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The average force is given by mass*acceleration.  The acceleration here is -30/25 m/s² (it is negative because it is a deceleration), so the force is -1000*30/25 = -1200 Newtons.  It is acting in the opposite direction to the direction of travel.

Why is no onw helping me?

We only have a few people who know Physics on here......none of them are online at the moment!!!

Sorry!!!!

 truck with a mass of 1.000 kg is movingat a constant speed of 30 més. It is brought to rest in 25 s.What is the magnitude and direction of the force acting to stop the car?

I think you have to use the "Impulse" formula for your problem. You also have to specify the DIRECTION that the truck is movig,i.e., Eastward, Westward.......etc.because momentum is a Vector quantity. So, you can try this formula:F = (mass * velocity change)/time=(1,000 X 30)/25=1200 Newtons, would be the magnitude of force acting on the truck. The DIRECTION is something you have to specify in advance.

You may discuss it with your teacher or instructor, if it's wrong. Good luck.