P1D1 - forces and moments Flashcards
State 4 ways that forces can affect an object
- Change its shape
- Change its size
- Change its speed
- Change its direction of motion
Define friction/drag and give examples
A force between two surfaces which makes motion more difficult and always results in production of heat.
E.g.
Air resistance - proportional to velocity
Explain effect of balanced and unbalanced forces
Unbalanced forces = resultant force - change in speed
Balanced forces = no resultant force - constant speed
Formula for force
mass (kg) x acceleration (m/s^2) = force (n)
State Hooke’s law equation
spring constant (n/m, k or the force needed to stretch a spring by one unit of length) x extension (m) = force (N)
What is the limit of proportionality?
The highest force that can be applied before the extension of a spring is no longer proportional to the force applied. Deformation occurs above this point.
To what precision should answers in physics be given unless stated otherwise?
3 significant figures!
What is a moment?
A turning force, such as when we open a jar of food.
What is the pivot?
The center of rotation
Formula for size of moment
force (n) x perpendicular/right angle distance to the pivot (m) = moment (Nm)
The larger the distance from the pivot, the bigger the moment
Why are door handles always on the opposite side to the hinges?
If the door handle is a large distance away from the pivot (the hinges), then the force we need to open the door is much smaller.
What if more than one force is trying to rotate an object?
We add up the moments, not forces! Their direction is called clockwise or anticlockwise
When is an object at equilibrium?
When there are no net forces and no net movements/moments.
What is the principle of moments?
An object will not rotate if the clockwise and anticlockwise moments are equal
What is the center of gravity/mass and how is it represented?
The imaginary point within an object that the mass and weight of the object is evenly dispersed around.
For regular objects/shapes, it is found along the line of symmetry.
Represented by one arrow drawn from the center of the object
What affects the stability of an object?
- Center of gravity - the lower the center of mass, the more stable an object is
- Size of base - the wider the base, the more stable an object is
When will an object fall?
When a vertical line down its center of gravity falls outside the base of an object
Name 8 forces
- Force of gravity/weight
- Normal reaction - the force the surface (on which the object lies) exerts upon it. Acts perpendicular to the surface
- Tension - when ropes,springs, etc are stretched. Away from the object being pulled
- Buoyancy/upthrust - the upward force a fluid exerts on an object. Affected by density
- Friction/drag - e.g. air resistance. Direction always opposite of motion
- Uplift - upward pressure
- Applied force - external push or pull
- Thrust - from engine
Newton’s first law
Law of inertia:
When all forces acting upon an object are balanced…
- an object at rest will stay at rest
- moving objects will continue moving at constant speed in a straight line
Newton’s 2nd law
An unbalanced force will cause acceleration. Acceleration = force/mass
Newton’s 3rd law
Every action has an opposite and equal reaction
How can tilting a pile of bricks (arranged in a regular rectangular position) make a truck more stable?
When the bricks are stacked upright, the height of the load is greater, so it’s center of gravity is higher. Tilting the bricks (to the leftl) reduces the overall height and brings the center of gravity more to the left so that it is above the support (truck). A lower center of gravity = more stability
How does a ticker timer work?
A ticker timer makes 50 dots per second, meaning that 1 space in between the dots = 0.02 seconds. If we know the length of the tickertape, we can do velocity=distance/time. E.g. the distance is 6cm/0.02 x 5 (because there are 5 gaps) = 6cm/0.1 second or 60cm/second
What does it mean when a spring is stiffer?
It will require more force for the same extension
How to find center of gravity of irregular shape?
- Cut out an irregular shape of card.
- Make a hole near the edge of the shape and place a pin through it. 3. Clamp the pin so that it is horizontal, and make sure the card can rotate.
- Tie a mass suspended by cotton from the pin. This must be free to swing backwards and forwards like a pendulum.
- Once the cotton and card have stopped moving, carefully mark where the cotton is aligned on the card by drawing a line along its length (shown as a black solid line in Figure 2).
- Repeat this procedure for three different positions
State the aim and method of Newton’s second law experiment
Plan: To show that force is directly proportional to acceleration
Method:
Set up experiment, with the falling weight attached to a string which is wrapped around the pulley and connected to the glider.
1. Measure the force of the falling weight with a spring balance
2. Activate the air track and let the weight fall (to pull the glider)
3. Measure the acceleration with a light gate
4. Repeat with different values of force
