Forces Between Objects.
A force can change the shape of an object or change its state of rest or its motion.
Equal and Opposite forces
Whether 2 objects push or pull on each other, they exert equal and opposite forces on one another.
The unit of force is the newton (N).
The driving force on a car is the force on the car which makes it move. This is sometimes referred to as the engine force or the motive force. This force is caused by friction between the tyre of each drive wheel and the ground. Friction acts when the tyre is in contact with the ground.
When the car moves forward, the force of friction of the road on the tyre is in the forward direction, and the force of friction of the tyre on the ground is in the reverse direction.
Zero Resultant Forces- P2.2
The resultant force is a single force that has the same effect as all the forces acting on the object.
Zero Resultant Force
When the resultant force on an object is zero, the object remains stationary if it was at rest, or the object continues to move at a constant speed in the same direction if it was already moving.
If only 2 forces act on the object, they must be equal to each other and act in opposite directions.
When a heavy crate is pushed across a rough floor at a constant velocity, the resultant force on the crate is 0. The push force on the crate is equal in size but acts in the opposite direction to the force of friction of the floor on the crate.
A glider on a linear air track floats on a cushion of air. As long as the track stays level, the glider moves at a constant velocity along the track, as friction is absent, and therefore the resultant force is zero.
Non-zero Resultant Forces.
When the resultant force on an object is not zero, the movement of the object depends on the size and direction of the resultant force.
When a jet plane is taking off, the thrust force of its engines is greater than the force of air resistance on it. The resultant force on it is the difference between the thrust force and the force of air resistance on it. The resultant force is therefore non-zero. The greater the resultant force, the quicker the take-off is.
When a car driver applies the brakes, the braking force is greater than the force from the engine. The resultant force is the difference between the braking force and the engine force. It acts in the opposite direction to the cars direction, so the car deccelerates.
The examples above show that if an object is acted on by 2 unequal forces acting in opposite directions, the resultant force is:
- equal to the difference between 2 forces
- in the direction of the 2 forces.
If 2 forces act in the same direction, the resultant force is equal to the sum of the 2 forces and acts in the same direction as the 2 forces.
Force and Acceleration
Resultant Force = Mass x Acceleration, or F = ma, where F is the force in newtons, m is the mass in kg, and a is the acceleration in m/s².
If the velocity of an object changes, it must be acted on by a resultant force. Its acceleration is always in the same direction as the resultant force.
The velocity of the object increases if the resultant force is in the same direction as the velocity. We say its acceleration is positive because it is in the same direction as the velocity,
The velocity of the object decreases if the resultant force is in the opposite direction to its velocity. Decceleration is due to the fact that friction is acting on the velocity of the object. We say it has a negative velocity, because it is in the opposite direction to the velocity.
The greater the resultant force on the object, the greater the objects acceleration.
The greater the mass of an object, the smaller its acceleration for a given force.