Definition of force
Force is defined as anything that changes the state of rest or motion of an object moving in a straight line.
Force is a vector quantity which has both direction and magnitude
The S.I. unit for force is N or kg m s-2
Force is measured by using a Spring Balance
The effects of a force
A force can
- move a stationary object
- stop a moving object
- accelerate a moving object
- decelerate a moving object
- change the direction of a moving object
- alters the size of the object
- alters the shape of the object
NEWTON ‘S LAWS OF MOTION
Newton’s First Law : An object will remain at rest or continue with a constant speed in a straight line unless acted on by an unbalanced force.
Newton’s Second Law : The acceleration of a body ,a, is directly proportional to the net force acting on it, F , and inversely proportional to its mass, m.
Newton’s Third Law : For every action there is an equal and opposite reaction.
The relationship between the acceleration,a ,the net force ,F and the mass , m.
Based on the Newton’s Second Law of motion,
a a F
a a 1
we obtain a a F
a = k F
F = kma
In S.I. units , the definition of a force of 1 N is given as the amount of force which is applied on a mass of 1 kg and which causes it to experience an acceleration of 1 ms–2
1 N = k ( 1 kg ) ( 1 ms-2 )
Thus k = 1
Therefore F = ma
Where F = net force
m = mass
a = acceleration
Two forces act horizontally to a block of wood of mass 4 kg as shown in figure above. What is the acceleration and the direction of motion of the block.
An object of mass 2 kg is pulled on the floor by a force of 5 N and having a constant velocity.
(a) What is the frictional force between the object and the floor.
(b) Calculate the acceleration of the object if the object is pulled by a 17 N force?
A bus of mass 2 000 kg travels with a uniform velocity
40 ms-1 for a distance 2500 m before it comes to rest. Calculate
(a) the average deceleration of the bus
(b) the average force applied by the brakes to bring the bus to a standstill
When traveling at 120 km h-1 a car has to overcome a drag force due to air resistance of 900 N. If the car has a mass of 1000 kg, determine the average force that the motor needs to apply if it is to accelerate at 5.0 ms-2.
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