8.1 - force + acceleration

what is difficult / dangerous about driving on ice?

moving from standstill and stopping

why can’t you easily stop a vehicle on ice?

your car will continue moving (sliding) when the brakes are applied as its tires have little / no grip on the ice, meaning friction (from the tires) cannot oppose the push force of your car and stop its movement

why can’t you easily move a vehicle from standstill on ice?

tires have little / no grip on the ice, meaning friction (from the tires) cannot get enough grip from the ice to move the car

why is it difficult / dangerous to drive on ice?

tires have little / no grip on the ice, meaning friction (from the tires) cannot oppose the push force of your car, or get enough grip from the ice to move the car forward

when pushing an object across a rough floor, what prevents the object from sliding after you stop applying a force?

friction

which direction does friction act?

opposite the push force

what does friction oppose?

the push force

what can be used to observe motion in the absence of friction?

a linear air track

what is a linear air track?

a device that allows motion to be observed in the absence of friction

how does a linear air track work?

the glider on the air track floats on a cushion of air. provided the track is level, the glider moves at constant velocity along the track because friction is absent

what is newton’s first law?

objects either stay at rest or move with constant velocity unless acted on by a force

which one of newton’s laws is this? - objects either stay at rest or move with constant velocity unless acted on by a force

newton’s first law

according to newton’s first law, what is an object moving at constant velocity?

either an object -

• acted on by no forces

• or the forces acting on it are balanced (so the resultant is zero)

what is the resultant force of an object moving at constant velocity?

zero

what are the forces acting on an object moving at constant velocity?

either no forces act on the object, or the forces are balanced (so the resultant is zero)

for an object moving at constant velocity, either no forces act on the object, or the forces are balanced (so the resultant is zero) - which one of newton’s laws does this apply to?

newton’s first law

what is newton’s second law?

• f = m a

• f ∝ m a

which one of newton’s laws is this? - f = ma

newton’s second law

for which mass type does newton’s second law apply to?

constant mass

how can we investigate the change of an object’s velocity when it is acted on by a constant force?

using a dynamics trolley and a motion sensor connected to a computer

what does the experiment investigating the change of an object’s velocity when it is acted on by a constant force prove?

f = m a, therefore i will call it the experiment investigating newton’s second law !

what equipment is used in the apparatus of the experiment investigating newton’s second law?

• dynamics trolley

• motion sensor

• computer

• sloping runway

• elastic bands

• interface

• card

here

apparatus for the experiment investigating newton’s second law here

experiment investigating newton’s second law

what is the point of the dynamics trolley in the experiment investigating newton’s second law?

the trolley’s velocity and how it changes with time is measured and used to display a velocity-time graph

what is the point of the motion sensor in the experiment investigating newton’s second law?

to track the movement of the card mounted on the trolley and work out the velocity of the trolley, sending a signal to the computer its connected to so the computer can display a velocity-time graph of the trolley

what is the point of the computer in the experiment investigating newton’s second law?

to receive a signal from the motion sensor and display a velocity-time graph of the trolley

what is the point of the sloping runway in the experiment investigating newton’s second law?

to compensate for friction so the trolley’s movement can be controlled (i.e., it won’t slide), ensuring the trolley is moving due to a constant force

why must there be a source of friction in the experiment investigating newton’s second law?

so the trolley’s movement can be controlled (i.e., it won’t slide), ensuring the trolley is moving due to a constant force

what is the point of the elastic bands in the experiment investigating newton’s second law?

to pull the trolley and be a way of varying force applied over repeated experiments

what is the point of the interface in the experiment investigating newton’s second law?

to help transmit the signal from the motion sensor to the computer idk

what is the point of the card in the experiment investigating newton’s second law?

to be tracked by the motion sensor so the trolley’s movement and therefore velocity can be measured

how does the experiment investigating newton’s second law work?

• the trolley is pushed with constant force up the sloped runway

• the motion sensor, tracking the card, sends signals to computer, which displays a velocity-time graph of the trolley

• the velocity-time graph shows that velocity increases a constant rate, meaning its gradient is constant and therefore acceleration is constant. accelerations is recorded

• repeat measurements are taken and recorded with varying force and mass (done by changing the number of elastic bands and trolleys, respectfully)

• by multiplying the mass and acceleration of your results, it will prove force is proportional to mass x acceleration, therefore proving newton’s second law

what is required for the experiment investigating newton’s second law to successively prove newton’s second law?

that acceleration (gradient of displayed velocity-time graph) is constant in each repeated measurement

how does an experiment investigating newton’s second law prove newton’s second law?

acceleration, mass, and force are recorded with varying mass and force. mass and acceleration can then be multiplied to show proportionality with force, therefore proving newton’s second law

what is the purpose of measuring velocity in the experiment investigating newton’s second law?

to display a velocity-time graph, which should give a constant gradient, therefore constant acceleration, which can be used to prove f = ma

how do we change the force the trolley is pulled with in the experiment investigating newton’s second law?

by changing the amount of elastic bands

in the experiment investigating newton’s second law, what happens if we increase the number of elastic bands used to pull the trolley?

the force the trolley is pulled with increases

in the experiment investigating newton’s second law, what happens if we decrease the number of elastic bands used to pull the trolley?

the force the trolley is pulled with decreases

how do we change the mass of trolley is pulled with in the experiment investigating newton’s second law?

by changing the amount of trolleys

in the experiment investigating newton’s second law, what happens if we increase the number of trolleys?

the mass of the system will increase

in the experiment investigating newton’s second law, what happens if we decrease the number of trolleys?

the mass of the system will decrease

in the experiment investigating newton’s second law, what length are the elastic bands?

stretched to the same length as each other

in the experiment investigating newton’s second law, why are the elastic bands stretched to the same length?

to ensure the force pulling the trolley is distributed evenly along the trolley

in the experiment investigating newton’s second law, how should the trolley be pulled?

with constant force

in the experiment investigating newton’s second law, why should be trolley be pulled with constant force?

• we are testing how the velocity of an object changes if its acted on by a constant force, so force should be constant (duh)

• we are testing to see if force is proportional to mass x acceleration (newton’s second law), therefore force needs to be constant so it can be accurately compared with different values of mass x acceleration

how do we find the right runway to use in the experiment investigating newton’s second law?

see if the trolley should move down the runway at a constant speed after being a given a brief push

what does the computer show in the experiment investigating newton’s second law?

a velocity-time graph of the trolley’s movement, with velocity increasing at a constant rate (constant gradient), therefore showing the trolley moves with constant acceleration

what are the typical measurements for the experiment investigating newton’s second law?

here

what can we do with the graph shown by the computer for the experiment investigating newton’s second law?

record acceleration (gradient of the velocity-time graph displayed) at different amounts of force and mass, to be multiplied with mass to prove f = ma (newton’s second law)

in the experiment investigating newton’s second law, how do we know that acceleration is constant?

the gradient of the velocity-time graph displayed by the computer is constant

in the experiment investigating newton’s second law, how do we know the trolley is accelerating rather than decelerating?

the gradient (acceleration) is positive, meaning the trolley is accelerating

in the experiment investigating newton’s second law, is acceleration constant?

yes

in the experiment investigating newton’s second law, why is acceleration constant?

because the trolley is being pulled with a constant force and no other forces are acting on it (other than the friction from the sloped runway, but that is to control the movement of the trolley with better accuracy)

how can f ∝ ma be rewritten as f = ma?

• f ∝ ma

• define the newton (the unit of force) as the amount of force that will give an object of mass 1 kg an acceleration of 1 ms^-2

• 1 ∝ 1 × 1

• 1 = 1

• therefore f = ma

what is the unit of measurement for mass?

kilogram (kg)

what direction is acceleration in?

always the same direction as the resultant force

why is acceleration always in the same direction as the resultant force?

because the resultant force is the push force, therefore the object accelerates towards the direction of the push force

what did galileo prove about the rate objects fall at depending on their mass?

objects fall at the same rate, regardless of their mass

what did galileo say about the rate at which two objects of different masses will fall if connected by a rope?

they would initially fall at different rates, until the faster one would be slowed down by the slower one, which would be sped up by the faster one until they fell at the same rate

how did newton explain galileo’s theory on the rate at which object’s fall?

• a = mg / m

• (acceleration = force of gravity / mass)

• mass would cancel out, therefore the acceleration of an object, regardless of mass, would always be g, which is a constant

a = mg / g

newton’s proof of galileo’s theory on the rate at which object’s fall

how does a = mg / g prove galileo’s theory on the rate at which object’s fall?

• a = mg / m

• (acceleration = force of gravity / mass)

• mass would cancel out, therefore the acceleration of an object, regardless of mass, would always be g, which is a constant

what is weight?

• the force exerted on an object’s mass due to the earth’s gravitational field strength

• w = m g

what is the equation linking weight, mass, and gravitational field strength?

w = m g

what is the unit for weight?

newtons (N)

what is the acceleration of an object in free fall?

gravitational field strength, g (9.81 ms^-2)

why is the acceleration of an object in free fall 9.81 ms^-2 ?

because the force of gravity is the only force acting on the object when in free fall

what is the support force of an object in equilibrium?

equal and opposite to the object’s weight

if you put an object on a surface, what is the support force exerted on that object by that surface?

equal magnitude to the object’s weight

how does a weighing balance measure the weight of an object?

when an object is in equilibrium, the support force is equal and opposite to the object’s weight, therefore an object placed on a weighing balance exerts a force on the balance equal to the weight of the object, thus the balance measures the weight of the object

here

what are examples of weighing balances?

• newton-metres

• top pan balance

what is a newton-metre an example of?

a weighing balance

what is a top pan balanced an example of?

a weighing balance

what is the gravitational field strength (g)?

the force of gravity per unit mass on a small object at a given position

why does the gravitational field strength only apply to small objects?

it means small as in, yk, not cosmic. if it were a large object (another planet, star, comet), then the forces would interact differently

how does a newton-metre weigh an object?

• there is a cylinder inside the balance tube that slides out when weight is added, and the end of the cylinder correlates to a newton scale on the tube’s surface

• since a newton-metre is an example of a weighing balance, it measures the weight of an object since the object (if in equilibrium) will exert a support force on the newton-metre that is equal and opposite to it’s weight

what does the gravitational field strength depend on?

a given position (distance away) from a large, cosmic mass creating the gravitational field

which is constant, weight or mass?

mass

what is mass?

• amount of matter making up an object

• a measure of an object’s inertia

why is mass constant?

because the amount of matter making up an object is the same (providing parts of the object are not removed or added) regardless of where the object is

why isn’t weight constant?

because it depends on the gravitational field strength, which varies depending on cosmic position

what is the gravitational field strength on the earth’s surface?

9.81 ms^-2

what is the gravitational field strength on the moon’s surface?

1.62 ms^-2

what is the weight of an 1kg object on the earth’s surface?

• w = mg

• = 1 × 9.81

• = 9.81 N

what is the weight of an 1kg object on the moon’s surface?

• w = mg

• = 1 × 1.62

• = 1.62 N

9.81 ms^-2

gravitational field strength on the earth’s surface

1.62 ms^-2

gravitational field strength on the moon’s surface

what is inertia?

an object’s resistance to change of motion

why is mass a measure of an object’s inertia?

because more force is needed to give an object a certain acceleration than to give an object with less mass the same acceleration

what is a demonstration of inertia?

a coin on a card on top an empty glass

here

how does a coin on a card on top an empty glass demonstrate inertia?

here

when the card is flicked, the coin drops into the glass because the force of friction on the coin due to the moving card is too small to shift it sideways

what unit of measurement is the scale on a top pan balance calibrated to?

grams (g) or kilograms (kg)