P5: Forces Flashcards
What do vector quantities have?
Magnitude and direction.
What do scalar quantities have?
Just magnitude.
Give 5 examples of vector quantities.
Force, volocity, displacement, acceleration, momentum.
Give 5 examples of scalar quantities.
Speed, distance, mass, temperature, time.
What are the two categories of forces?
Contact and non-contact.
How are weight and mass different?
Mass is a measure of the amount of matter in an object, so remains the same everywhere. Weight is a force which acts on an object due to gravity. A measurement of weight could change with location.
Mass and weight are directly proportional. What formula links them?
Weight = mass x gravitational field strength
What is the unit for weight?
Newtons (N).
What is the unit for mass?
Kg.
Work done =
energy transferred (and force x distance).
What formula relates work done, force and distance?
Work done = force x distance
What is elastic deformation?
Where an object returns to its prior shape after the force deforming it is removed.
What is inelastic deformation?
Where an object doesn’t return to its prior shape after the force deforming it is removed.
What is meant by the limit of proportionality (in the context of springs)?
The point where a spring’s extension stops being proportional to the force applied to it. A plotted graph would become non-linear at this point.
What equation is used to find the force applied to a spring when stretching/compressing it?
F = ke (force = spring constant [N/m] x extension [m])
Practical:
Plan an investigation to show the relationship between force and extension for a spring.
- Use a stand to suspend a spring. Clamp a mm ruler in line with the spring.
- Record the natural length of the spring, taking the reading at eye level.
- Hang a mass on the spring and measure its length. Calculate the extension by substracting the initial length.
- Repeat this process, adding one identical mass each time, until you have 6 measurements.
- If one extension is larger than the previous, the limit of proportionality has been succeeded. You’ll need to repeat the experiment, with a new spring and lighter masses.
- Calculate the force exerted on the spring per mass (weight = mass x gravitational field strength).
- Plot force exerted on the spring (x axis) against spring extension. You should see a directly proportional / linear relationship.
- You could find the spring constant using the equation F = ke.
What is the moment (or torque) of a force?
Known as the turning effect of a force, it is the rotational equivalent of linear force, i.e. a twisting force.
What equation is used to find the moment of a force?
M = Fd (moment [Nm] = force x perpendicular distance from pivot to line of action of force)
What is the unit for moment?
Newtonmeters (Nm)
What is pressure?
Force per unit area.
What equation is used to calculate the pressure a fluid exerts on a surface it touches?
Pressure (Pa) = force normal to surface/area of surface
What is the unit for pressure?
Pascals (Pa).
Pressure in a liquid depends on what?
Depth, density and gravitational field strength.
What is upthrust?
An upward resultant force on a submerged object which is equivalent to the weight of fluid it displaces.
What is velocity?
Speed in a given direction.
At what speed does a person usually walk?
1.5m/s
At what speed does a person usually run?
3m/s
At what speed does a person usually cycle?
6m/s
At what speed does a car usually travel?
25m/s
What is acceleration?
Rate of change of velocity (speed)
What equation is used to calculate acceleration?
acceleration = change in velocity (speed) / time
Friction always acts in what direction relative to motion?
The opposite direction.
What is drag? Name one type of drag.
1) The friction when something moves in a fluid (liquid/gas). 2) Air resistance
Friction in fluids (drag) increases with __.
Speed.
Explain how falling objects reach terminal velocity.
- When an object first falls, the downward force of gravity is much more than the upward frictional force, so it accelerates.
- As speed increases, friction increases, which gradually reduces the object’s acceleration until the frictional force = the accelerating force (resultant force = 0).
- It has therefore reached its terminal velocity and falls at a steady speed.
What does the frictional force of air resistance depend on?
An object’s shape and area.
Newton’s 3rd law states that, when 2 objects interact, they exert equal and opposite forces on each other. Why does anything move in relation to anything else?
1) The forces act in opposite directions. 2) Objects have varying masses, so move varying amounts when experiencing the same force.
What is stopping distance?
The combined thinking and braking distance.
What 2 factors affect thinking distance?
Speed and reaction time.
What 4 factors affect reaction time?
Tiredness, drugs, alcohol and distractions.
What 5 factors affect braking distance?
Speed, weather, tyre condition, condition of brakes, and road conditions (e.g. leaves).
How do brakes work?
- Force is applied to the brakes of a vehicle
- Work is done by the friction force between the brakes and the wheel
- This reduces the kinetic energy of the vehicle
- The temperature of the brakes increases
Why is applying the brake too hard/quickly dangerous?
Very large decelerations may cause brakes to overheat or cause the wheels to skid.
