Chapter at a glance
- A force is a push or pull on an object resulting from the object’s interaction with another object.
- At least two objects must interact for a force to come into play; the SI unit of force is newton (N).
- A force can make an object move from rest, change its speed, change its direction of motion, change its shape, or cause some or all of these effects together.
- Forces are of two types: contact forces (require physical contact) and non-contact forces (act from a distance).
- Contact forces include muscular force and frictional force; non-contact forces include magnetic force, electrostatic force and gravitational force.
- Friction always acts opposite to the direction of motion (or attempted motion) and arises due to irregularities on surfaces in contact.
- The force with which the Earth attracts an object towards itself is called the weight of the object (SI unit N); gravitational force is always attractive.
- Mass of an object remains the same everywhere while its weight can vary; liquids exert an upward buoyant force (upthrust) on objects immersed in them.
Definitions and laws
- “A force is a push or pull on an object resulting from the object’s interaction with another object. The SI unit of force is newton (written with a small ‘n’) and its symbol is N.”
- “The force that comes into play when an object moves or tries to move over another surface is called the force of friction or simply friction. Friction always acts in a direction opposite to the direction in which the object is moving or trying to move.”
- “The force exerted by a magnet on another magnet or a magnetic material is called magnetic force.”
- “The force exerted by a charged body on another charged body or an uncharged body is called electrostatic force.”
- “The force with which the Earth attracts objects towards itself is called the gravitational force. The gravitational force exerted by the Earth is also called force of gravity or simply gravity.”
- “The force with which the Earth pulls an object towards itself is called the weight of the object. … SI unit of weight is also newton (N).”
- “The force applied by a liquid on an object in the upward direction is known as upthrust or buoyant force.”
Important diagrams and activities
- Fig. 5.1 (pushing, pulling, lifting a box) – shows different ways muscular force is applied.
- Fig. 5.5 & 5.6 (object sliding on surface + surface irregularities) – demonstrates friction opposing motion.
- Fig. 5.7 (ring magnets on a stick) – shows magnetic force acting without contact (like poles repel).
- Fig. 5.8 & 5.9 (charged plastic scale attracting paper; charged balloons repelling) – illustrates electrostatic force and like/unlike charges.
- Fig. 5.11 (object dropped/thrown vertically) – shows effect of gravitational force on vertical motion.
- Fig. 5.12 & 5.13 (spring with objects; spring balance scale) – demonstrates measurement of weight using extension of spring.
- Activity 5.3 & 5.4 – investigate friction on different surfaces.
- Activity 5.9 & 5.12 – use spring balance to compare weights of different objects.
Common misconceptions and exam pitfalls
- Confusing mass (constant, in kg) with weight (force, in N) – many students write “weight in kg”.
- Thinking friction is always undesirable; NCERT emphasises it is necessary for walking, writing, etc.
- Believing no force acts on a stationary object – balanced forces may be acting.
- Assuming non-contact forces require no interaction at all – interaction still occurs (field concept introduced later).
- Writing that gravitational force can be repulsive – text states it is always attractive.
- Overlooking that friction acts opposite to the direction of motion or attempted motion.
Formula sheet
No numerical formulas or equations are given in the chapter. The only quantitative information is the SI unit of force and weight: newton (N).