Unit 4 - Mechanics Flashcards
What is a scalar?
A quantity which only has a magnitude
What is a vector?
A quantity which has a magnitude and a direction
Where should the arrows start on a scale diagram?
At the centre of mass
What is the resultant force?
The difference between forces that originates when there are multiple forces all acting on the same object
If an object is stationary then the resultant force on the object is __ and the forces acting on it are ________
- 0
- Balanced
How can you calculate the resultant force from a scale diagram?
Arrange the arrows top-to-tail and then use trigonometry to determine the resultant force
What is resolving vectors?
The process of splitting up a resultant force into its separate components
What two components are you most likely to resolve into?
Horizontal and vertical (unless the object is on a slope then the normal is included)
Horizontal Force = ?
Vertical Force = ?
- Resultant Force x cos(x)
- Resultant Force x sin(x)
What will happen to an object if the forces acting on it are balanced?
It will remain stationary or moving in the same direction at a constant speed
What is a moment?
The turning effect of a force about a point (the pivot)
What is the equation and unit for the moment?
Moment (Nm) = Force (N) x Distance (m)
What angle does the force have to be to the distance when using the moments equation?
The force has to be perpendicular to the distance
What do you do if the force is not perpendicular to the distance when calculating moments?
You can resolve the force into its horizontal and vertical component and the use the one which is at right angles to the distance
What does it mean if the moments are balanced?
There is no resultant moment: sum of clockwise moments = sum of anticlockwise moments
What is the line of action?
The line along which a force is considered to act
What happens if the pivot point is on the line of action?
There is no turning effect
What is torque?
The force produced when two equal and opposite forces which are parallel and act along different lines act about the same point
What is the difference between speed and velocity?
Speed is a scalar quantity and velocity is a vector quanitity
T/F: You can have negative velocity
True, because velocity is a vector it can be negative if acting in a negative direction
What are the two main types of speed?
Average and instantaneous
How do you determine the average speed from a distance-time graph?
Calculate the gradient of the whole graph
How do you determine the instantaneous speed from a distance-time graph?
Draw a tangent to the curve at the point when you want to know the velocity and then calculate the gradient of that tangent
T/F: Distance-time graphs can increase and decrease
False, distance-time graphs are always increasing because distance covered can never decrease
Define acceleration
The rate of change of velocity
Is acceleration a vector or a scalar?
A vector
What can we infer about the velocity if there is uniform acceleration?
There is a constant change in velocity
How can we calculate the acceleration from a velocity-time graph?
Calculate the gradient
What is projectile motion?
The motion an object experiences as the result of a vertical force and no other horizontal force in a gravitational field
How does an object move when experiencing projectile motion?
An object will maintain horizontal velocity whilst accelerating downwards
What are Newton’s 3 laws of motion?
- An object will remain at rest or moving at a constant velocity unless acted upon by an external force
- An unbalanced force acting on an object will cause it to accelerate in the direction of that force, this acceleration will be directly proportional to to the unbalanced force and inversely proportional to the mass of the object (F = ma)
- If object A exerts a force on object B then object B exerts an equal and opposite force on object A
Why do we draw free body diagrams?
We draw free body diagrams so that we can have a visual representation of the forces that are acting on an object and their magnitude and direction relative to each other and the centre of mass of the object
What would happen if you are standing on scales in a lift and it is:
a) moving down
b) moving up
c) stationary
a) The number of the scales will decrease as it uses springs to measure the reaction force of the person and, since weight can’t change, in order for the lift to move down the upward force has to decrease
b) It will increase for a similar reason as the reaction force has to increase in order for the lift to move up
c) The number will stay the same as if the person was standing on normal ground as the person is stationary
What is terminal velocity?
The maximimum velocity an object falling through a fluid can achieve
Why does a skydiver reach a point where they are no longer accelerating?
This is because the velocity and the air resistance are directly proportional. As the velocity increases so will the air resistance which means that a point will be reached where the drag caused by the air resistance will be equal to the weight of the person so they will move downwards in a uniform manner
Drag Force = ?
6 x Pi x Dynamic Viscosity x Radius of Sphere x Velocity
What does Newton’s third law state?
“If object A exerts a force on object B, then object B exerts an equal and opposite force on object A. Both forces are of the same type.”
What is a third law pair?
Two objects that have a relationship due to Newton’s third law because they have the same type of force acting on them
What type of force is reaction force?
Electrostatic
Why is reaction force electrostatic?
When a force is applied to the ground it pushes the atoms in the solid closer together, the opposing force is cause by the electrostatic repulsion between the nuclei of the two atoms
What is impulse?
Force x Time (= Change in momentum)
T/F: The rate of change of momentum is not equal to the unbalanced force applied
False: the rate of change of momentum is equal to the unbalanced force applied because if you link impulse and Newton’s second law you end up with this equation: Force = (Mass x Change In Velocity) / Change In Time
What are the three rules for calculating the momentum after a collision?
1) V1 = V2
2) M1V1 = M1V2 + M2V3
3) M1V1 = (M1 + M2)V3
What is an elastic collision?
A collision in which all of the kinetic energy is conserved
What is an inelastic collision?
A collision in which not all of the kinetic energy is conserved
What do safety features in vehicles aim to do and why?
They aim to increase the time of the collision which decreases the force on the passengers
What is work done equal to?
Energy transferred
