Download Forces - Deans Community High School

Movement
Forces
Forces are all around us; they affect our surroundings and us. We
cannot see forces but we can feel them or see their effects.
Forces can:
Change the speed of an object.
Change the shape of an object
Change the direction of a moving object.
Measuring forces
The most common method of measuring forces in
the laboratory is to use a Newton balance. This
device makes use of a spring in a casing. the
greater the force on the spring the greater the
extension of the spring.
The force caused by the pull of the Earth’s gravity
is given a special name. This force is called
WEIGHT.
It is a word that is commonly used, but is not normally used in its
correct physics definition. If you buy a 1kg bag
of sugar it does not have a weight of 1kg, the
correct term is that it has a MASS of 1kg. To
find the weight you need to multiply the mass by
ten.
WEIGHT = MASS (Kg) x 10
Weight is measured in the unit the newton (N),
named after Sir Isaac Newton, a famous
scientist.
Friction
Friction is a force that tries to oppose motion. This means it tries to
stop stationary objects from starting to move or it causes moving
objects to slow down or stop.
Sometimes we want to increase friction other times we want to
decrease it, it just depends on the situation.
When we want to increase friction.
Grip for tyres when
driving in snow. Grit
put down on roads.
Friction between the
brake pads and wheel
on a bicycle.
When we want to decrease friction.
Lubricating an engine to make it run
more freely.
Oiling the hinges of a
door to make it open
more easily.
One type of friction that affects many vehicles is air friction. A
process called streamlining can reduce this.
Streamlined
Smooth curves
Pointed shape in direction it
moves.
NOT Streamlined
Boxy shape
Blunt shape in direction it
moves.
Lift force from
athlete
Balanced Forces
If equal forces are acting in
opposite directions they are
called BALANCED FORCES.
In the example opposite the
forces are balanced because the
force from the athlete acting up
is the same as the weight force
acting down.
Weight
air resistance
For the parachutist, the air resistance
force is equal to the weight but in the
opposite direction.
weight
Speed and acceleration
You may remember from the sound and music topic how to
measure an average speed. To measure any speed you must
measure a distance and measure the time it takes to cover that
distance. The average speed can then be calculated using the
equation
Average speed =
distance
time
Measured
distance
In the example above the students time the car travelling between
the two lamposts. They are then able to check if the car is breaking
the local speed limit.
If the time to be measured is very small [less than 1 second] it is
difficult to do this accurately using a stopwatch. In these cases
some type of electronic measurement will be used.
Speeds measured when the time is small are called instantaneous
speeds.
The police measure
instantaneous speeds when
using radar speed guns.
Acceleration is a measure of how quickly an object changes its
speed. This information can often be found in magazines which
give performance figures for cars. The accelerations can be
compared by looking at the time it takes the car to go from 0 to 60
mph.
The table below shows these time for four different cars.
Car
Time for 0-60 mph
Aston Martin DB7
5.7 s
Ford Ka 1.3
14.7 s
Vauxhall Vectra 1.8
10.2s
Mercedes Benz SLK 200
8.2s
The information in the table shows us that the Aston Martin DB7
has the greatest acceleration. This is because the Aston Martin
takes the shortest time to change its speed from 0-60 mph.
Moving bodies
One unfortunate consequence of having cars and lorries on the
roads is that occasionally they crash. If everyone involved is very
lucky then there will be no injuries.
In this part of the book we will look at the effect a crash will have
on a wall.
There are three situations
1. A car crashes into a wall at 20 mph
2. The same car crashes into a wall at 50 mph
3. A bus crashes into a wall at 20 mph
Before you turn over the page think about which situation will
cause most damage, and which one will cause least damage.
Crash situations
1.
20 mph
2.
50 mph
3.
20 mph
You can see that: (a) crash 1 causes less damage than crash 2
(b)crash 1 causes less damage than crash 3.
Reason: (a) The car in crash 2 has the same mass but is travelling
faster.
(b) The bus in crash 3 is travelling at the same speed but
has a larger mass.
The effect of a collision increases as the mass and speed increase.
When an object changes its speed there has to be a force acting on
it. The change in speed depends on:
(a) The size of the force acting
(b) The time the force acts.
(a) The harder
[more force]
the golfer hits
the ball the
faster it will
travel off the
tee.
(b) The longer
[more time]
the wind
blows the
faster the
windsurfer
travels.
When someone or something throws a ball, the distance the ball
travels is called the RANGE.
The range depends on how fast the ball is thrown and what angle it
is thrown at.
For any fixed angle of
throw, the range increases
as the speed increases.
For a fixed speed of throw
the range increases as the
angle increases- up to 45°
then reduces as the angle increases.
Dropping ball
When a ball lands after being dropped it will generally bounce
back upwards. For any ball, the height that is bounces back to
depends on two factors.
(a) The speed it hits the ground
(b) The surface it lands on.
Fast
Slow
rebound
height
If an experiment is
conducted using
identical balls landing
on the same surface,
then the faster the ball
hits the ground the
higher it will rebound.
When the experiment is
conducted using identical balls
landing on the different surfaces
at the same speed the result is
different. Generally the harder
the landing surface the higher
the rebound height.
rebound
height
Hard
Soft
Movement Learning Outcomes.
You should know or be able to do the following
Forces
 Describe how you would use a Newton balance to measure a
force.
 The Earths pull on an object causes a force acting downwards
 The force caused by the Earth’s pull is called WEIGHT.
 Weight can be calculated using the equation
Weight = 10 x mass
 Friction is a force that opposes motion
 It will try to stop a stationary object from starting to move
 It will cause a moving object to slow down
 Friction can be increased by
(a) Making contact surfaces rougher [gritting icy roads]
(b) Adding grips to surfaces [studs on sports shoes]
 Friction can be decreased by
(a) Lubrication [oiling moving parts]
(b) Smoothing surfaces [sanding drawer runners]
(c) Using an air cushion [hovercraft]
(d) Streamlining [shape of performance cars]
 Streamlining is done to reduce the effect of air friction.
 Objects which have been streamlined have
(a) Had rough surfaces smoothed
(b) Been made into a narrow shape
(c) Had corners rounded
 When the forces acting on an object are equal and act in
opposite directions they are called Balanced Forces.
Speed and acceleration
 Describe how you would measure an average speed.
(a) measure distance travelled
(b) measure time to cover distance
 Calculate average speed using the equation
distance
Average speed =
time
 Describe how to measure instantaneous speed.
(a) instantaneous speed measured over a very short time
(b) some electronic timing device needed [light gates]
 Know that acceleration is when an object changes its speed.
Moving bodies
 Know that the effect of a collision increases as mass and speed
increase.
 Know that the change in speed of an object increases as
(a) the force acting on it increases
(b) the time the force is acting increases
 Know that the range of a ball depends on
(a) the speed the ball is thrown
(b) the angle at which the ball is thrown
 Know that when a ball bounces its rebound height depends on
(a) the speed the ball hits the ground
(b) the type of surface the ball is bouncing on
Questions
1. What is the name given to the force that the Earth exerts on an
object?
2. A student has a mass of 55 kg. What is the size of the force that
the Earth exerts on them?
3. A toy car is pushed along the floor.
(a) Describe the motion of the car when it is
released.
(b) Explain your answer to (a).
4. Give an example of a situation where you would want to
increase friction.
5. Give an example of a situation where you would want to
decrease friction.
6. (a)Describe what you would do to make a boat go through the
water more easily.
(b) What is this process called?
7.
4N
4N
The diagram shows forces acting on
a block.
What name is given to forces
producing this situation?
8. Mr. Letham wants to calculate the average speed of the players
in the football team when they run from touchline to touchline.
Describe how he would do this, you should include a desciption of
measurements and the method of calculatying the speed.
9. Paula Radcliffe won the 10 000 m at the European
championships in August 2002 in a time of 30 min 01 sec.
Calculate her average speed for the race in metres per second.
10. The Wright brothers were the first people to fly an aeroplane.
Their first flight in 1903 lasted only 12 seconds and covered
just 36 metres.
Calculate the average speed of the plane during that first journey.
11. Describe how you could measure the instantaneous speed of a
naughty student as they run past the classroom door. You should
include a desciption of measurements and the method of
calculatying the speed.
12. The table below shows the performance of different cars.
Car
Time for 0-60 mph Top speed (mph)
W
5.0s
145
X
8.0 s
120
Y
7.0 s
118
Z
6.0 s
153
(a) Which car has the greatest acceleration?
(b) Explain your answer to (a).
(c) Which car can travel fastest?
13.A car pushing competition is set up at a charity event. The aim
of the event is to make the car travel as fast as possible by pushing
it.
Teams can have up to four members.
Give two ways in which the speed of the car could be made
greater.
14. What factors do you have to take into account if you want to
throw a ball to a friend who is some distance away from you?
15. John drops his football out of the window. What factors will
affect the rebound height of the ball after it hits the ground?