P8: Forces (Y10 - Autumn 1) Flashcards
🟢 How are displacement and distance different?
Distance is how far an object moves. Distance does not involve direction. (Scalar)
Displacement includes both the distance and the way the object moves, measured in a straight line from the start point to the finish point and the direction if that straight line. (Vector)
🟢 What are Scalars and Vectors
The size of a quantity is is called it’s magnitude
Physical quantities that have size/magnitude but no direction are scalars.
Physical quantities that have size/magnitude and direction are vectors. (They can be show with scaled arrows showing size and direction)
🟠 Speed, Velocity and Force Equations:
Speed = Distance / Time
Velocity = Displacement / Time
Force = Mass x Acceleration
🟢 What is a Force?
As force is a push or a pull that acts on an object. Force is measured in Newtons (N).
🟢 Contact Forces and Non-Contact Forces definitions:
If two objects must touch to interact then these are contact forces. If they don’t need to directly touch to interact then these are non-contact forces.
🟢 Examples of Contact and Non-Contact Forces?
Contact Forces:
- Friction
- Air Resistance
- Tension
(A normal contact force is the support force - or reaction force - exerted on an object when it comes into contact with another stable object. Like a book resting on a table.
Non-Contact Forces:
- Magnetic Force
- Gravitational Force
- Electrostatic Force
🟠 Force Interaction - Newtons Third Law
When two objects interact with each other they exert equal and opposite forces on each other. (Newton’s Third Law)
E.g A person of pushes a wall with a force of 50N, the wall exerts an equal opposite force of 50N back.
🟠 Force Interaction - Newtons Second Law
In order to make an object move from rest, the forces must be unequal (Newton’s Second Law)
E.g If a person pushed with a greater force than the resistive forces from a block it will then move in the direction of the greater force (with a relative force)
🟠 An object will remain at rest or continue to move at a constant speed when…
The forces are balanced, and if they get unbalanced the object will accelerate/decelerate
🟠 What is Resultant Force?
Resultant Force is the difference between two opposite forces and the sum of two forces going in the same direction. When they are at other angles, use a parallelogram of forces.
❌ Definition of a Moment
A moment is a turning force.
The turning effect of a force is called the moment of the force.
To calculate the size of a moment, use the equation:
Moment (Mn) = Force (N) x Perpendicular Distance from Pivot (m)
❌ How can moments be balanced if two people of different weight are on a see-saw?
These two people will have to change how close or far away they are from the pivot, to be able to balanced out the clockwise and anti-clockwise moments.
For the smaller force the even out a bigger force, it must have a larger pivot (which is why we use levers).
❌ How to show moments in equilibrium mathematically
The sum of anticlockwise moments = The sum of clockwise moments
M1 = M2 = 0
Which means: F1 x d1 = F2 x d2
Or alternatively: W1 x d1 = W2 x d2
(Each number is a subscript)
❌ How are Gears Linked with Moments
Gears, like levers, can transmit turning forces. They can transfer turning effects whilst increasing them or decreasing them.
Moment = Force x Gear Radius
❌ Gears and Moments Example Question:
A gear of radius 15mm turns with a force of 100N. What is the moment produced by the gear?
100N x 0.015m = 1.5Nm
(Also, if Gear B is twice as big as Gear A, then Gear B will make one full turn for every two full turns of Gear A. Therefore, Gear B (the bigger gear) will turn slower than Gear A (the smaller gear)).
❌ What Are Low and High Gears Inputs and Outputs (Amounts)
For a Low Gear: The Output is low speed and provides a high turning effect/moment
For a High Gear: The Output is at high speed provides a low turning effect/moment
🟠 What is meant by Centre of Mass (CoM)?
For different objects, there is a point within it that can be considered as the central point at which the mass of the object acts through, centre of mass.
🟠 How is an object stable?
The stability of an object is affected by two factors:
- The width of the base of the object
- The height of the Centre of Mass
(Objects with a wide base, and a low centre of mass, are more stable than those with a narrow based and high centre of mass)
🟠 What happens if the centre of mass goes from inside to outside of an object?
If the centre of mass falls out of an object where it is meant to be inside of, the object will most likely topple over.
🟠 How to find the centre of mass of an irregularly shaped object
Equipment:
- Stand
- Plum line (string with weight at the bottom)
- Your Irregular Shape
Method:
1. Put a hole in one corner of the shape and suspend from the stand rod.
- Use a plum lime placed by the card to draw a vertical line down it.
- Repeat the procedure by hanging the card in different corners until you find where all of the points meet.
(To test this, try and balance this object on it’s centre of mass on a slender flat surface)
🟢 Step-by-step Parallelogram of Forces Method
Step 1:
The vectors must be drawn to scale, for example: 1cm = 20N.
Step 2:
The two force vectors must then be drawn again, parallel to each other. Vector ‘a’ (top right) is parallel to vector ‘A’ (bottom left) and vector ‘b’ (top left) is parallel to vector ‘B’ (bottom right).
Step 3:
Finally the distance can be measured of the resultant vector and also the angle of it.
❌ Step-by-step Resolving Forces in Equilibrium Method
Step 1:
First we can draw a geometric ‘scaled’ free body diagram with our weight acting straight down to the ground
Step 2:
Draw the force acting normal to the slope and the force acting along side/parallel to the slope to form a parallelogram.
❌ What are the key conditions for an object to be in equilibrium?
The key conditions for an object to be in equilibrium are:
- The resultant force on the object is zero
- The forces acting on the object have no overall turning effect.
❌ How to work out whether or not an object is in equilibrium
- If the lines of force are parallel, the sum of forces in one direction must be equal to the sum of the forces in the opposite direction. This means that the resultant force on the object is zero.
- If the line of action of the forces are not parallel, the forces can be resolved into two components along the same perpendicular lines. The components along each line must balance out if the resultant force is zero.
🟢 What can the Parallelogram of Forces be used for
We can use the parallelogram of forces to find the resultant force acting on an object, when the two forces are working in different directions.
