1c forces, movement, shape and momentum Flashcards
What are forces measured in?
Forces are measured in Newtons (N).
We measure forces using a newtonmeter
What are Vectors?
A quantity that has both a size (magnitude) and a direction eg. velocity, force
What is a Scalar?
A quantity that has size but not a direction eg. mass, time
Examples of Vectors
Force
Velocity
Acceleration
Displacement
Examples of Scalars
Time
Speed
Distance
Energy
Mass
Temperature
What is the resultant force?
The resultant force is the total or “overall” force acting on an object
What is the equation for resultant force?
Resultant Force = Mass x acceleration
N = kg x m/s2
What is weight?
Weight (gravity) is the gravitational force so is measured in Newtons. Weight depends on the strength of the gravity and so it is different if you are not on earth
What is mass?
Mass is measured in kilograms (kg). Your mass is the same wherever you are
What is your mass and weight on the earth compared to the moon?
My mass on Earth is 80kg and my weight is 800N on the moon my mass is still 80kg but my weight is now 130N because the gravity is weaker
What is the equation for weight?
Weight = mass x gravitational field strength
N = kg x N/kg
earth = 10N/kg
What is the equation for stopping distance?
Stopping distance = thinking distance + braking distance
What is the thinking distance?
The distance travelled between the time when the driver sees that they need to brake and the time when they apply the brakes
What is the breaking distance?
The distance travelled while the brakes are applied (ie. the distance travelled between applying the brakes and coming to rest)
What are some factors that affect thinking distance?
- speed
- alcohol/drugs in a person’s system
- distractions
What are some factors that affect stopping distance?
- speed
- condition of speed
- condition of tyres
- condition of brakes
- mass of vehicle
What is the equation for moment?
Moment (turning effect of a force)
= perpendicular distance from point x force
What are moments?
A moment is a turning effect of a force
Moments are either clockwise or anticlockwise
Total clockwise moment = total anticlockwise monet
What is the principle of moments?
For an object that is balanced (in equilibrium), the total clockwise moment must equal the total anticlockwise moment.
eg. the turning of fore
To be balanced, both clockwise and anticlockwise are equal
What is the centre of gravity?
- The centre of gravity of a body is that point at which the weight of the body acts
- If the object is symmetrical and uniform (ie. the same density throughout) then the CofG is in the centre of the object
- If suspended a body will come to rest with the centre of gravity directly below the point of suspension
- An object will topple if its centre of gravity is not over the base of the object.
Centre of Gravity and Weight
We can imagine the centre of gravity as being a point where the weight of the object acts. So if we are drawing a force diagram, the weight should be drawn from the centre of gravity
Centre of Gravity and Moments
If an object is symmetrical then it’s centre of mass will be in the middle of it (if it isn’t symmetrical, then we would need to find the centre of mass by suspending it and drawing vertical lines
What is the equation for momentum?
Momentum = mass x velocity
p = m x v
Is momentum a vector or a scalar?
Momentum is a vector because it has a direction and a size
What is the conservation of momentum?
In any collision / explosion, total momentum before = total momentum after
What is momentum in collisions?
The momentum of an object is equal to its mass multiplied by its velocity. Momentum has direction, the same as the velocity, and is measured in kilogram metres per second.
In any interaction of bodies, where no external forces act on the bodies, momentum is conserved.
In snooker, a head-on collision of a white ball with a red ball can result in the red ball moving off with the same initial velocity as the white ball. This is an example of momentum conservation.
What is the equation for resultant force?
R fore = change in momentum/time
N = kgm/s / s
What is Hooke’s law?
Hooke’s law states that the extension of a spring is proportional to the force used to stertch the spring?
What is change in momentum?
Change in momentum = final momentum - initial momentum
What does proportional mean?
That if the force is doubled then the extension also doubles
What are elastics?
If a material being stretched or compressed is behaving elastically, this means that when the deforming force is removed, it will return to its original length
What is the elastic limit?
- Spring and wires will deform elastically until the elastic limit. Beyond this point, the spring or wire will be permanently stretched.
- Hooke’s law is no longer obeyed if the spring is stretched too far
Does an elastic band obey Hooke’s law?
An elastic band does not obey Hooke’s law. However, it does return to its original length when the stretching force is removed
What is Newton’s third law?
If an object exerts a force on another object, the second object exerts a force on the first of equal size but in the opposite direction
What will happen to the box if you push it at 100 N?
If I push a box with a 100N force, it pushes me with a 100N force in the opposite direction
How will these safety features help? (car)
These features will increase the time taken for a collision. The momentum change will be the same. Therefore the force on the person is reduced, reducing the risk of injury
In a car what are the safety features that reduce the risk of injury?
- seatbelts
- crumple zone
- aribags
- collapsible steering wheel
