Transcript Forces

Forces
Distance, Speed
and Time
D
Speed = distance (in metres)
time (in seconds)
S
T
1) Dave walks 200 metres in 40 seconds. What is his speed?
2) Laura covers 2km in 1,000 seconds. What is her speed?
3) How long would it take to run 100 metres if you run at 10m/s?
4) Steve travels at 50m/s for 20s. How far does he go?
5) Susan drives her car at 85mph (about 40m/s). How long does it
take her to drive 20km?
Distance-time graphs
2) Horizontal line =
40
4) Diagonal line
downwards =
30
Distance
(metres)
20
10
0
Time/s
20
1) Diagonal line =
40
60
80
100
3) Steeper diagonal line =
40
Distance
(metres)
30
20
10
0
Time/s
20
40
60
80
100
1) What is the velocity during the first 20 seconds?
2) How far is the object from the start after 60 seconds?
3) What is the velocity during the last 40 seconds?
4) When was the object travelling the fastest?
Acceleration
S
Acceleration = change in speed (in m/s)
(in m/s2)
time taken (in s)
A
T
1) A cyclist accelerates from 0 to 10m/s in 5 seconds. What is her
acceleration?
2) A ball is dropped and accelerates downwards at a rate of 10m/s2
for 12 seconds. How much will the ball’s velocity increase by?
3) A car accelerates from 10 to 20m/s with an acceleration of 2m/s2.
How long did this take?
4) A rocket accelerates from 1,000m/s to 5,000m/s in 2 seconds.
What is its acceleration?
Velocity-time graphs
1) Upwards line =
80
Velocity
m/s
4) Downward line =
60
40
20
0
10
2) Horizontal line =
20
30
40
50
3) Steeper line =
T/s
80
60
Velocity
m/s
40
20
0
T/s
10
20
30
40
1) How fast was the object going after 10 seconds?
2) What is the acceleration from 20 to 30 seconds?
3) What was the deceleration from 30 to 50s?
4) How far did the object travel altogether?
50
Balanced and unbalanced forces
Consider a camel standing on a road.
What forces are acting on it?
Reaction
These two forces would be equal –
we say that they are BALANCED.
The camel doesn’t move anywhere.
Weight
Balanced and unbalanced forces
Reaction
What would happen if we took the
road away?
Weight
Balanced and unbalanced forces
What would happen if we took the
road away?
The camel’s weight is no longer
balanced by anything, so the camel
falls downwards…
Weight
Balanced and unbalanced forces
What would happen if we took the
road away?
The camel’s weight is no longer
balanced by anything, so the camel
falls downwards…
Balanced and unbalanced forces
1) This animal is either
stationary or moving with
constant speed…
2) This animal is getting
faster…
3) This animal is getting
slower….
4) This animal is also
either stationary or moving
with constant speed..
Force, mass and acceleration
1) A force of 1000N is applied to push
a mass of 500kg. How quickly does
it accelerate?
F
2) A force of 3000N acts on a car to
make it accelerate by 1.5m/s2. How
heavy is the car?
3) A car accelerates at a rate of
5m/s2. If it weighs 500kg how
much driving force is the engine
applying?
4) A force of 10N is applied by a boy
while lifting a 20kg mass. How
much does it accelerate by?
m
a
Terminal Velocity
Consider a skydiver:
1) At the start of his jump the air
resistance is _______ so he
_______ downwards.
2) As his speed increases his air
resistance will _______
3) Eventually the air resistance will be
big enough to _______ the
skydiver’s weight. At this point
the forces are balanced so his
speed becomes ________ - this is
called TERMINAL VELOCITY
Stopping a car…
Tiredness
Too many
drugs
Thinking
distance
Poor
visibility
Wet roads
Icy roads
Tyres/brakes
worn out
Too much
alcohol
Stopping
distance
Driving too
fast
Work done
When any object is moved around work will need to be
done on it to get it to move (obviously).
We can work out the amount of work done in moving an
object using the formula:
Work done = Force x distance moved
in J
in N
W
in m
F
D
Work done against frictional forces is transferred mainly as heat.
Force and acceleration
If the forces acting on an object
are unbalanced then the object
will accelerate, like this bear:
Force (in N) = Mass (in kg) x Acceleration (in m/s2)
F
M
A
Kinetic energy
Any object that moves will have kinetic energy.
The amount of kinetic energy an object has can be found
using the formula:
Kinetic energy = ½ x mass x velocity squared
in J
in kg
KE =
½
in m/s
mv2
Some example questions…
1) A 70kg boy is running at about 10m/s. What is his
kinetic energy?
2) A braking force of 1000N is applied by a driver to
stop his car. The car covered 50m before it stopped.
How much work did the brakes do?
3) What is the kinetic energy of a 100g tennis ball being
thrown at a speed of 5m/s?
4) A crane is lifting a 50kg load up into the air with a
constant speed. If the load is raised by 200m how
much work has the crane done? (The answer isn’t
10,000J)
Momentum
• Depends on mass
• And velocity
• Momentum = mass x velocity
mom = mv
• Is a measure of how difficult it is to change the
movement of an object.
Momentum
• Is conserved (kept the same) in all collisions.