Forces and Motion Flashcards
Explain distance-time graphs?
Time on the x-axis, distance on the y-axis.
Gradient shows speed, if it is curved it is accelerating, if it is flat it is stationary, if there is a straight line (not flat) it is moving at a constant speed.
Relationship between speed, distance and time?
speed = distance/time
Practical: investigate the motion of everyday objects such as toy cars and tennis balls?
- Set up two light gates 0.5m apart
- Select a (tennis ball) and measure and record its diameter
- Roll it through both gates
- Record the time it took to go through each gate and the time it took to travel between gates to calculate speed and acceleration
The relationship between acceleration, change in velocity and time
taken?
acceleration= change in velocity/time taken a = (v-u)/t
Explain velocity−time graphs?
Time on the x-axis, velocity on the y-axis.
Gradient is acceleration, if the line is flat then it is moving at a constant speed, if it is straight (not flat) then it is accelerating at a constant rate, if it is curved then it not accelerating at a constant rate - a tangent to the curve can be taken.
The area under the graph = distance travelled
Use the relationship between final speed, initial speed, acceleration and distance moved?
(final speed)^2 = (initial speed)^2 + (2 × acceleration × distance moved)
v^2 = u^2 + (2 × a × s)
Three ways that forces can effect objects?
Changing shape, changing speed, changing its direction of motion
How do vector and scalar quantities differ?
Scalar describes magnitude, e.g. size, speed.
Vector describes magnitude and direction. e.g. velocity, displacement. Force is a vector quantity.
How to calculate resultant force along a line?
If forces are in the same direction they are added together, if they are in different directions then you must find the difference.
How do resultant forces cause an object to act?
If there is a unbalanced resultant force on an object it will accelerate in the direction of the resultant force. If the forces are balanced then a stationary object (weight = normal contact force) will remain stationary. If the object is moving then it will continue to move in the same direction.
What is friction?
A force that opposes motion
What is the relationship between unbalanced force, mass and acceleration (Newton’s Second Law of Motion)?
force = mass x acceleration
Acceleration is proportional to the resultant force
Relationship between weight, mass and gravitational field strength?
weight = mass x gravitational field strength W = mg
How to calculate stopping distance?
stopping distance = breaking distance + thinking distance
Factors affecting stopping distance?
Breaking distance: water on the road reduces friction, speed, mass
Thinking distance: Alcohol, tiredness, distractions
Factors affecting stopping distance?
Breaking distance: water on the road reduces friction, speed, mass, poor brakes
Thinking distance: Alcohol, drugs, tiredness, distractions
How to calculate thinking distance?
thinking distance = speed x reaction time
Describe the forces acting on falling objects (and explain why falling objects reach a
terminal velocity)?
Skydiver analogy:
- When the skydiver jumps out the plane weight is much greater than air resistance, so they accelerate.
- As they accelerate, air resistance increases.
- Eventually they reach a constant speed as air resistance has increased to equal weight. This is called terminal velocity.
- As the parachute is opened air resistance increases dramatically due to an increase in surface area, so a new, lower terminal velocity is reached.
Practical: investigate how extension varies with applied force for helical springs, metal
wires and rubber bands?
- Set up a retort stand with a ruler clamped vertically in place
- At the top of the stand attach the helical spring
- Measure the position of the bottom of the spring on a ruler (L_0)
- Add a 1N weight to the bottom of the spring
- Measure the new position of the bottom of the spring (L_1)
- Calculate the extension of the spring by doing L_1 - L_0
What are the types of deformation?
Elastic deformation is when a change in shape is temporary and a shape returns to it s original shape after the force causing the deformation have been removed.
Inelastic or plastic deformation is permanent deformation.
(Paper 2) Relationship between momentum, mass and velocity?
momentum = mass x velocity p = mv
(Paper 2) Use the idea of momentum to explain safety features?
Safety features such as seatbelts are built to stretch, so if a sudden crash happens and a person is thrown forward the seatbelt stretches, so the person stops moving gradually, not suddenly. As the momentum decreases over a longer period of time, the force on the person decreases.
(Paper 2) Explain conservation of momentum?
Total momentum before the crash is the same as total momentum after the crash
mv + mv = total momentum before the crash
total momentum before the crash = total momentum after the crash
total momentum after the crash = mv + mv
(Paper 2) Relationship between force, change in momentum and time taken?
force = change in momentum / time taken F = (mv-mu) / t
(Paper 2) What is Newton’s Third Law?
Every action has an equal and opposite reaction.
i.e forces come in pairs that are equal in size, e.g. weight and normal contact force for an object on the table.
(Paper 2) Relationship between moment, force and perpendicular distance from the pivot?
moments = force x perpendicular distance from the pivot
(Paper 2) What is the centre of gravity?
All the weight of a body acts through its centre of gravity.
It is the point at which we can assume all of an objects mass is concentrated.
The centre of gravity for all regular objects is in its centre.
(Paper 2) How to find the centre of gravity for an object of irregular shape?
- Hang up the object
- Suspend a mass from the same place
- Mark the position of the thread
- Repeat steps one to three with the object suspended from a different place
- The point at which the two lines cross is the centre of gravity
(Paper 2) Explain forces on a beam?
The beam is not turning, so the moments to from the start to the end are equal to the moments from the start to the mass.
F1d1 = F2d2
F2 = F1d1 / d2
