Learning OutcomesA student should be able to :• State the effects of balanced force actingon an object.• State the effects of unbalanced forceacting on an object.•Determine the relationship betweenforce,mass and acceleration. F=ma
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
Situation 1A bird perching on a branch of a tree R (Reaction of the branch) W (Weight of the bird)
A bird perching on a branch of a treeR (Reaction of the branch) 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. W (Weight of the bird)
Situation 2A cyclist use a bigger force to pedal his bicycle and increased its speed.
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.
Situation 3 Push the stationary toy car A toy car collides with a wall with the hand
Situation 3 Toy car starts to move Toy car stop the moving Push the stationary toy car A toy car collides with a wall with the hand The toy car is stop by the wall The toy car start to move The forces can stops the motion The forces can moves a of an object. stationary object.
A golf buggy moving in a golf course R (Reaction of the ground)T (Engine Force) F (Frictional Force) W (Weight of the golf buggy)
A golf buggy moving in a golf course R (Reaction of the ground)T (Engine Force) 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
Situation 5 Lorry moves from smooth surface to a rough surfacesmooth surface rough surface Move to
Lorry moves from smooth surface to a rough surface smooth surface rough surface Move to 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)
Bending a straight rulerStraight ruler A straight ruler forms an arc when bending forces are applied to Change to both ends. The forces can change the shape of an object. Bend ruler
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 forcesA book resting on a hard surface. The weight, W book is actingvertically downwards. At the same time, an equal and oppositeforce (normal reaction, R) from the surface acted on the books.The net force acting on the book is zero. The book remain at rest
Example of balance forces: 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:An airplane is flying horizontally at a constant height with aconstant velocity. The engine provides a forward thrust,T. Thewind and air resistance provides a drag, F against the forwardsmotion. The wings of the plane provide a lift, L vertically tobalance its downward weight, W. When these four forces arebalanced, the net force acting on the plane is zero. This meansthat the lift force is equal to its weight, and the forward thrust isequal 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 moving when 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 withzero acceleration. Suddenly, at the arc ofhighway, this two car hit together. In yourgroup, discuss what effects of balanced andunbalanced forces occur before, during andafter the collision?
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 unbalanced forces occur before, during and after the collision? Balanced force Unbalanced force 1. The cars move 1. Change of state. with constant ( from move to stop) velocity. 2. Change of shape. ( in front of the car damaged) 3. Change of velocity (decelerate)
RELATIONSHIP BETWEEN FORCE, MASS AND ACCELERATION F 2F F m m 2m m (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
• Newton’s second law of motion states the 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 6N (b) the acceleration (a) Solution: 15N (a) F = (15 - 6)N = 9N (b) (b) Acceleration, a = F ÷ m = 9 N ÷ 3 kg = 3 N kgˉ¹ @ m sˉ²
Its time to do exercise!Exercise 2.5 the effect of force