General Physics 1.5 Forces Flashcards
Define ‘resultant force’
The sum of all the individual forces acting on an object (taking directions into account).
What happens if a resultant force acts on an object?
It causes a change of momentum in the direction of the force.
How can a resultant force change the motion of an obiect?
It can change…
• Speed
• Direction
How can the resultant force be found?
Adding force vectors together, tip (arrow) to tail.
What happens if there is zero resultant force?
The object will remain stationary, or (if moving) will continue to move in the same direction with the same speed.
Define friction.
Friction is the force providing resistance to the motion of two surfaces sliding past each other.
Give an example of a frictional force.
Air resistance.
When work is done overcoming friction, what occurs?
Energy is dissipated, resulting in heating.
When does circular motion occur?
When a force is acting perpendicular to the motion of an object.
Describe the speed and velocity of an object in circular motion
• The speed is constant
• The velocity is always changing, since it is constantly changing direction
Give an example of a force causing circular motion
Gravity, which causes the earth to orbit the sun.
State Hooke’s law
The force on an object (eg. a wire or spring) is directly proportional to its extension.
Give the equation for Hooke’s law
F=kx
Where…
F = force (N)
× = extension (m)
k = spring constant (N/m)
At what point does Hooke’s law no longer apply?
The limit of proportionality.
What does the limit of proportionality look like on an extension-load graph?
Where the graph stops being linear.
What is a moment?
The rotational (or turning) effect of a
force.
Give the equation for moments.
moment (Nm) = force (N) x distance (m)
(note: distance is the perpendicular distance from the force to the pivot)
When does rotational equilibrium occur?
When the sum of clockwise moments = the sum of anticlockwise moments
This is the principle of moments.
When is equilibrium reached?
When there is no resultant force or turning effect.
(clockwise = anticlockwise moments)
What is a centre of mass?
A single point through which the force of an object’s weight acts. (This is a modelling assumption).
Describe how to find the centre of mass of a plane lamina.
• Suspend the object and a plumb line from the same point
• Trace the thread of the plumb line with a pencil
• Repeat using different points
• The centre of mass is where all the lines cross
What is a vector quantity?
A vector quantity has both magnitude (size) and direction.
What is a scalar quantity?
A quantity that has only magnitude, not direction.
Give examples of vectors
• Velocity
• Force
• Acceleration
• Displacement
Give examples of scalars
• Speed
• temp
Mass
Energy
Describe how resultant forces can be represented visually
Use scale drawings of vector diagrams.
• Add force vectors tip to tail
• Connect them for the resultant force
