Post on 26-May-2015
PREPARED BY:
PHYSICS DEPARTMENT
SBPI TEMERLOH
What is a force?
Example of activities
that require force?
2.5 Understanding the effects of force
A student should be able to :
• State the effects of balanced force
acting on an object.
• State the effects of unbalanced force
acting on an object.
• Determine the relationship between
force,mass and acceleration. F=ma
Learning Outcomes
The action of pushing or pulling is a force.
It is a vector quantity
Its SI unit is Newton (N) or kg m sˉ²
A force is not visible but the effect of a force
can be observed
A bird perching on a branch of a treeR (Reaction of the branch)
W (Weight of the bird)
Situation 1
A bird perching on a branch of a treeR (Reaction of the branch)
W (Weight of the bird)
The forces of gravity, W pulls it
downward
and the reaction of the branch, R
pushed
the bird upward to balance the weight.
Total force acting on the bird is
zero and the bird is at rest.
A cyclist use a bigger force to pedal his bicycle and increased its speed.
Situation 2
A cyclist use a bigger force to pedal his bicycle
and increased its speed
F
The forces to the pedal is greater
than the frictional forces acting
on
the bicycle.
The resultant force, F causes the
bicycle to accelerate forward.
Push the stationary toy car with the hand
A toy car collides with a wall
Situation 3
Toy car starts to move Toy car stop the moving
Push the stationary toy car with the hand
The toy car start to move
The forces can moves a stationary object.
A toy car collides with a wall
The toy car is stop by the wall
The forces can stops the motion of an object.
Situation 3
A golf buggy moving in a golf course
T (Engine Force)
R (Reaction of the ground)
F (Frictional Force)
W (Weight of the golf buggy)
A golf buggy moving in a golf course
T (Engine Force)
R (Reaction of the ground)
F (Frictional Force)
W (Weight of the golf buggy)
T – drives the buggy forward T has the same magnitude with F (included air resistance), but acts in the opposite direction. The net horizontal force acting on the buggy is 0
W is balanced by the R on the buggy The net vertical force on the golf buggy is 0
So the buggy moves with a constant velocity
Lorry moves from smooth surface to
a rough surface
Move to
smooth surface rough surface
Situation 5
Lorry moves from smooth surface to
a rough surface
Move to
smooth surface rough surface
When a lorry moves from a smooth surface to a rough surface,
the forces of friction retards the motion of the lorry and slow down
a lorry moving (decelerate)
A tennis ball hit by the racket
Situation 6
A tennis ball hit by the racket
When a tennis ball is hit
by a racket, the
reactive
force causes the tennis
all to change its
direction.
Bending a straight ruler
Change to
Straight ruler
Bend ruler
A straight ruler forms
an arc when bending
forces are applied to
both ends. The forces
can change the
shape of an object.
BALANCE FORCES
An object may have several forces acting on it
But if the forces are in balance, they cancel each other
out (no net forces)
Then, the object behaves as if no force is applied to it.
With balance forces on it, an object is either at rest or
moving at a constant velocity. (Newton’s first law of
motion).
Example of balance forces
A book resting on a hard surface. The weight, W book is acting
vertically downwards. At the same time, an equal and opposite
force (normal reaction, R) from the surface acted on the books.
The net force acting on the book is zero. The book remain at rest
A car moving at constant velocity along a straight road. The engine provides a forward driving force, T. The wind and frictional force, F. The net force acting on the car is zero. The car travels with its original constant velocity
Example of balance forces:
Example of balance forces:
An airplane is flying horizontally at a constant height with a constant velocity. The engine provides a forward thrust,T. The wind and air resistance provides a drag, F against the forwards motion. The wings of the plane provide a lift, L vertically to balance its downward weight, W. When these four forces are balanced, the net force acting on the plane is zero. This means that the lift force is equal to its weight, and the forward thrust is equal to the drag.
UNBALANCED FORCE
• When two or more forces acting on a body are not balance, there must be a net force acting on it.
• This net force is known as the unbalanced force or the resultant force.
• An object will accelerate if the forces acting on it are not balance.• Unbalanced forces acting on an object can cause the object:
(i) to start or stop movingwhen a golfer hits a stationary golf ball, a force acts
on the ball and causes it to fly off from rest. The net force or the unbalance force causes the golf ball to
accelerate.
(ii) to accelerate or decelerate
-when the engine of a moving car is shut down, the car will slow down and finally stop. The net force due to friction is acting against the direction of the motion of the car. It causes the car to decelerate
(iii) to change its direction
-when a footballer kicks a fast moving ball towards him, the ball bounces off and moves in other direction. The unbalanced force causes the ball to change its direction.
• The motion of a body due to an unbalanced force is closely related to Newton’s second law of motion
• the acceleration of moving body is related to the net force applied on it and the mass of the body
Discussion group.
Two cars with different direction move with zero acceleration. Suddenly, at the arc of highway, this two car hit together. In your group, discuss what effects of balanced and unbalanced forces occur before, during and after the collision?
Balanced force Unbalanced force
1. The cars move with constant velocity.
1. Change of state. ( from move to stop)2. Change of shape. ( in front of the car
damaged)3. Change of velocity
(decelerate)
Initially, two cars with different direction move with zero acceleration.
Suddenly, at the arc of highway, this two car hit together. In your group, discuss what effects of balanced and
unbalancedforces occur before, during and after the collision?
RELATIONSHIP BETWEEN FORCE, MASS AND ACCELERATION
(a) (b) (c)
(a) When a net force, F acts on a mass, m it causes an accelerate, a
(b)When the force, F on the same mass is doubled, its acceleration also doubled, 2a
(c) The same force applied to twice of the mass results is only half of the acceleration, ½ a
F
m
2F
m
F
m2m
• Newton’s second law of motion statesthe acceleration produced by a net force on an object is directly proportional to the magnitude of the net force applied and is inversely proportional to the mass of the object. The direction of the acceleration is the same as that of the net force
• Combining the relation ship, a ∞ F and a ∞ 1 , we get
(i) The graph of a against F and
(ii) The graph of a against m
Example:
• A wooden block of mass 3 kg is pulled along a table with a constant velocity by a force of 6 N as in Figure 1(a). If the pulling force is increased to 15 N as in Figure (b), what is
(a) the resultant force(b) the acceleration
Solution:
(a) F = (15 - 6)N = 9N
(b) Acceleration, a = F ÷ m= 9 N ÷ 3 kg= 3 N kg ¹ @ m s ²ˉ ˉ
6N
(a)
15N
(b)
Its time to do exercise!
Exercise 2.5 the effect of force