True/ False questions

(1) During the collision of a car with a large truck, the truck exerts an equal size force on the car as the car exerts on the truck.
(2) If two object have the same acceleration, they are under the influence of equal forces
(3) An object's velocity will change if a net force acts on the object
(4) The net force which acts on an object which maintains a constant velocity is zero
(5) If a net force acts on an object, the object's speed will change.

1) True

2) False

3) True

4) True

5) False

Explanation:

1)

According to the Newton's third law of motion every action has an equally opposite reaction. Hence the force acted upon by the truck and car is equal in magnitude and in opposite direction when they collide with each-other.

2)

It is not necessary that the two objects having same acceleration are being acted upon by equal force because acceleration also depends upon the mass of moving body.

Mathematically given as:

where:

a = acceleration

F = force

m = mass of the moving body

So, we can have various combinations of force and mass for which the acceleration of the body is equal.

3)

Velocity being a vector quantity possesses both the magnitude and the direction and according to the Newton's first law of motion a body continues to move with uniform velocity until acted upon by an external net force. So, when an external force acts on the body then the state of uniform motion of the body is changed which may involve the change in magnitude of the velocity or change in the direction of the velocity or both of these.

4)

The state of uniform motion under the influence of forces resulting zero is observed when a body is pushed on a rough surface to have a uniform motion. The applied force in this case nullifies the frictional force acting on the body due to the relative motion between the surface of the body and the other fixed surface.

This is in accordance with the Newton's second law of motion which states that the rate of change in momentum is directly proportional to the force applied.

5)

When a net force acts on a moving body then it is not necessary that the speed of the body will change, it may also happen that only the direction of the motion changes whereas its speed remains constant as in the case of a turning of a vehicle at a constant speed.

the velocity you are looking for is made up of two parts: horizontally velocity, vh, and its vertical velocity, vv. the actual velocity of the ball will be these two parts added together, and because velocity is a vector then they will need to be added vectorially.  

neglecting air resistance etc etc.,  

the horizontal velocity component will be constant (because there is no accelerating force in the horizontal direction.  

vh = 8 ms-1  

the vertical velocity component has an accelerating force: 'g'.  

using v = u +at where a = g = 9.81 ms-2.  

the vertical component of velocity, vv = 0 + gt = vv =9.81 x 2  

vv = 19.62 ms-1 . the zero appears because the virtical component of velocity at time t=0 is zero. remember, the ball was thrown horizontally.  

how fast it will be moving (it actual path (being a parabola )) will be the vector sum of these two.  

adding vectorally gives: -  

v = sqr root (8^2 + 19.62^2) = sqr root(64+384.9) = 21.18 or 21.2 ms-1