Forces

FORCESFORCESSec 4Sec 4

http://www.uq.edu.au/_School_Science_Lessons/PushPull0.GIF

BTEC, you should be able to:BTEC, you should be able to:

Describe the effects of a forceRecall the relationship resultant force =

mass x acceleration (F = m x a)Solve related problems with above

relationshipsUnderstand mass, weight and accelerationUnderstand weight and able to differentiate

weight and massRecall the relationship weight = mass x

gravity Understand Inertia

ForceForce

• Think of force as a PUSH or PULL

• The block was pushed/pulled by tractor• Both blocks experience a force to the right.• Force felt by block is provided by tractor

PUSH PULL

Types of ForcesTypes of Forces• Magnetic force

– force due to magnetic attraction and repulsion

• Weight– force acting on a body due to gravitational pull

• Tension– force at the ends of a stretched string, spring or

rope

• Contact force– force that appears only when there is contact

between two bodies

• Friction– force due to roughness of surfaces

ForceForce• When a force acts on an object, it tends to

cause the object to accelerate.

• Other effects of force are:– Decelerate an object– Change the direction of motion of a body– Change the shape and size of a body

Effect of force Effect of force • Make a stationary object move

Effect of force Effect of force – Change the speed of a body

Effect of force Effect of force – Change the direction of motion of a

body without its speed

Effect of force Effect of force • Change the shape and size of a body

Understanding Net ForceUnderstanding Net Force

• What happens if an object is pushed by two set of forces as shown above?

• The result is that the object experiences a SUM of the forces, which is 10 + 5 = 15 N.

• In this case, we say that the net force is 15 N.

10 N

5 N

=15 N

Understanding Net ForceUnderstanding Net Force

• What happens if an object is pushed by two set of opposite forces as shown above?

• The result is that the object experiences a SUM of the forces, which is 10 - 5 = 5 N.

• In this case, we say that the net force is 5 N.

10 N

5 N

=5 N

Net forceNet force• What is the net force acting on objects below?

A2N 8N A 6N

B2N

10NB 12N

C8N 8N CNo net force

D3N

5ND2N

10N

Resolving Forces in 2DResolving Forces in 2D

ForceForce• Force can be calculated with this

simple equation:

amF Net Force Mass Acceleration (N) (kg) (m/s2)

SI Unit: newton ; symbol : N

Quick CheckQuick Check• A car of mass 1500 kg accelerates at

2 m/s2. Calculate the forward force provided by the engine.

F = m x a = 1500 x 2 = 3000 N

Quick CheckQuick Check• A 1800 kg van is reversing. Backward

force provided by the engine is 2400 N. Calculate the backward acceleration of the van.

F = m x a 2400 = 1800 x a a = 2400 / 1800 = 1.33 m/s2

Quick CheckQuick Check• A boat of mass 500kg is moving against the flow of a

river. Current of river produces a force of 1000 N. Engine of the boat produces a forward force of 3000 N. What is the net force acting on the boat?

Net force = 3000 – 1000 = 2000 N

• What is the acceleration of the boat? F = m x a 2000 = 500 x a a = 2000 / 500 = 4 m/s2

3000 N

1000 N

Quick CheckQuick Check• A small rocket with mass 800

kg has a weight of 8000 N. At take-off, the rocket engine produces an upward force of 10 000 N.

• What is the net upward force acting on the rocket during take-off?Net force, F = 10,000 – 8000

= 2,000 N 8000 N

10,000 N

Quick CheckQuick Check

• What is the acceleration of the rocket?F = m x a a = F/m = 2000 N / 800 kg

= 2.5 m/s2

8000 N

10,000 N

Mass and WeightMass and Weight• In the study of mass and weight, you first

need to forget the day-to-day understanding of weight.

• For example, “the weight of a pupil is 50 kg” is WRONG! FORGET THIS, at least temporarily.

• Mass of an object can be understood as the amount of atoms that makes up the object.*

* This is not strictly correct, but as a beginner it is useful to think of it this way

MassMass• Mass of a handphone is always 0.2 kg,

in Singapore, or US or Brazil or even on the Moon or Mars.

• A 1.5 liter PET bottle of Coke will always have the same mass even if the drink is poured out into a container. (shape of liquid has changed but not the mass)

MassMass• In Physics is defined as (this means that if

asked in exams, please write this statement down)

A measure of the amount of matter in a body

• Symbol: m• SI Unit: kilogram (kg)

• Mass of an object cannot be changed by its:– Shape– Location

EraserEraser

WeightWeight• Weight of an object is the effect of gravity

on it.• For instance, a box has a mass of 1 kg.

(this means that the box has a great number of atoms worth 1 kg)

• For a 1kg box on Earth, it weighs 10 N. However if the box is on the Moon, where the gravity is only 1/6 of Earth, the box weighs only 10/6 N.

WeightWeight• In other words, you might not be able

to lift a 60kg dumb-bell on Earth but on the Moon, you possibly can because it is now appears lighter.

• On Earth, the dumb-bell weighs 600 N while on the Moon is only 100 N.

WeightWeight• Weight of an object can be measured

by simply multiplying the mass of a object with the gravity of its location.

gmW Weight = Mass x Gravitational of object of object strength (N) (kg) (N/kg)

Quick checkQuick check• A book has a mass of 1.5 kg. What is its

weight on Earth?W = m x g = 1.5 x 10

= 15 N• If gravitational acceleration of Moon is 1/6 of

Earth’s, how much is the book’s weight on the Moon.

W = m x g = 1.5 x (1/6 x 10) = 2.5 N

Ask yourself, what is the mass of the book on the Moon? Has it

changed? Can it change?

Quick checkQuick check• What is the mass of a boy who

weighs 450N on the surface of the Earth?

W = m x g m = W/g = 450/10 = 45 kg

FrictionFriction

• A force that opposes motion

• Block is pulled to the right by force F• So, block tends to move to the right• But friction acts on the block towards

the left

Force, FFriction

FrictionFriction

• Consider that the friction is constant. This means for two particular surfaces in contact, the friction force between is fixed. For other surfaces, the value of frictional force could be different. *

* not strictly true, but for N-levels, consider it so

Force, FFriction

FrictionFriction

• When force applied is more than friction, the box will start moving. In fact it will accelerate.

• If the force applied is removed, the box will stop moving due to friction, since it opposes motion.

• However if the force is reduced such that it is equal to friction, the result is that the box will move at constant speed.

Force, FFriction

Friction – What’s Good about Friction – What’s Good about it?it?

• Allows us to walk• Allows a car to move• Allows us to hold a chopstick

Friction – What’s bad about it?Friction – What’s bad about it?• Causes wear and tear – brake pads of

vehicles, soles of shoes• Reduces efficiency of performance –

traveling

Solutions to overcome Solutions to overcome frictionfriction

• Moving on air cushion or electromagnetic fields

• Use rollers or wheels• Lubricate surface• Smoothen surface• Using materials with very low frictional

resistance

Quick checkQuick check• A 5kg box is pushed with a forward force of

10N on a rough surface with friction of 4N, what will be the acceleration of the box?

Net force on box = 10 – 4 = 6N F = m x a a = F/m

= 6/5 = 1.2 m/s2

5 kg 10 N4 N

Quick checkQuick check• Tom pushes a crate of mass 20kg

across a floor at constant velocity of 0.2m/s by exerting a horizontal force of 50N.

• What is the resultant force acting on crate?Since velocity is constant, there is no acceleration, so (by Newton’s 2nd Law)

resultant force is 0.

20 kg 50 NFriction? N

0.2m/s

Quick checkQuick check

• What is the frictional force acting on crate?

Since resultant force = 0 N, frictional force must also be 50 N, but in opposite direction.

20 kg 50 NFriction? N

0.2m/s

Quick checkQuick check

• Tom now increases his force to 80N.• What is the new resultant force on

crate?Frictional force remains at 50 N.Resultant force = 80 – 50 = 30 N

20 kg 80 NFriction? N

Quick checkQuick check

• What is the acceleration of the case?F = m x a a = F/m

= 30 / 20 = 1.5 m/s2

20 kg 80 NFriction? N