Transcript Force, Work and Power

Force, Work and Power
Chapter 41
What is a force?
• A force is anything that causes an object to
change its velocity.
• Force is measured in Newton’s(N)
Examples of Forces and their
effects
Push
Pull
Weight
Friction
Electric
Magnetic
Changes the velocity of an Object
Changes the velocity of an Object
Moves objects towards the ground
Slows down moving objects
Causes charged particles to move
Causes magnetic materials to move
Force can be measured using a spring balance.
Forces always occur in Pairs.
For every force there is an equal and opposite force.
Forces always occur in Pairs.
For every force there is an equal
and opposite force
The Effects of force on a
spring(Hook’s Law)
• The extension of a spiral spring is directly
proportional to the force stretching it.
• It is found that by adding equal weights to
the spring will extend it in equal amounts.
Hook’s Law
Friction
• Friction is the force which prevents easy
movement between two objects in contact.
High friction- sandpaper, car
tyres, rough soled shoes.
Low Friction- wet soap, oil
glass.
To reduce friction we use a
lubricant.
Work
• Work is done when a force moves a body.
Work = Force(N) x Distance(m)
Work and Energy are measured in Joules.
Examples:
Find the work done when a force of 6N
moves a glass a distance of 0.5m across a
counter?
Answer
Work done = Force x Distance
= 6N
x 0.5m
= 3 Nm
= 3 Joules
Example
Find the work done by a weightlifter who has
lifted 90kg from the floor to a height of 1.6m.
Answer
Mass lifted = 90kg
--> WEIGHT LIFTED = 90 kg X 10 = 900 N
Work done = Force x Distance moved
= 900 N x 1.6m
=
1440 N m
=
1440 J
Power
Power is the rate at which work is done.
Power is measured in watts(W) where 1W
= 1J/s
• Question:
• What is the work done by a shopper who uses a
force of 120 N to push a trolly a distance of 15 m?
• If the shopper takes 9s to carry out this work, what
power has been used?
Answer
What is the work done by a shopper who uses a force of 120
N to push a trolly a distance of 15 m?
Work done
= Force x Distance
= 120N x 15m
=
1800 J
If the shopper takes 9s to carry out this work, what power has
been used?
Power
= Work done
Time taken
=
1800J
9s
=
200 W
Textbook Chapter40 Force,Work and Power
A1.
A2.
A3.
A4.
A5.
A6.
A7.
A8.
A9.
A10.
A11.
A12.
A13.
A14.
Push, pull, newtons.
Direction.
Gravity.
Electric.
Spring balance.
Friction, newtons.
Lubricants, friction.
Force, object.
Energy.
Force, newtons, distance, metres, joule.
1200J.
600 joules.
Work.
Rate, work, watts
B1.(a) Anything that causes an object to move or change its
velocity; newtons.
(b)
Spring balance.
(c)
Bullet from a rifle
Two people on roller skates
Balloon
jet
(d) 8N
B2.(a) A force that prevents easy movement between two
objects in contact.
High friction: wheels of a car; brakes of a car; thread of shoes
,sand paper. Low friction: wet surfaces; wet soap; ice; gears on
a bicycle.
(b)See Experiment 41.3,p.277.
(c)Helpful: shoes on the ground; wheels on the road; brakes on a
car; sandpaper. Not helpful: hinges on a door; when skiing;
pushing large weights; inside a car engine.(d)Lubrication.
B3.
(a)
When a force moves an object; joules.
(b)
Energy.
(c)
The rate at which work is done; watts.
(d)
(i)1500J;
(ii)150W.
(e)
(i)1000J;
(ii)50W.
(f)
Chemical energy in food.