Physics Flashcards
What does the gradient of a distance-time graph represent?
The speed of the object.
The steeper the gradient, the greater the speed.
What is a distance time graph?
A graph to help us describe the motion of an object.
How do we work out speed?
Speed = distance / time
m/s = m / s
What is the speed of an object?
The distance travelled each second.
What does a horizontal line on a distance-time graph mean?
The object is stationary.
What does a straight vertical line on a distance time graph mean?
The object is moving at a constant speed.
What is the speed of a runner who covers 400m in 50 s?
8 m/s
What is velocity?
The velocity of an object is it’s speed in a given direction.
If an object changes direction it changes velocity, even if it’s speed stays the same.
What do we call the change in velocity?
Acceleration
What is the unit for acceleration?
m/s 2
What is the equation to work out acceleration?
Acceleration = (final velocity - initial velocity) / time A = V - U / T
m/s 2 = m/s - m/s / s
(V - U also means change in speed)
What happens if the value for acceleration is negative?
It’s decelerating or negatively accelerating.
What does a horizontal line on a speed-time graph mean?
A steady, constant speed.
What does a stationary object look like on a distance-time and speed-time graph?
D.T. - a horizontal line
S.T. - no line at all
What does a constant acceleration look like on a distance-time and speed-time graph?
D.T. - an upwards curve (which goes right then up)
S.T. - a positive, straight line
What does steady deceleration look like on a distance-time and speed-time graph?
D.T. - a downwards curve (which goes up then right and stays horizontal)
S.T. - a negative, straight line
What does an object going backwards at a constant speed look like on a distance-time and speed-time graph?
D.T. - a straight, negative line
S.T. - a horizontal line
What does an object constantly accelerating backwards look like on a velocity and speed-time graph?
S.T. - a positive, straight line
V - a negative, straight line
What is acceleration?
Change of velocity per second
What does the area under the line (between the line and the x axis) represent in a velocity time graph?
Distance travelled.
What does the gradient of a velocity time graph mean?
Acceleration.
The steeper the line, the greater the acceleration. If the line is horizontal, there is no acceleration so it’s travelling at a constant speed.
What does calculating the gradient of a D.T and S.T graph tell us?
D.T. - speed
S.T. - acceleration
What is average speed of a car that travels 1.2 km in 1 minute?
1.2 km = 1200 m
1 minute = 60 seconds
1200m / 60s = 20
20 m/s
What are action and reaction forces?
Objects always exert equal and opposite forces on each other.
If object A exerts a force on object B, object B exerts an equal and opposite force on object A.
These are called action and reaction forces.
What is the resultant force?
The single force that would have the same effect on the object as all the original forces acting together.
The combined effect of all forces acting upon an object.
A resultant force always causes an acceleration so if there’s no acceleration, the resultant force must be 0.
The bigger the resultant force, the greater it’s acceleration.
The greater the mass of an object, the smaller it’s acceleration for a given force.
A resultant force always causes acceleration.The greater the resultant force on an object is the greater its acceleration. The big of a mass of an object the bigger the force needed to give it a particular acceleration
If there are 3 cars
1) driving force 200N - resistance force 200N
2) driving force 300N - resistance force 200N and
3) driving force 100N - resistance force 200N
which car would start moving?
Car 2 as the driving force is greater than the resistance force.
What is the resultant force of a 4N force and a 3N force acting in the same direction?
7N
What is the equation for force?
Force = mass x acceleration
F = m x a
N = kg x m/s 2
Which object has the greatest force acting upon it?
A 5kg rock accelerating at 3 m/s 2 or a 7kg boulder accelerating at 2 m/s 2?
N = kg x m/s 2
1) N = 5 x 3
N = 15
2) N = 7 x 2
N = 14
A 5 kg rock a” at 3 m/s 2 has the biggest force.
What is the stopping distance?
The stopping distance if a vehicle is the distance it travels during the driver’s reaction time (the thinking distance) plus the distance it travels under the braking force (the braking distance).
Stopping distance = thinking distance + braking distance
What is the thinking distance?
How long it takes to process the response.
The thinking distance is increased if the driver is tired or under the influence if alcohol or drugs.
What is the braking distance?
The distance travelled while braking. This is increased by poorly maintained roads, speed of the car, bad weather or poor car conditions e.g. Worn out tyres or brakes.
If an object falls freely, what is the resultant force acting upon it?
Gravity. We call the force of gravity weight.
The equation F = m x a becomes:
Weight = mass x acceleration due to gravity
W = m x g
N = kg x m/s 2
What is Hooke’s law?
The law that states that the extension of springs and elastic objects is directly proportional to the force applied, provided the limit of proportionality is not exceeded.
What is Hooke’s law’s equation?
Force = spring constant x extension N = N/m x m
(Spring constant measures how stiff the spring is. The larger the spring constant, the stiffer the spring.)
A spring has a spring constant of 30 N/m. If the extension is 0.30 m, what is the applied force?
F = k x e F = 30 x 0.30 F = 9N
What’s work?
Energy transferred by a force
What done to overcome friction is transferred as energy that heat the object that rubs together and the surrounding
What’s the equation for work done?
W = F * d
Work done = force * distance moved
J = N * m
What is the work done on an object if a force of 300N moves it at a distance of 8m?
W = F * d J = 300 * 8
= 2400J
What’s GPE?
Gravitational potential energy is energy stored in an object because of its position in the Earth’s gravitational field.
Whenever an object is moved vertically upwards, it gains gravitational potential energy equal to the work done on it by the lifting force.
(It depends on the objects weight and how far it moves vertically)
What’s the equation for GPE?
E = m * g * h
Energy = mass * gravitational field strength * height
J = kg * N/Kg * m
What’s the increase in GPE when a mass of 40kg is lifted 8m?
If it takes 2 seconds, what’s the power developed?
GPE = 40 * 8
= 320J
P = 320 / 2
= 160 watts
What’s kinetic energy?
Movement of an object measured where the greater the mass and faster the speed of the object, the more kinetic energy it has.
What’s the equation for kinetic energy?
E k = 1/2 * m * v2
= 1/2 * mass * speed 2
= 1/2 * kg * m/s 2
What’s the equation for momentum?
P = m * v
Momentum = mass * velocity
Kg m/s = kg * m/s
Objects have momentum when they’re moving
Whenever objects interact the total momentum before the interaction is equal to the total momentum afterwards - this is the law of the conservation of momentum
Like velocity, momentum has size and direction - while one direction is positive, the other must be negative.
Objects at rest have a momentum of 0. In an explosion, they move apart with equal and opposite forces.
What is the momentum of a 1000 kg car travelling at 30 m/s?
P = m * v
P = 1000 * 30
= 30,000kg m/s
What is impact time?
Time taken for a collision to take place
The the impact time is short, the forces on the object are large - as impact time increases, the force becomes less.
How have cars been made safe?
Seat belts and air bags spread the force across the chest and they also increase the impact time.
Side impact bars and crumple zones give way in an impact so increasing the impact time.
What happens to a passenger in a collision if they’re not wearing a seatbelt or have a very narrow seat belt?
Without one, they will continue forwards and hit the windscreen.
If it’s narrow, the force isn’t spread and may cut them.
What is the equation for power?
Power = energy / time
OR
Power = current * voltage
P = I / V
How does an insulator become negatively charged?
It gains electrons
Objects with opposite electric charges attract
Objects with the same electric charges repel