CP2 Forces and Motion Flashcards
Resultant force
The sum of all forces acting on an object
What does it mean if an object has a resultant force of 0N ?
The forces are balanced
The object is moving at a constant velocity, or is at rest
What is the resultant force of an object traveling at a constant velocity?
0N
What does it mean if the resultant force is not 0N ?
The object can either be accelerating, decelerating, or changing direction
Weight
The force acting upon an object due to gravitational attraction
Where does the force of gravity on Earth always pull to?
The centre of the Earth
The effect of the force of gravity in an object is called the object’s ….
Weight
What are forces measured in?
Newtons (N)
Mass
The mass of an object is the amount of matter an object is made from
What is mass measured in?
Kilograms (kg)
What is weight measured in?
Newtons (N)
How can we calculate the weight of an object?
Weight = Mass * Gravitational Field Strength
What’s the gravitational field strength of Earth (to the nearest while number) ?
10N/kg
What’s the weight of a 5kg mass on Earth’s surface?
5kg * 10N/kg = 50 N
What piece of equipment can be used to measure the weight of an object?
Newton metre or Force metre
How does an object move if the forces acting on it are balanced? (two answers)
The object stays at rest, or moves at a constant velocity
Calculate the weight of an object when the mass is 50kg, and the gravitational field strength is 1.6N/kg.
50 kg * 1.6 N/kg = 80 N
Acceleration
A measure of how much an object’s velocity changes in a certain amount of time
How do you calculate acceleration?
Acceleration = change in velocity / time
a = (v-u) / t
Name the two factors that affect acceleration
- the size of the force
- the mass of the object
What’s the equation that states the relationship between force, mass, and acceleration?
Force = mass * acceleration
f = m * a
What is acceleration measured in?
m/s^2
What force will make a mass of 4kg accelerate at 5 m/s^2 ?
Force = 4 * 5 = 20 N
A force of 50N acts on a 2kg mass. What will be the acceleration of the mass?
Acceleration = 50/2 = 25 m/s^2
A force of 2000N accelerates a car at 2.5 m/s^2.
What is the mass of the car?
Mass = 2000/2.5 = 800 kg
Newton’s First Law
“An object at rest stays at rest, and an object in motion stays at a constant speed and in a straight line unless acted upon by force.”
Newton’s Second Law
“The acceleration of a moving object depends upon its mass and the force acting on it.”
Newton’s Third Law
“If one object exerts a force on another object, then the other object exerts an equal and opposite force on the first object.”
The acceleration of an object is :
- Directly proportional to the resultant force on the object
- Inversely proportional to the mass of the object
What does directly proportional mean?
If two things are directly proportional, they increase and decrease together
What does inversely proportional mean?
If A is inversely proportional to B, this means that as A increases, B will decrease by the same factor
Inertial mass
A measure of how difficult it is to change the velocity of an object (the ratio of force over acceleration)
How are Action-Reaction forces different to Balanced forces?
Action-reaction forces act of different objects, whereas balanced forces act on the same object
How are Action-Reaction forces similar to Balanced forces?
The forces are equal in size, and act in opposite directions
For two forces to satisfy Newton’s third law, they need to:
- Act on different objects
- Be equal in magnitude
- Act in opposite directions
- Be the same type of force
Momentum
A measure of the tendency of an object to keep moving, or how hard it is to stop moving
What two things does the momentum of an object depend on?
The object’s mass and velocity
How can momentum be calculated?
Momentum = mass * velocity
p = mv
A 500 kg wrecking ball is moving at 10 m/s when it hits a building.
What is its momentum?
500 * 10 = 5,000 kg m/s
What is momentum measured in?
kg m/s
A 500 kg wrecking ball has a momentum of 1500 kg m/s.
What is its velocity?
1500 / 500 = 3 m/s
A football is kicked at a velocity of 6 m/s. It has a momentum of 240 kg m/s.
What is its mass?
240/6 = 40 kg
Why does a motorcycle travelling at 30 m/s have less momentum than a car travelling at the same speed?
Momentum depends on mass as well as velocity - a motorcycle usually has a smaller mass than a car, so will have less momentum than a car at the same velocity.
Conservation of Momentum
When moving objects collide, the total momentum of both objects is the same before the collision as it is after the collision - if there are no external forces.
Which formula expressed the relationship between momentum, force, and time
Force = change in momentum / time
F = (mv-mu) / t
A car has a mass of 2000kg. Its velocity changes from 5 m/s to 10 m/s in 5 seconds.
What force causes this change in velocity?
F = (20,000 - 10,000) / 5 = 2,000 N
A toy car has a mass of 2 kg. Its velocity changes from 2m/s to 5m/s, with a force of 4N.
How long does this change in velocity take?
t = ((2kg * 5m/s) - (2kg*2m/s)) / 4N = 1.5 s
How do you calculate stopping distance?
Stopping distance = thinking distance + braking distance
Stopping distance
The total distance a car travels until it stops
Thinking Distance
The distance the car travels between the driver seeing a danger and them acting upon it
Braking Distance
The distance a car travels until it stops, once the brakes have been applied
Name some factors that affect thinking distance
- Tiredness
- Distractions
- Alcohol
- Drugs
Name some factors that affect braking distance
- Weather
- Condition of the car’s tyres or brakes
- Road conditions
- Speed of the car
A car travels 7m during a driver’s reaction time, then a further 23m whilst the car is braking.
What was the car’s stopping distance?
7m + 23m = 30 m
A car travels 46m whilst the car is braking. The total stopping distance was 56m.
What was the driver’s thinking distance?
56m - 46m = 10 m
Name three safety features a car has to reduce force in a collision
- Seat belts
- Air bag
- Crumple zone
Velocity
The speed of an object in a given direction
What happens to an objects velocity if they are moving in a circular path, and why?
Its velocity is always changing.
The speed might be constant, but the direction is always changing so therefore the velocity changes too.
What do we call the resultant force acting on an object travelling in a circular path?
The Centripetal Force
Equilibrium
When a situation is not changing because all the things that affect it balance out
