P8 - Forces in balance Flashcards
What is a force? Name and describe 2 types:
-a vector quantity, a push/pull that acts on an object due to the interaction with another object
contact - objects are physically touching
non-contact - objects are physically separated
What is weight? Give its equation, and how it is measured:
a force acting on an object’s centre of mass due to gravity
W = m g
-using a calibrated spring balance (ie a newton meter)
What is a resultant force? How can you find it?
-a single force that has the same effect as all of the original forces acting on an object added up
-draw a parallelogram of forces
How can a single force be resolved?
use a free body diagram to resolve a force into 2 components at right angles to each other (or on a slope as shown below)
How can a force do work on an object? Give its equation:
-a force does work on an object when the force causes a displacement of the object (movement)
-1 joule (or Nm) of work is done when a force of 1N causes a displacement of 1 metre
W = F s
s is distance moved along the line of action of the force
Define a moment, and give its equation:
-the turning effect of a force
M = F d
d is the perpendicular distance from the pivot to the line of action of the force
What is the centre of mass?
the point from which an object’s weight may be considered to act from
How do gears transmit a force?
-if a force is a applied to a gear, the rotational effect of the force causes the gear’s teeth to transmit an equal force to the other gear’s teeth that are in contact with it
-however the moment may change depending on the other gear’s radius
M = Fd, if the radius is larger than the driver gear then it produces a larger moment but turns slower, which is the principle of a low gear
Explain what a high gear is and how it works:
-high gears produce high speeds
-the input gear is bigger and the output gear is smaller
-as the input gear is rotated, the output gear rotates multiple times for every one revolution of the input
-this results in a higher speed
However due to the smaller moment on the output gear, the acceleration would be less
Explain the frictional forces acting on a bike on both wheels:
-consider a frictionless surface - the rear wheel would just rotate and not move the bike
-in order to create a forward motion, the surface must have friction, and the friction must oppose the direction of the relative motion of the rear wheel (which is backwards at the point of contact), therefore the frictional force on the rear wheel must be forwards
-the front wheel is translated forwards but not rotated due to the lack of friction - to rotate it, the frictional force must oppose the relative motion of the front wheel to the ground (which is forwards), therefore the frictional force on the front wheel is backwards
Describe how the force acting on the pedal causes a moment about the rear axle:
-force on pedal causes a moment about the pedal axle
-causes a force on the chain, which then causes a moment about the rear axle
Suggest some factors that will affect the distance an electric car can travel before it has to be recharged:
-terrain (eg going up/downhill)
-battery capacity
-speed
-mass
this was the very first thing in the physics mock and you messed it up
Explain why the forces acting on a brick dropped in a pool cause it to be stationary at the bottom:
-weight force of the brick causes it fall to the bottom of the pool
-upthrust acts upwards on the brick
-normal contact force from the floor also acts upwards on the brick
-the weight of the brick equals the upthrust plus the normal contact force, and because they are opposite in direction, the resultant force is 0
-by Newton’s 1st Law, the brick will remain stationary
Why does an object accelerate when dropped?
there is a resultant force of its weight acting downwards due to gravity, so it accelerates
When asked about why anything accelerates, always mention the resultant force as it is the only reason why things can accelerate (Newton’s 1st Law)
