VIDEO OF MOTION

What cause motion….?A force also can cause:

a. a moving object to change its speed

c. a moving object to change its direction of motion

d. an object to change in size and shape

FORCE

THE EFFECTS OF A FORCE

FORCE

1. A force is a pulling or a pushing action on

an object.

2. We make use of forces in our daily life all

the time.

3. Forces have magnitude and direction.

4. The unit of force is newton (N) or (kg m s-2).

5. A force is not visible but the effects of

forces which can be observed.

Balanced Force:

1.An object may have several

forces acting on it. If the forces

are in balance, they cancel

each other out. Then, the

object behaves as if no force is

applied to it (net force or

resultant force is 0).

The effects of balanced forces on object:

2. With balanced forces on

it, an object is either at

rest, or moving at a

constant velocity. (net

force or resultant force is

0).

EXAMPLES:Balanced Force

1. Balance forces on a stationary cup

Another ex.

2. Balance forces on the aircraft result in it moving at constant velocity

Another Ex.

Unbalanced Force:

1.An object will accelerate when

forces acting on it are not balanced.

how to know balanced

force---- it has net force (resultant

force).

The effects of unbalanced forces on object:

EXAMPLES:Unbalanced Force

Example

Another Example

Relation Between Force, Mass, and Acceleration:

A Question

SOLVING PROBLEMS1. When a force acts on a mass of 3 kg,

the resultant acceleration is 2 m s-2. Calculate the force………

3 kgF (N)

a = 2 m s-2

F = m a m = 3 kg

a = 2 m s-2

F = 3 kg 2 m s-2

F = 6 kg m s-2 = 6 N

Solution:

2. Azhari applies a force of 50 N to move a 12 kg box at a constant velocity. What is the frictional force acting on the box?

Constant velocity ------ acceleration, a=0

Net force in the direction of velocity, F = F1 - f

= 50 N - f

from F = ma

50 N – f = 12 kg 0 m s-2

f = 50 N – 0 = 50 N

12 kg F1 =50 N

f

Solution:

3. What force is required to move a 2 kg object with an acceleration of 3 m s-2, if:

a. the object is on a smooth surface? (f=0)

b. the object is on a surface where the average force of friction acting on the object is 2 N?

Solution:

a. From F = ma

F1- f = m a

F1- 0 = 2 kg 3 m s-2

F1 = 6 N (a force of 6 N is required to accelerate the object at 3 m s-

2)

b. Net Force (F) = F1 – f = F1 – 2 N

From F = ma

F1 – 2 N = 2 kg 3 m s-2

F1 = 6 N + 2 N = 8 N (a force of 6 N is required to accelerate the object at 3 m

s-2)

summary