Forces 2 - weight, force diagrams, work done Flashcards
Define weight
Weight is the force acting on an object due to gravity.
Why is the force of gravity so close to the earth.
The force of
gravity close to the Earth is due to the gravitational field around the
Earth.
What does the weight of an object depend on
The weight of an object depends on the gravitational field strength
at the point where the object is.
State the equation for weight
weight = mass x gravitational field strength
State the equation that links weight, mass and gravitational field strength
W = mg
weight - in newtons N
mass in kg
gravitational field strength - g in newtons per kilogram N/kg
What is an object’s centre of mass
An object’s centre of mass is the weight of an object (due to gravity) that can be considered to act at a single point
why is mass a scalar quantity
Mass is a scalar quantity because it has no direction (magnitude only)
Complete the sentence
The weight of an object is directly proportional to the ____ of the object
The weight of an object is directly proportional to the mass of the object
Weight ∝ Mass
(As mass doubles, weight also doubles)
How can we determine an objects weight
How can weight be measured
Weight can be measured using a calibrated spring-balance (a newton meter)
What is the gravitational field strength
The gravitational field strength is a measure of the force of gravity in a particular location
(the gravitational field strength depends on where you are)
Define mass
The mass of an object tells us how much matter the object has in it.
Why are forces vector quantities
Forces are vector quantities because they have both magnitude (size) and direction
Define resultant force
The resultant force is a single force that has the same effect as all of the original forces acting together
A man is applying a force of 20N acting to the right
A friction force of 10N is acting to the left
Work out the resultant force
Resultant force = 20 - 10 = 10N acting to the right
(DIRECTION IS NEEDED)
Label the forces acting on a car travelling at a constant speed
air resistance/friction<-car-> driving force
Weight acting downward (e.g. 3cm arrow)
2 1.5 cm arrows for the wheels - normal contact force
Arrow for air resistance/friction should = length of arrow for driving force
0 4 Hailstones are small balls of ice. Hailstones form in clouds and fall to the ground.
A hailstone falls from a cloud and accelerates.
Why does the hailstone accelerate?
there is a resultant force acting
(note:remember f=ma)
Forces that are balanced should have be of the same length
What happens, if the resultant force acting on an object is 0
If the resultant force acting on an object is zero, then all the forces are said to be balanced
The skydiver is falling through the air at a constant velocity
The skydiver experiences a force of 800N acting downwards (this is his weight due to gravity)
At the same time the skydiver experiences an upward force of 800N due to air resistance
Calculate the resultant force
Resultant force = 800 - 800 = 0N -
In this case the forces are balanced
Draw a free body diagram of:
The skydiver is falling through the air at a constant velocity
The skydiver experiences a force of 800N acting downwards
At the same time the skydiver experiences an upward force of 800N due to air resistance
Describe the forces acting on an aeroplane flying at a constant velocity at a constant altitude
The weight of the aeroplane is acting downwards towards the Earth (due to gravity)
However, the aeroplane is at a constant altitude, therefore there must be a force the same magnitude as the weight but acting in the opposite direction.
That force is called lift
The aeroplane experiences a forward force provided by the engines, that force is called force
At the same time, the aeroplane also experiences the force of air resistance acting in the opposite direction
Because the aeroplane is moving at a constant velocity, the forward and backwards forces must be balanced
Draw a free body diagram of an aeroplane flying at a constant velocity and constant altitude
Vector diagrams
Two forces are acting on an object
One force has a magnitude of 10N and the other force has a magnitude of 8N.
The angle between the two forces is 30 degrees.
Calculate the resultant force
1cm = 1N
Use a ruler to draw a 10cm long arrow to represent the 10N force
Using a protractor - measure 30 degrees
Use a ruler to draw an 8cm arrow to represent the 8N force
Now create a parallelogram
To do that copy the 8cm line and position at the head of the 10cm force vector
Do the same for the 10cm line
Then draw a line from the tails of the force vectors to the other side of the parallelogram
Measure the length of the vector and work out the resultant force that it represents
Answer vector = 17.5cm = resultant forc1ce = 17.5N
A force of 280N and a force of 320N are acting on an object
There is an angle of 20 degrees between the forces
Calculate the resultant force
1cm = 40N
Vector = 14.8cm = 592N
Resolving forces
A 100N force is acting at an angle of 35degrees with the horizontal
Determine the horizontal and vertical components of this force
Scale = 1cm = 10N
Draw faint lines to show the horizontal and vertical axes
Using a protractor, measure 35 degrees from the horizontal
Use a ruler to draw the vector showing the 100N force (10 cm)
Now draw dotted lines from the head of the vector to the horizontal and vertical axes
At this point draw horizontal and vertical components
Use a ruler to determine the magnitude
Rub out any faint lines used to show the vectors in the final answer
Horizontal component = 8.3cm = 83N
Vertical component = 5.7N = 57N
A 75N force is acting at an angle of 20 degrees with the horizontal
Determine the horizontal and vertical components of this force
Scale: 1cm = 10N
Horizontal component = 7cm = 70N
Vertical component = 2.5cm = 25N
https://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2021/november/AQA-84632H-QP-NOV21.PDF
question 7.5
look at diagram above q 7.4
Determine the magnitude and direction of the resultant force of the towbar on the
trailer by drawing a vector diagram
The resultant force of the towbar on the trailer has a horizontal component and a
vertical component.
horizontal force = 200 N
vertical force = 75 N
m
Magnitude of force = _______N
Direction of force = ____________N
https://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2021/november/AQA-84632H-MS-NOV21.PDF
PAGE 19
Define work
When a force causes an object to move through a distance, work is
done on the object.
What happens when work is done against frictional forces acting on an object
Work done against the frictional forces acting on an object causes a
rise in the temperature of the object.
Describe the energy transfer involved when work is done when a man is pushing a box
The chemical energy store in the man’s muscles has been transferred to the thermal energy store of the box
(the man is applying a forward force to the box, the force of friction is acting between the bottom of the box and the floor. The friction cause the temperature of the box to increase)
What is transferred when force is used to move an object
Whenever a force is used to move an object, energy is transferred.
equation that links work done, force and distance
W = F x s
Work done = Force x Distance
Work done - Joules - J
Force - Newtons
Displacement - METRES
distance must be in the line of action of the force
(e.g. a person walking up stairs - because weight is acting downwards - only vertical distance is relevant
1 joule = ___
1 joule = 1 newton metre
When is one joule of work done
One joule of work is done when a force of one newton causes a
displacement of one metre.
A car is travelling with a velocity of 20m/s
The driver applies the brakes and the car comes to a stop
What happens when the car brakes?
During braking, the brake presses against the wheel
The force of friction now acts between the brake and the wheel
The kinetic energy store of the car is transferred to the thermal energy store of the brakes.
This causes the temperature of the brakes to increase and at the same time, the car slows down as it loses kinetic energy
What type of energy store does a moving object have
A moving object has a kinetic energy store
