# Moving Things, People And Ideas

## Force and Pressure

Force: A push or pull on an object is called force.

• At least two objects need to interact for the force to come into play. Mutual interaction between two objects results in a force between the two objects.
• When two forces are applied on an object in the same direction, there is an add-on effect of both forces.
• When two forces are applied in opposite directions, the difference between two forces gives the value of the net force.
• Force is expressed in terms of its magnitude and the direction in which it is working. Any change in direction or magnitude of the force, there is a change in the effect of the force.
• Generally, more than one force is acting on an object at any given time.

## Effect of Force on State of Motion

A force can change the state of motion. Both rest and motion are called states of motion. When an object is at rest, its speed is zero.

Effect of force on an object at rest: A force can move an object at rest. For example; if a force is applied on a book the book can move in the direction of the force.

Effect of force on a moving object: A force can have following effects on a moving object.

• When the applied force is in the same direction as the direction of motion, there is an increase in the speed of the object.
• When the applied force is in opposite direction of motion, there is a decrease in the speed of the object. The object may also come to rest when the applied force is in opposite direction to the direction of motion.
• When the applied force is at some acute or obtuse angle to the direction of motion, there is a change in the direction of motion.

Effect of Force on Shape: Force can change the shape of an object. When force is applied on a ball of dough, there is a change in the shape of the dough. Similarly, a potter changes a lump of clay into pots by applying force. When air is filled in a balloon; the shape of the balloon changes because of the applied force.

## Types of Force:

There are two main types of forces, viz. contact force and non-contact force.

• Contact Force: Contact force is a force which can produce its effect only by making a contact with the object. Following are the contact forces:
1. Muscular Force: It is the force applied by muscles of humans or animals. A Bullock is able to pull a cart because of muscular force. The bullock needs to be harnessed to the cart for applying the force.
2. Friction: When one surface is moving over another surface, a force comes into play and opposes their relative motion. This force is called friction or force of friction. Friction always opposes the motion because it acts in opposite direction to the motion. Force of friction arises due to contact between two surfaces. If there is no contact between two surfaces there would be no friction.
• Non-contact Force: The non-contact force is a force which can produce its effect without making a contact with the object. Following are the non-contact forces:
1. Magnetic Force: The force applied by a magnet is called magnetic force. A magnet can exert its force on an object even from a distance and does not need to touch that object.
2. Electrostatic Force: It is the force applied by a charged body on another charged or non-charged body. When you brush a comb in your hairs and then bring the comb near small bits of paper, the comb attracts the bits of paper. This happens because of electrostatic force.
3. Gravitational Force: Every object in the universe exerts a force on another object. This force is called gravitational force. It is the gravitational force of various celestial bodies which keeps them in their position in the space.

## Applied Force Affects Motion of Object

An applied force affects the motion of an object . An applied force can be a push, pull, or dragging on an object.

The push can come from direct contact, like when objects collide or from a force field like magnetism. The pull seems to only come from a field at a distance, like gravity or magnetism. Dragging can occur when sliding an object over the surface of another.

The action from a force can cause an object to move or speed up (accelerate), to slow down (decelerate), to stop, or to change direction. Since any change in velocity is considered acceleration, it can be said that a force on an object results in the acceleration of an object.

## Applied force can cause acceleration

When a force acts on an object that is stationary or not moving, the force will cause the object to move, provided there are no other forces preventing that movement. If you throw a ball, you are pushing on it to start its movement. If you drop an object, the force of gravity causes it to move.

If an object is initially stationary, it accelerates when it starts to move. Acceleration is the change in velocity over a period of time. The object is going from v = 0 to some other speed or velocity.

Likewise, if an object is already moving and a force is applied in the same direction, the object will speed up or accelerate. For example, a gust of wind can speed up a sailboat.

## Applied force can cause deceleration

If an object is moving and there is an applied force in the opposite direction of the motion, the object will decelerate or slow down. If you throw a ball up at a given velocity, it will slow down as it travels upward due to the force of gravity. Likewise, an aeroplane will decelerate if flying into a strong headwind.

A decelerating force can cause a moving object to stop. This can be seen when you apply the brakes on your car.

## Applied force can cause change in direction

A force applied at an angle to the direction of motion of an object can cause it to change direction. A side wind will cause an aeroplane to change its direction.

It is possible that the object keeps going at the same speed if the force is applied perpendicular to the direction of motion. But the velocity of the object changes. Speed is how fast the object is going, while velocity is speed plus direction.