel direction to each other about the same pivot.
The formuale to calculate torque then becomes:
Magnitude of Force x Perpendicular distance between the line of action of both forces
Define center of mass and center of gravity.
The center of mass of an object is a singular point where all it’s mass can be assumed to be situated.
The center of gravity is a singular point in a body where all the weight of the body can be assumed to act
Where must a force be applied for a body to move in a straight line?
The force must act directly through the center of mass of the body.
How do you find the center of mass of an irregular shaped object?
What happens when your results consist of a triangle rather than a single point?
Draw three random points on the shape. Clamp the body from one of the points and allow the body to settle. Hang a plumb line directly in front of the shape and trace the line.
Repeat this for the other points and the intersection of the three lines made would be the center of mass.
Sometimes a triangle appears at the intersection, rather than a point. The center of mass is assumed to be the center of the triangle.
