UNIT NO.8
Force and Motion
(Solved Exercise)
8.1 Encircle the correct option.
1. An example of contact force:
a.Electrostatic force b.Muscular force c.Magnetic force d.Gravity
2. The force between two negatively charged particles is:
a.Gravitational b.Frictional c.Repulsion d.Attraction
3. Which statement is true?
a. for every action force, there is a smaller reaction force in the opposite direction
b. for every action force, there is a smaller reaction force in the same direction
c. for every action force, there is an equivalent reaction force in opposite direction
d. for every action force, there is an equivalent reaction force in the same direction
4. The Earth’s pull on the objects is called:
a.Gravity b.Frictional force c.Electrostatic force d.Magnetic force
5. A batsman hits the ball back towards the bowler, which effect of force is seen:
a. change of shape b. change of direction
c. change of state from motion to rest d. change of state form rest to motion
6. Which one of the following statements is correct?
a. 1 m = 1000 km b. m/s is the unit of displacement
c. N is the unit of force d. 1hr = 60 s
7. Which one of the following statements is not correct?
a. Displacement is the shortest distance between two points.
b. Displacement is the distance covered in one second.
c. Metre (m) is the unit of displacement.
d. Rate of change of distance with time is called speed.
8.2 Define the following:
1. Contact forces
Contact force are those forces that act only when objects are in physical contact with other objects and bring about necessary changes.
2. Non-contact forces
Some forces do not involve physical contact between the objects on which they act. They act through the space between them. Such forces are called non contact forces.
3. Gravitational force
The Earth pulls the things towards itself with a specific force called gravity. Force that act on the body due to gravity is called gravitational force.
4. Displacement
Displacement is define as the shortest distance between two points.
5. Speed
Speed is defined as the distance travelled per unit time.
Speed = Distance / Time
8.3 Constructed Response Questions
1. A rocket has been fired upward to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.
1. Action Force
2. Reaction Force
Rocket engine pushes out hot gases as an action forces and the hot gases exert a reaction force on the rocket, which makes the rocket lift up.
2. Briefly describe the effect of non-contact force on each of the following:
a. Falling of apple from the tree
The falling of an apple from the tree is caused by the non-contact gravitational force, which pulls the apple towards the Earth.
b. A foot ball that is kicked
A football that is kicked experiences the non-contact force of gravity, pulling it downward while it travels through the air.
c. Iron pins are attracted towards a magnet
The iron pins experience a non-contact magnetic force, which attract them towards the magnet.
d. A porter carrying a load
A non-contact force, such as gravity, pulls the load downward, increasing the weight the porter must carry.
e. Charged comb attracts small pieces of paper
A charged comb attracts small pieces of paper due to the non-contact electrostatic force, which induces opposite charges in the paper, pulling them toward the comb.
f. Bringing like poles of two magnets near each other
Bringing like poles of two magnets near each other results in a non-contact repulsive magnetic force, causing them to push away from each other.
3. The detail of a 12 hours journey of a car with an average speed of 60 km/hr is given below:
| Duration | Distance Covered |
| 0 – 2 hours | X |
| 2 – 6 hours | 240 km |
| 6 – 12 hours | 240 km |
Find the value of X.
Solution:
Time = 2 hrs
Velocity = 60 km/hr
Distance = velocity x time
Distance = 60 x 2
Distance = 120 km
So value of X will be 120 km.