Presentation on theme: "Forces A force is when an object is pushed, pulled, stretched or squashed. Forces can change the: (a) speed of an object (b) shape of an object (c) direction."— Presentation transcript:
1ForcesA force is when an object is pushed, pulled, stretched or squashed.Forces can change the:(a) speed of an object(b) shape of an object(c) direction an object is travelling.A force can cause an object to accelerate, decelerate or remain at a steady speed.
2Action Force (N) Measuring Force Force is measured in units called Newtons (N).A Newton Balance is the instrument used to measure force.ActionForce (N)lifting a 0.5 kg masslifting a schoolbagopening a door
3The Newton Balance When a force is applied to a Newton Balance: the spring stretches becoming longerthe pointer indicates on the scale how big the force is.The bigger the force, the more the spring stretches.
4Mass and WeightMass is the amount of matter (atoms and molecules) that an object has.Mass is measured in kilograms (kg).Weight is the gravitational force that the Earth exerts on an object.Weight is measured in Newtons (N).
5Mass (kg)Weight (N)0.51.01.52.02.551015202510On Earth we found that:
6Gravitational Field Strength The weight of an object is given by:QuantityUnitWeight ( W )Mass ( m )Gravitational Field Strength( g )Newtons ( N )Kilograms ( kg )Newtons per Kilogram( N kg-1 )x÷WmgThe gravitational field strength is the force acting on each kilogram of mass.On earth, the gravitational field strength (g) is 10 N kg-1.
7My Mass and WeightMeasure your mass in kilograms (kg) by standing on bathroom scales.scalespersonResultMy mass = kgNow calculate your weight using the equation:
8Example 1A persons mass is measured to be 78kg. Calculate the weight of the person on Earth.
9Gravitational Field Strength The gravitational field strength is the force acting on each kilogram of mass.The value of g depends on where you are in the universe.LocationGravitational Field Strength (N kg-1)EarthMoonJupiterMarsDeep Space101.6264
10Example 1Calculate the weight a 23 kg package(a) on the Moon(b) on Mars(c) in deep space(a)
12Friction Friction is a force which acts against moving objects. Air friction is called air resistance.air resistancekicking forceAs the speed of an object increases, the force of friction opposing it increases.
13Decreasing FrictionFriction can be reduced by:oiling moving parts (engine).waxing the surface in contact with the ground (ski’s).streamlining to lower air resistance (cyclist).Increasing FrictionFriction can be increased by:applying brakes.increasing the tread of a tyre.using a parachute to increase air resistance.
14Balanced ForcesBalanced forces are when forces are equal and opposite.They look like this:3 kgtension in string = 30 N12 N12 Nweight = 3 x 10 = 30 N
15Newton’s First Law Newton’s first law states: “ an object will remain at rest or continue in the same direction at the same speed, unless acted upon by an unbalanced force.”This means if a car is travelling at a steady speed of 10 ms-1, the engine force equals the force of friction.force of friction: 1000 Nengine force: 1000 N
16SpaceshipA rocket in space can switch off its motors, and it will travel at a constant speed.This is because no forces are acting on it.(no weight or resistance)Maximum SpeedWhen a car reaches its maximum speed, the force of friction equals the maximum force of the engine.It can therefore go no faster.
17Unbalanced ForceAn unbalanced force is a single force that can replace all other forces.The unbalanced force is sometimes called the resultant force.Example 1State the resultant force for the following:3 kg4 N3 N5 N3 kg12 N4 Nunbalanced force:8 N to the rightunbalanced force:6 N to the right
18Seat BeltsA seat belt in a car provides a backwards force to prevent the passenger from moving forward if the car stops suddenly.If there was no such backward force present the passenger would continue moving forward at a steady speed (from Newton’s first law).The seatbelt provides an unbalanced force to decelerate the driver (from Newton’s second law)...\Seatbelt.mpg
19Yellow BookResultant Force – Page 66Q46, 47, 48, 49
20Newton’s Second Law F m a Newton’s second law states: “ an object acted upon by a constant unbalanced force, moves with constant acceleration in the direction of the unbalanced force.”This law relates the unbalanced force, mass and acceleration.x÷Fma
21Metres per second per second ( ms-2 ) QuantityUnitUnbalanced Force ( F )Mass ( m )Acceleration ( a )Newtons ( N )Kilograms ( kg )Metres per second per second ( ms-2 )Example 1Calculate the acceleration of the following 3 kg mass.2 N3 kg14 N
22The frictional force is 800 N. Calculate the mass of the car. Example 2The engine of a car exerts a force of 1,700 N, causing the car to accelerate at 1.5 ms-2.The frictional force is 800 N.Calculate the mass of the car.800 N1,700 N
23Newton’s Second Law – Pages 66 - 69 Yellow BookNewton’s Second Law – PagesQ50, 51, 55 (a) (b), 57, 61, 63*